Vaccine Research
Vaccine Studies
Here you’ll find a list of peer-reviewed journal articles covering vaccine safety and the impact of vaccination.
This information can help you navigate questions about specific vaccines and address common concerns.
Annals of Internal Medicine: Measles, Mumps, Rubella Vaccination and Autism: A Nationwide Cohort Study (3/5/19)
A large study in Denmark followed more than 657,000 children from 1999 to 2010. Researchers wanted to know if the MMR vaccine had anything to do with autism. They used Denmark’s national health records to see which children were vaccinated, which ones were diagnosed with autism, and whether autism ran in their family.
During the study, doctors diagnosed more than 6,500 children with autism. The researchers then compared children who got the vaccine to those who did not. They found that children who got the MMR vaccine were not more likely to get autism. In fact, slightly fewer vaccinated children had autism.
Researchers also looked at children in the study who were considered more likely to get autism, such as those with a brother or sister who had it. Even in those children, the MMR vaccine did not raise the risk of autism.
The authors said their findings back up what scientists agree on: the MMR vaccine does not cause autism—not even in children whose families have a history of it.
JAMA: Autism Occurrence by MMR Vaccine Status Among US Children With Older Siblings With and Without Autism (4/21/15)
Surveys showed that nearly 1 in 5 parents of a child with autism put off or skipped the MMR vaccine for their younger children. Many worried that the vaccine might cause autism in a child they thought was more likely to get it.
To check this, researchers looked at health insurance records for 95,727 children born in the U.S. between 2001 and 2007. Each child had an older brother or sister included in the same records. About 2% of the children had an older brother or sister with autism. These children were more likely to have autism because it runs in the family.
Researchers compared children who got the MMR vaccine to children who did not, and they found no higher risk of autism in either group. In fact, among children with an older brother or sister who had autism, fewer of the vaccinated children had autism.
The study had some limits, though. It used insurance records, which leave out vaccines given at schools or public health clinics. And children in the study all had private insurance. But even so, the authors concluded that the MMR vaccine does not raise the risk of autism, even in children whose families already have a history of it.
Journal of Pediatric Pharmacology and Therapeutics: Thimerosal-Containing Vaccines and Autism: A Review of Recent Epidemiologic Studies (July–Sept 2010)
This article looked at whether thimerosal causes autism. Thimerosal is a preservative made with a type of mercury called ethylmercury. It used to be added to many childhood vaccines to keep them from going bad.
In 1999, the American Academy of Pediatrics (AAP) and the U.S. Public Health Service (USPHS) said thimerosal should be taken out of childhood vaccines just to be safe. In 2004, the Institute of Medicine (IOM) had rejected the idea that vaccines with thimerosal cause autism.
The review found:
- No link between vaccines with thimerosal and autism. The few studies that said there may be a link were not well done or were altered, so their findings could not be trusted.
- A real link is very unlikely. The kind of mercury in thimerosal is not the same kind that harms the brain and nervous system. Mercury in thimerosal breaks down quickly and leaves the body; it does not build up inside the body.
The authors found that the science does not support a link between vaccines with thimerosal and autism.
NEJM: Early Thimerosal Exposure and Neuropsychological Outcomes at 7 to 10 Years (9/27/07)
In a 2007 study, researchers tested 1,047 American children between the ages of 7 and 10. They gave each child 42 tests to measure memory, attention, language, reading, fine motor skills, and behavior. For each child, the team added up all the thimerosal mercury the child had taken in before birth and during their first seven months of life. Researchers gathered this information from vaccine records, medical records, and interviews with parents.
The researchers found that children who had more exposure to thimerosal in early life did just as well on all 42 tests years later. They found no link between thimerosal and problems with the brain or development.
In fact, the results went in the other way. Children with more exposure to thimerosal scored slightly better on one test of fine motor skills and on one test of attention.
This study found no evidence that thimerosal caused any problems in how children’s brains worked at ages 7 to 10. This is one of several large studies that support the safety of vaccines with thimerosal.
Lancet: Mercury Concentrations and Metabolism in Infants Receiving Vaccines Containing Thiomersal: A Descriptive Study (11/30/02)
Thimerosal contains a type of mercury called ethylmercury, but most of what scientists knew about mercury and the growing brain came from studies on a different type of mercury, called methylmercury. This kind comes mainly from some kinds of fish, and it stays in the body for weeks or months. People worried that ethylmercury might act in the same way and build up in babies after each vaccine dose.
To find out, researchers studied 40 healthy babies (newborn to 6 months old) who got routine vaccines with thimerosal. They also studied 21 other babies who did not get vaccines with thimerosal. Then, they measured the mercury in each baby’s blood, urine, and poop at different times after each dose.
The study found three main things:
- The mercury in the babies’ blood stayed below the safety limits that U.S. safety agencies set, even after the largest doses of ethylmercury.
- Ethylmercury leaves the blood quickly. Its half-life (the time it takes for half of it to leave the body) is about 7 days. For methylmercury, it takes about 6 months. This means most of the vaccine mercury left the body within a week or two after each dose.
- The body actively gets rid of mercury, mostly through the babies’ poop. There is strong proof that thimerosal does not build up to dangerous levels after repeated doses.
This study shows that ethylmercury acts very differently from the methylmercury in fish. It does not stay in the body or build up.
Medscape: ‘Nearly Every’ Child With Measles Suffers This Hidden Threat (2/27/26)
More people are getting measles across the U.S. After cases rose in 2025, experts warned of a hidden problem called “immune amnesia.” This 2026 research looked at that health problem.
The measles virus attacks the memory cells in the immune system. These are the cells that remember the viruses and bacteria from past infections and vaccines. When the measles virus destroys these cells, the body forgets how to fight off some infections. A child can lose between 11% and 73% of their immune memory. This leaves the child open to getting sick from illnesses their body could once fight off.
The effects can last for up to five years. During that time, the child has a higher chance of catching other infections, going to the hospital, and missing school. Doctors did not fully understand immune amnesia until 2015. But long before that, doctors in Scotland noticed that after measles spread through an area, many people died in the following years from other sicknesses. Records of this go back to the 1700s.
Measles is far more than a childhood rash. Vaccination protects not only against the disease itself. It also protects against the silent immune damage measles can leave behind.
Science: Measles Virus Infection Diminishes Preexisting Antibodies That Offer Protection from Other Pathogens (11/1/19)
In this study, scientists used a blood test called VirScan to check disease-fighting antibodies in children before and after they caught measles. Then they compared the results to those of children who got the measles vaccine.
They found that measles wipes out much of the immune system’s memory of past diseases. This leaves children open to diseases their bodies could once fight off—even ones their vaccines protected them from. They found that the measles vaccine did not cause this loss of immune memory and is therefore much better than catching measles.
Almost everyone who gets measles will experience some immune memory loss. Measles weakens adult immune systems, too. See the following resources for more:
- University of Colorado Anschutz Dept. of Medicine: Immune Amnesia: When Measles Makes Your Body Forget How to Fight Infections
- Cleveland Clinic: Does Your Adult Patient Need a Measles Vaccine?
Science Immunology: Incomplete Genetic Reconstitution of B Cell Pools Contributes to Prolonged Immunosuppression After Measles (11/1/19)
Researchers studied blood samples from children before and after they caught measles. They found two ways measles damages the immune system:
- The body struggles to rebuild its supply of B cells (the immune cells that learn to spot and fight new threats). This leaves the immune system weaker and less developed.
- Measles destroys “memory” B cells. These are the cells that remember past infections and the diseases a child’s vaccines cover.
These changes can last even after a child seems fully recovered. This leaves children open to infections and diseases their immune systems once fought off. Together, these findings show why measles vaccination matters. It prevents measles and protects everything else a child’s immune system has learned over time.
The BMJ: Aluminum Adjuvants in Vaccines and Potential Health Effects: Systematic Review (5/6/26)
Scientists did a large review to find out whether the small amounts of aluminum in some vaccines cause any serious or lasting health problems. Vaccine makers have added tiny amounts of aluminum to some vaccines for almost 100 years to help them work better and last longer. These vaccines protect against diphtheria, tetanus, pertussis (whooping cough), hepatitis, HPV, and meningitis.
The team reviewed 59 studies—including high-quality trials and large studies that followed groups of people over time. These studies checked for possible health problems, such as autism, asthma, type 1 diabetes, headaches, muscle pain, and skin reactions. After looking at each study, the researchers found that the evidence shows no connection between vaccines with aluminum and any serious or lasting health problem. The most common reaction was a small, harmless lump where the needle went in that went away on its own.
Some weaker studies raised concerns about a rare muscle condition. But these studies were too small and had too many mistakes to prove anything. Overall, the researchers said these findings back up decades of safety data. They also support keeping aluminum in vaccines that are given around the world.
Annals of Internal Medicine: Aluminum-Absorbed Vaccines and Chronic Diseases in Childhood: A Nationwide Cohort Study (7/15/25)
A large study looked at more than 1.2 million children born in Denmark between 1997 and 2018. The goal was to find out whether the small amounts of aluminum in some childhood vaccines might lead to long-term health problems.
Vaccine makers have put small amounts of aluminum in some vaccines for decades to help them work better. But parents and researchers have asked whether it could lead to lasting health problems.
To look into this, researchers followed these children through national health records for more than 20 years. They looked at 50 different long-term health problems. The health problems included autism and other brain and development problems, type 1 diabetes and other autoimmune diseases, asthma, hay fever, and food allergies. They compared children who got more aluminum through their early vaccines with children who got less. They also considered many other factors that could affect health.
Researchers found that vaccines with aluminum did not raise the risk of any of the 50 health problems they studied. But they could not fully rule out very small effects on some rarer problems. The findings add to the evidence that supports keeping these vaccines in vaccination schedules.
Vaccine: Pharmacokinetic Modeling as an Approach to Assessing the Safety of Residual Formaldehyde in Infant Vaccines (6/7/13)
Researchers at the U.S. Food and Drug Administration used a computer model to check whether the tiny amount of formaldehyde in some childhood vaccines is safe. Vaccine makers use formaldehyde when making certain vaccines, such as DTaP, polio, and hepatitis B. Formaldehyde kills viruses and stops poisons (toxins) from bacteria from working. They remove almost all of it after making the vaccine. But a very small amount (no more than 0.02% per dose) can remain.
To test safety, researchers built a model of how formaldehyde moves through a baby’s body. They used it to predict what would happen if a baby got a typical round of vaccines with formaldehyde (about 200 micrograms total) in one visit.
The model showed that the most formaldehyde moving through the body at any time would be less than 1% of what the body naturally makes every day. The body makes formaldehyde as a normal part of how it works. It builds, uses, and gets rid of large amounts of it every day as its cells do their work.
The researchers found that the tiny amount of formaldehyde in childhood vaccines is safe. It is well below the levels the body handles every day without harm. Many people now point to this study as proof that formaldehyde left in vaccines does not put children at risk.
Journal of Cellular and Molecular Medicine: The Cellular Function and Molecular Mechanism of Formaldehyde in Cardiovascular Disease and Heart Development (5/10/21)
This review explains that formaldehyde is a natural part of how the human body works. Every cell makes small amounts of it. Healthy blood normally holds a small, measurable amount at all times. At these natural levels, formaldehyde does important jobs in the body. But at high levels, like the levels some workers breathe in over many years, or the levels found in polluted air, formaldehyde can hurt blood vessels and heart muscle cells. This raises the chance of heart attacks, heart failure, and clogged arteries.
The review’s main point is that the amount and length of time of formaldehyde exposure matter. Small, natural amounts help the body. But breathing in high amounts for a long time can hurt it.
How this relates to formaldehyde in vaccines:
Health problems can come from breathing in high doses of formaldehyde for years, usually at a job or in a factory. The body clears the leftover formaldehyde from a vaccine within about 30 minutes. It does not push blood levels above the body’s normal level.
This same review that warns about formaldehyde’s risks at extreme levels also shows why the amount found in vaccines is too small to matter. It falls far below any level known to cause harm.
BMJ Paediatrics Open: Understanding Adverse Events Following Immunisation: Evidence of Vaccine Safety and Coincidental Events from 3 Years of Surveillance (3/23/26)
In this study, researchers spent three years carefully looking at health events that happen after routine childhood vaccinations. They studied both events the vaccine causes and those that happen by chance in the days and weeks that follow.
The study found that routine childhood vaccines are very safe overall. Most people reported mild events, like soreness where the needle went in, a low fever, or crankiness. These are normal, passing side effects that go away on their own. Serious events were rare. Most of the health problems were “chance events,” which are problems that show up after getting vaccinated but that are not caused by the vaccines.
The researchers wrote things down carefully, checked back with families, and compared how often kids in general get sick. They pointed out that this kind of careful watching matters. It reassures families that when a problem happens after a vaccination, it most likely is by chance and has nothing to do with the vaccine.
Journal of Allergy and Clinical Immunology: Risk of Anaphylaxis After Vaccination in Children and Adults (March 2016)
In this study, researchers looked at the Vaccine Safety Datalink. This is a group of large U.S. healthcare systems that links each person’s medical records to their vaccination records.
The researchers looked at every dose given over three years (January 2009 to December 2011). That added up to more than 25 million vaccine doses in children and adults. The team searched the records for any signs of anaphylaxis (severe allergic reaction that can be deadly). Then doctors checked each case closely. They used strict international guidelines to make sure the reaction was really anaphylaxis. They also checked that the vaccine likely caused it.
In the end, only 33 cases met these strict guidelines across 25 million doses. That is about 1 in every 1 million doses. This shows that serious allergic reactions after vaccines are very rare. The rate was about the same in kids and adults. Most people who reacted already had a history of asthma or other allergies. All recovered fully. Rates for each vaccine were just as low.
Experts now use this 1-in-a-million number as a guide when they judge the safety of newer vaccines.
NEJM: HPV Vaccination and the Risk of Invasive Cervical Cancer (9/30/20)
Researchers in Sweden used real-world data to show for the first time that the HPV vaccine prevents cervical cancer (not just the early cell changes that come before cancer). Before this study, people trusted the vaccine mostly because it could stop lasting HPV infections and serious cell changes. These problems can turn into cervical cancer over time.
This study followed nearly 1.7 million girls and women in Sweden between the ages of 10 and 30 using the country’s health records from 2006 to 2017. It tracked who got the four-strain HPV vaccine and who got cervical cancer. More than half a million of the women got the vaccine. Among them, doctors only found 19 cases of cervical cancer. Doctors found 538 cases among women who did not get vaccinated. The researchers made sure age and other factors did not explain the difference.
Vaccinated women had about a 63% lower risk of getting cervical cancer. The study also found that when women got the vaccine mattered. Women who got the vaccine before age 17 had an 88% lower risk of cervical cancer. Women who got it between ages 17 and 30 still had a 53% lower risk.
The findings backed up earlier research showing the HPV vaccine is a powerful tool against cervical cancer. Getting it earlier gives women more protection. Experts around the world now point to this study when they push to grow HPV vaccination programs. Their goal is to wipe out cervical cancer as a public health threat.
Lancet: Cervical Cancer Mortality Trends Following HPV Vaccination in England, 2001–24: An Analysis of Population-Based Mortality Data (7/11/26)
Researchers looked at cervical cancer death rates in England from 2001 to 2024. They focused on women ages 20 to 34. These women were the first generation to get the HPV vaccine through England’s national program, which began in 2008.
Among women ages 20 to 24 who got the vaccine at age 12 or 13, no one died of cervical cancer. Based on past rates without vaccination, researchers would have expected around 23 deaths. Among women who got the vaccine at older ages, deaths also dropped a lot. But some of these women had already come into contact with HPV before getting vaccinated, which means the vaccine was not able to fully protect them.
Researchers estimated that HPV vaccination prevented about 200 cervical cancer deaths in England through the end of 2024. These findings support vaccinating girls between age 12 to 13, before they come into contact with HPV. This gives them the greatest protection against cervical cancer.
Frontiers in Public Health: Optimizing HPV Vaccine Effectiveness: Impact of Vaccination Age and Dose Schedule on Immunogenicity and Cervical Cancer Prevention (5/7/25)
This review looked at what age is best to get the HPV vaccine and how many doses are needed. The researchers pulled together results from 28 studies published between 2013 and 2023. The studies covered all three HPV vaccines (Cervarix, Gardasil, and Gardasil 9), different age groups, and age at vaccination.
When children got vaccinated between ages 9 and 14, the vaccine worked 74–93% of the time. But when children got vaccinated between ages 15 and 18, it worked 12–90% of the time, depending on the study. The youngest children had the best response because their bodies made more HPV-fighting cells and their protection lasted longer.
The review also looked at how many doses were needed. Studies show that two doses given before age 15 work just as well as three doses given to older teens or adults. They also show that a single dose may protect almost as well as two or three doses against the most dangerous types of HPV (HPV-16 and HPV-18). These types cause most cervical cancers.
For women under age 25, three doses work nearly 100% of the time at preventing infection and changes in cells that can turn into cancer. Strong evidence shows that the vaccine prevents most cervical, anal, and vaginal cancers that HPV can cause.
The authors note that there is only about 10 years of data so far. More studies will help show how long that protection lasts. Overall, the authors recommend that young people get vaccinated early—ideally before they become sexually active—and that they get the right number of doses for their age.
GIRLS
International Journal of Epidemiology: Quadrivalent Human Papillomavirus Vaccination in Girls and the Risk of Autoimmune Disorders: The Ontario Grade 8 HPV Vaccine Cohort Study (5/28/18)
This large study followed 290,939 girls ages 12 to 17. All the girls could get the vaccine through Ontario’s school-based HPV vaccination program between 2007 and 2013.
The researchers looked at how often each girl got a new autoimmune disorder (when the body attacks itself by mistake) in the weeks after vaccination (days 7–60). The researchers found no increased risk of autoimmune disease after HPV vaccination. The researchers found that the HPV vaccine did not raise the risk of these illnesses. This stayed true even for girls who were already sick with other health problems.
These results give strong proof that the vaccine does not cause autoimmune illness in teen girls.
BOYS
International Journal of Epidemiology: Quadrivalent Human Papillomavirus Vaccination in Boys and Risk of Autoimmune Diseases, Neurological Diseases and Venous Thromboembolism (4/1/18)
This national study followed 568,410 boys born in Denmark between 2006 and 2016. The researchers matched each boy’s HPV vaccination records to hospital records for 39 autoimmune diseases, 12 brain and nerve diseases, and blood clots.
The study found no sign that HPV vaccination raised the risk of these health problems in boys ages 10 to 17. Earlier studies showed that the HPV vaccine is safe in girls. This study shows it is safe in boys as well.
Frontiers in Public Health: The Effectiveness of COVID-19 Vaccines in Reducing the Incidence, Hospitalization, and Mortality from COVID-19: A Systematic Review and Meta-Analysis (8/26/22)
Researchers combined results from 54 studies. They wanted to see how well the main COVID-19 vaccines—Pfizer, Moderna, and AstraZeneca—protected people from three things: catching COVID-19, going to the hospital, and dying from it.
The results were strong and stayed steady across different groups of people and different vaccines. After one dose, vaccines stopped infection about 71% of the time, prevented hospital stays 73% of the time, and prevented death 68% of the time. After two doses, those numbers went up: they stopped infection 87% of the time, prevented hospital stays 89% of the time, and prevented death 92% of the time.
The authors found that vaccination is still one of the best ways to prevent severe illness and death. Getting all the recommended doses gives the best protection against the most serious dangers of COVID-19.
Nature Communications: A Systematic Review and Meta-Analysis of the Impact of Vaccination on Prevention of Long COVID (11/24/25)
This review pulled together the best studies to answer a big question about COVID-19 vaccines: do they lower the chance of getting of Long COVID?
The researchers looked at 31 studies and combined the results. They found that people who got a COVID-19 vaccine before they got sick were about 23% less likely to get Long COVID than people who did not get the vaccine. Booster doses helped even more. They lowered the risk by about 26% compared to no vaccination. They also protected better than the first vaccine doses alone.
The findings add to growing proof that vaccination not only protects against severe COVID-19 but also lowers the risk of long-term symptoms—such as feeling very tired, having trouble thinking clearly, and having trouble breathing. These symptoms can linger for months or years after you get sick.
The researchers concluded that keeping up with COVID-19 vaccination and boosters may be one of the best ways to lower the long-term harm from COVID-19.
Sage Open Pediatrics: Myocarditis Following COVID-19 Vaccine: What Did We Learn? (6/20/25)
This review looked at more than 40 studies about myocarditis (when the heart muscle gets swollen and irritated) after mRNA vaccination. The researchers found that myocarditis after COVID-19 vaccination is rare—only 1.7 cases for every 100,000 people who get vaccinated. This happens most often in teen boys and young men, usually within 2 to 4 days of the second dose. The risk is about five times higher after the second dose than the first. It is also much higher in boys and men than in girls and women of the same age.
The good news is that most cases are mild. Problems like chest pain, trouble breathing, or a fluttering heartbeat usually go away with basic care. Most people fully recover.
The researchers also pointed out that catching COVID-19 is much more likely to cause myocarditis than the vaccine. This means the vaccine offers benefits for heart health.
The researchers found that even though the vaccine can cause myocarditis in rare cases, the mRNA COVID-19 vaccines are very safe overall. The benefits of vaccination outweigh the risks.
Nature Communications: Safety Outcomes Following COVID-19 Vaccination and Infection in 5.1 Million Children in England (5/27/24)
In this study, researchers looked at health records in England for more than 5.1 million children and teens ages 5–17. They wanted to directly compare the safety of COVID-19 vaccination with the risks of getting sick with COVID-19. They used the country’s vaccine, hospital, and testing records from December 2020 to August 2022.
Researchers looked at how often kids got rare but serious health problems—including heart swelling (myocarditis), brain and nerve problems, and other swelling problems—both after vaccination and after getting sick with COVID-19. They found that for children ages 5–11, the vaccine did not raise the risk of these problems at all. In children ages 12–17, the vaccine came with a small rise in myocarditis—about 5 cases for every million doses after a second Pfizer dose. Most cases were mild and went away quickly. No child under 18 died from myocarditis.
On the other hand, catching COVID-19 caused more serious problems in children who did not get the vaccine. For example, a dangerous swelling condition called multisystem inflammatory syndrome (MIS-C) showed up in 137 out of every million unvaccinated children ages 5–11. In unvaccinated children ages 12–17, it showed up in 84 per million. That is much higher than the risk from the vaccine.
Kids who got the vaccine before catching COVID-19 mostly did not get these serious problems. The researchers said the COVID-19 vaccine is safe for children, and it is less risky than catching COVID-19 itself.
Lancet Regional Health – Europe: COVID-19 Vaccination and Birth Outcomes of 186,990 Women Vaccinated Before Pregnancy: An England-Wide Cohort Study (8/13/24)
In this study, researchers looked at health records for almost 187,000 pregnancies in England. They wanted to know if women who got a COVID-19 vaccine in the year before getting pregnant had different birth results than women who did not get one.
They followed all pregnancies that reached at least 24 weeks between May 2021 and October 2022. Then they compared the women who had gotten a COVID-19 vaccine before pregnancy with those who did not. They found that women who got the vaccine before pregnancy had better results. Fewer babies came too early, and fewer were born too small. For women who got an mRNA vaccine (Pfizer or Moderna), fewer babies were stillborn compared to women who did not get the vaccine.
The only concern the researchers found was a slightly higher rate of blood clots during pregnancy in women who had got the AstraZeneca vaccine (not an mRNA vaccine). This did not happen with the mRNA vaccines.
This study gave strong proof that COVID-19 vaccination before pregnancy is safe and may lead to better birth results. This backs up the advice that women who plan to get pregnant should keep up with their COVID-19 vaccines.
MMWR: Effectiveness of 2024–2025 COVID-19 Vaccines in Children in the United States—VISION, August 29, 2024–September 2, 2025 (12/11/25)
This report looked at data from almost 98,000 visits kids made to emergency departments and urgent care clinics across nine U.S. states using CDC’s VISION Network. This system that links vaccination records with health data from 256 emergency rooms and urgent care sites. The goal was to see how well the 2024–2025 COVID-19 vaccines protected kids over the past year.
Researchers compared vaccinated and unvaccinated children who came in with COVID-19 symptoms between August 2024 and September 2025. The results showed strong protection for kids of all ages. For the youngest children (9 months to 4 years), the new vaccine stopped 76% of COVID-related emergency or urgent care visits within six months of getting vaccinated. Children and teens ages 5–17 saw 56% protection overall. Protection was about the same in both the 5–11 (51%) and 12–17 (61%) age groups. Looking further out, up to 10 months after getting the vaccine, protection in the youngest group stayed strong at about 77%. In older children, it dropped to about 45%.
The CDC said the 2024–2025 vaccines gave kids added protection, even to those who had been sick with COVID-19 before.
Vaccines: Influenza Viruses and Vaccines: The Role of Vaccine Effectiveness Studies for Evaluation of the Benefits of Influenza Vaccines (5/1/22)
This review looked at how flu vaccines work and how researchers can check whether they’re doing their job in the real world.
Getting vaccinated is the best way to reduce illness, hospital stays, and deaths from seasonal flu—especially for people who can get very sick, like young children and older adults.
Today’s flu vaccines help people from getting sick, but they are not perfect. Even when the vaccine is a good match for the flu that is going around that season, it usually works about 40–60% of the time. The authors say that flu vaccines clearly lower illness and death for people of all ages and risk levels.
Frontiers: Safety, Immunogenicity, Efficacy and Effectiveness of Inactivated Influenza Vaccines in Healthy Pregnant Women and Children Under 5 Years: An Evidence-Based Clinical Review (10/12/21)
Researchers looked at whether flu vaccines are safe and worth getting for pregnant women and young children. Because most vaccine research focuses on healthy adults, the researchers checked the evidence they could find for these two groups.
They looked through more than 3,700 studies and found 93 that met their standards. They focused on inactivated flu vaccines (the regular flu vaccine made from killed virus) rather than the needle-free nasal-spray version made from a live virus. Across the studies, the vaccine was generally easy for people to handle. Side effects were mild, such as a sore arm where the needle went in, or a low fever. These side effects didn’t last long.
The vaccine caused a strong immune response. Protective antibody levels showed up in 65–96% of pregnant women and 50–100% of young children. It also lowered the risk of lab-confirmed flu by about 50–70% in pregnant women and 20–90% in children.
How well it worked changed with the season and how well the vaccine matched the flu strains going around that year.
The authors found that the benefits of flu vaccination clearly outweigh the possible risks. They said both pregnant women and young children should get it. Vaccination protects mothers, their newborns—who get some protection passed to them through the placenta during pregnancy—and young children, who are among the biggest spreaders of the virus.
Reviews in Medical Virology: Relative Efficacy, Effectiveness and Safety of Newer and/or Enhanced Seasonal Influenza Vaccines for the Prevention of Laboratory‐Confirmed Influenza in Individuals Aged 18 years and Over: Update of a Systematic Review (2/24/25)
Researchers pulled together the results of many studies to ask whether newer (“enhanced”) vaccines protect adults better than the regular flu vaccine. They looked at four newer types of flu vaccines.
Most of the studies looked at older adults, because they can get sickest from flu. The authors looked at 59 studies in all. They found that some newer vaccines seemed to work a little better than regular flu vaccines at stopping the flu and keeping people out of the hospital.
More research is needed, but the newer vaccines may offer more protection against the flu for adults, especially older adults.
Human Vaccines & Immunotherapeutics: The Safety of Inactivated Influenza Vaccines in Pregnancy for Birth Outcomes: A Systematic Review (11/15/18)
In this study, researchers looked at whether getting a flu vaccine during pregnancy is safe for the baby. Pregnant women have a higher chance of getting very sick from flu. They are one of the top groups that should get vaccinated. Yet, many women choose not to, partly because they worry it might not be safe. To help answer those worries, the authors put together the results from 40 studies that compared how babies of vaccinated and unvaccinated mothers did at birth.
They looked at babies born too early, babies born too small, babies who were smaller than expected for their age, babies born with birth defects, and babies who died before birth. They found no sign that flu vaccination raised the chance of any of these things. In fact, vaccinated women were a little less likely to have a baby born too early or too small.
The authors said that these findings add to growing proof that flu vaccination during pregnancy is safe and may even be good for the baby.
JAMA Pediatrics: Maternal Vaccine Effectiveness Against Influenza-Associated Hospitalizations and Emergency Department Visits in Infants (12/18/23)
This 2023 study asked whether a flu vaccine during pregnancy protects newborns from serious flu in their first months of life when babies are too young to get their own flu vaccine.
Researchers used data from seven children’s hospitals across four flu seasons (2016–2020). They studied about 3,800 babies under 6 months old who went to the emergency room or hospital with breathing problems. They checked whether the babies who tested positive for flu were less likely to have moms who got the vaccine.
Researchers found that when moms got a flu vaccine during pregnancy, their babies were about one-third less likely to end up in the hospital or ER with the flu. The vaccine worked best for the youngest babies (under 3 months) and when moms got the vaccine in the third trimester (the last three months of pregnancy). In those cases, it cut the risk of the flu by about half.
The authors concluded that getting the flu vaccine during pregnancy protects babies during the months before they can get their own flu vaccine. That is a strong reason for pregnant women to get vaccinated.
Human Vaccines & Immunotherapeutics: Influenza Vaccines: Evaluation of the Safety Profile (1/30/18)
This review gathered the published evidence on how safe flu vaccines are across different groups of people. The authors looked at the two main types of flu vaccine—the injected vaccine and the live, weakened nasal-spray one. They studied how each works in children, healthy adults, older adults, and people with long-term health problems.
Researchers found that both types have a good safety record and people handle them well. Side effects are usually mild and short-lived: a red or sore arm with the injected vaccine, or a stuffy nose, mild fever, and tiredness for the nasal spray. Most go away within a few days. Serious reactions are very rare. For example, severe allergic reactions happen in fewer than one in a million doses. None of the studies tied a death to a flu vaccine.
The authors flag a few cautions. Children and teens with severe asthma or active wheezing should avoid the nasal-spray vaccine. Researchers still need to do more studies in children under two.
They concluded flu vaccines are very safe and remain the single best protection against the flu. Vaccine makers also update the vaccines every year, and no vaccine is completely risk-free for everyone. So, experts must keep watching the safety of the flu vaccine every year to keep the public’s trust.
Lancet: Respiratory Medicine: Safety, Immunogenicity, and Efficacy of a COVID-19 Vaccine (Nvx-CoV2373) Co-Administered with Seasonal Influenza Vaccines: An Exploratory Substudy of a Randomised, Observer-Blinded, Placebo-Controlled, Phase 3 Trial (February 2022)
This study looked at whether people could safely and effectively get the Novavax COVID-19 vaccine at the same time as a seasonal flu vaccine. The study was part of a larger clinical trial in the UK. It included about 430 adults who got either both vaccines together or the Novavax vaccine alone.
The researchers found that getting both vaccines together was safe. Serious problems were rare and similar in both groups. But people who received both vaccines at once found more minor reactions, such as a sore arm where the needle went in, tiredness, and achy muscles.
The flu vaccine produced a full immune response even when people got it alongside the COVID-19 vaccine. People made fewer antibodies against COVID-19 when they got both vaccines together, but this didn’t really change how well the vaccine protected them.
The Novavax COVID-19 vaccine worked 87.5% of the time in the group that got both, compared with the 89.8% in the group that got it alone. That difference small enough to be a chance in a study this size.
The authors said that giving both vaccines in a single visit is safe and effective. It is also stressed that it is more convenient to get both vaccines at once.
Lancet: Respiratory Medicine: Safety and Immunogenicity of a High-Dose Quadrivalent Influenza Vaccine Administered Concomitantly with a Third Dose of the mRNA-1273 SARS-CoV-2 Vaccine in Adults Aged ≥65 Years: A Phase 2, Randomised, Open-Label Study (April 2022)
Researchers tested whether adults 65 and older could safely get a high-dose flu vaccine and a Moderna COVID-19 booster at the same time.
The team signed up 306 U.S. adults in this age group and split them into three groups. Some got the flu vaccine alone, some got the COVID-19 booster alone, and some got both vaccines together.
Researchers found that getting both vaccines at once was safe. Serious side effects were rare in every group. More people had minor reactions, like a sore arm where the needle went in or tiredness. These two reactions were more common when vaccines were given together but were about the same as getting one vaccine on its own.
The vaccines did not weaken each other when given at the same time. The flu vaccine built strong antibody protection whether people got it alone or with the COVID-19 booster. The COVID-19 booster raised COVID-19 antibody levels just as much either way.
The researchers found that older adults can safely and effectively get both vaccines at the same visit. This can help keep more people in this higher-risk group up to date on their vaccines.
Vaccine: Safety of Co-Administration of mRNA COVID-19 and Seasonal Inactivated Influenza Vaccines in the Vaccine Adverse Event Reporting System (VAERS) During July 1, 2021–June 30, 2022 (March 2023)
This study looked at reports people sent to the Vaccine Adverse Event Reporting System (VAERS). Reports came from people who got an mRNA COVID-19 vaccine (Pfizer or Moderna) and a seasonal flu vaccine at the same visit during the 2021–2022 season. Researchers reviewed 2,449 reports over that year. Most came from people who got a COVID-19 booster and a flu vaccine in the same visit.
In the reports that weren’t serious, people described the same familiar side effects they get from either vaccine alone, including a sore arm, headache, pain, fever, and tiredness. These came up about as often whether people got the vaccines together or separately. Serious reports (15.8%) most often involved heart, infection, or breathing problems. Reports included 47 deaths, mostly from heart and blood vessel problems and infections. These are the same leading causes of death in the general U.S. population, which are not tied to the vaccines.
Reports of other problems researchers watched for were rare, including severe allergic reaction, heart inflammation, Bell’s palsy (temporary weakness or drooping on one side of the face), and Guillain-Barré syndrome (immune system attacks the body’s own nerves). When doctors reviewed them, most didn’t match the definition of those conditions.
The authors said that this large review found no unusual or unexpected safety warning signs from giving these vaccines together. These findings show that it is safe to get both vaccines at the same visit.
Pharmaceuticals: COVID-19 and Seasonal Influenza Vaccination: Cross-Protection, Co-Administration, Combination Vaccines, and Hesitancy (3/8/22)
This review looked at COVID-19 and seasonal flu vaccines given together from four angles. They looked at whether the flu vaccine might offer protection against COVID-19; how safe and effective it is for people to get both vaccines at the same time; the work on new combination vaccines that cover both viruses; and how the public feels about getting both vaccines at once.
Researchers found that people who got a flu vaccine were somewhat less likely to test positive for COVID-19. This may be because a vaccine can train the immune system to fight off new and unrelated threats better.
Three careful studies where people were randomly assigned to groups all showed that getting COVID-19 and flu vaccines at the same visit is safe. The study showed side effects were no different from either vaccine alone.
People also had a full immune response to all four flu strains. The COVID-19 antibody response stayed strong with most vaccine combinations. Where it dipped a little (as with the Novavax vaccine), protection against illness stayed high.
Public health agencies worldwide, including the World Health Organization (WHO) and CDC, recommend getting both vaccines at the same time. Still, the authors note that some people hesitate to get both at once. This remains a real challenge. But they note that the bigger challenge is vaccine hesitancy in general, not just worries about getting both vaccines at once.
Journal of Allergy and Clinical Immunology: Safety of Influenza Vaccines in Persons with Egg Allergy—A Systematic Review (February 2024)
This review shows that people with an egg allergy can safely get a flu vaccine. Researchers combined data from 31 studies of children and adults with egg allergies who got egg-based flu vaccines. This study included people with past severe allergic reactions to eggs.
Researchers looked at two things: giving the flu vaccine all at once (full dose) or splitting it into two smaller doses with a waiting period in between. Across every study they reviewed, no one died, no one had a severe allergic reaction, and no one went to the hospital because of a flu vaccine. A small number of people had mild reactions, but no more often than people without an egg allergy.
The current advice is that anyone 6 months or older with an egg allergy, no matter how severe their past reactions were, should get a flu vaccine every year. They can get any flu vaccine approved for their age without special steps, extra wait time, or a trip to an allergist.
This evidence backs up the current advice from major medical associations. Since the 2023–24 season, doctors no longer need to treat patients with egg allergies any differently when giving flu vaccines.
MMWR: Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2024–25 Influenza Season (8/29/24)
Most flu vaccines are still made in chicken eggs. For years, doctors told people with egg allergies to take special steps when getting a flu vaccine, such as getting the vaccine in a doctor’s office, waiting longer afterward, or choosing a specific egg-free brand of vaccine. That advice has changed.
The CDC and major health organizations now say that an egg allergy is no longer a reason to skip the yearly flu vaccine or take any extra steps, no matter how bad past reactions were. Many studies support this change.
They show that flu vaccines do not cause severe allergic reactions in people with egg allergies any more often than in those without egg allergies. This also includes people who have had severe reactions to eggs in the past.
Pediatrics: Hepatitis B Vaccination at Birth: Safety, Effectiveness, and Public Health Benefit (5/1/26)
This review looks at whether it’s better to give babies their first hepatitis B vaccine (HepB) at birth (“birth dose”)—what doctors in the U.S. have done since 1991—or to wait until later.
The question came up after a federal advisory committee voted in December 2025 to loosen the birth dose recommendation. The committee suggested that when a mother tests negative for hepatitis B during pregnancy, parents and their doctors could decide together whether and when to vaccinate the baby. If they do not vaccinate at birth, they should wait until the baby is at least two months old.
The authors gathered and studied a lot of evidence. This included disease-tracking data, clinical trials, systematic reviews, and vaccine safety records. They used it to compare the birth dose against a delayed first dose. They found strong proof that the birth dose is both safe and effective. They found no proof that waiting makes the vaccine any safer or work any better. They also found no proof for using blood tests to check a baby’s immunity before finishing the vaccine series.
Routine infant vaccination has already cut hepatitis B infections in children by 99%. Because of this, the authors said that no evidence supports changing the vaccine recommendation that has been around for a very long time. They warned that changing it could really harm children’s health.
Journal of Pediatric Pharmacology and Therapeutics: Why the Birth Dose Matters for Hepatitis B Vaccination (2/31/26)
In late 2025 and early 2026, federal health officials moved away from the long-standing recommendation that all newborns get a hepatitis B vaccine at birth. They shifted to a risk-based approach, where babies routinely get the vaccine only if the mother tests positive for hepatitis B or she doesn’t know if she has it. This study argues that the change is dangerous and unwise.
Newborns face unique danger. Up to 90% of babies who catch hepatitis B will get a lifelong infection, compared with only 5% of infected adults. And 25% of those children may die early from liver disease or liver cancer. The U.S. tried a similar risk-based plan before it started vaccinating all newborns in 1991. That plan failed to bring down infection rates among children.
The authors also warn about three problems. Many people carry hepatitis B without knowing it. The virus can survive outside the body for over a week. And testing during pregnancy misses around 15% of pregnant women. So, a baby who looks “low risk” may not be.
The American Academy of Pediatrics (AAP) and the American Academy of Family Physicians (AAFP) have not changed their immunization schedules. They still recommend the HepB birth dose for all newborns. The Pediatric Pharmacy Association urges all healthcare providers and pharmacists to keep recommending the hepatitis B vaccine at birth for every newborn.
American Journal of Preventative Medicine: Hepatitis B Birth Dose: First Shot at Timely Early Childhood Vaccination (10/1/21)
A study followed over 9,000 babies born at a Seattle-area hospital between 2008 and 2013. It found a link between getting the hepatitis B vaccine at birth and finishing all recommended childhood vaccines by 19 months of age.
About 75% of babies got the HepB birth dose as recommended. But only about half of the babies finished the full recommended vaccine series on schedule by 19 months. Of the babies who got the HepB vaccine before leaving the hospital, 60% finished the full series, compared with just 34% of those who did not.
The researchers also considered things such as race, income, insurance status, and how long the baby stayed in the hospital. Babies who got the birth dose were nearly three times as likely to be fully vaccinated by 19 months.
The researchers suggest that when a parent accepts or turns down that very first vaccine, the decision may set the tone for all future vaccination choices. They also said that a missed HepB birth dose can be an early signal for pediatricians. It may prompt them to reach out to families who may need extra support or information to stay on track with their child’s vaccines.
Vaccines: Herpes Zoster Vaccination: Insights into Efficacy, Safety, and Guidelines (4/28/25)
Shingles (herpes zoster) happens when the chickenpox virus wakes back up in the body. It is painful, and it becomes more common and more serious with age. Its most feared problem is nerve pain that can persist for months or years.
There are two vaccines that have been developed to prevent it: an older live vaccine (Zostavax) and a newer protein-based vaccine (Shingrix). Shingrix has replaced Zostavax because it offers better protection. Zostavax offered about 51% protection in adults 60 years and older. Shingrix protects over 97% of adults 50 years and older. And it keeps working well even in people over 70.
Shingrix is also safe and effective for people with weakened immune systems, like cancer patients and people who have had transplants. These patients couldn’t get the original vaccine. People get Shingrix in two doses. Side effects are mostly mild and short-lived, such as a sore arm, tiredness, and low fever. Serious problems are rare. But the FDA has flagged a small, possible link with a rare nerve condition called Guillain-Barré syndrome. New research also suggests that Shingrix may help protect against dementia, which is loss of memory and confusion over time.
Many countries, including the U.S., UK, Australia, and Germany, now recommend the vaccine. The agreement between countries shows how much experts value the vaccines for older adults and people with weakened immune systems.
Annals of Internal Medicine: Effectiveness of Recombinant Zoster Vaccine Against Herpes Zoster in a Real-World Setting (1/9/24)
A large study looked at how well the two-dose shingles vaccine (Shingrix) works in real life among nearly 2 million adults ages 50 and older. The researchers used data from four major U.S. health care systems.
The vaccine’s clinical trials showed 97% effectiveness. But researchers found somewhat lower, but still meaningful, protection in everyday life. A single dose cut the risk of shingles by about 64%. Finishing the full two-dose series raised that to 76%. Two-dose protection worked well over four years of follow-up. One-dose protection dropped a lot after the first year. It fell from 70% to roughly 45–52% in later years.
These findings confirm that Shingrix works well at protecting older adults from shingles. They also show that the second dose is key to lasting protection. It’s important that people do not skip the second dose or put it off for too long.
Clinical Infectious Diseases: Effectiveness of the Adjuvanted Recombinant Zoster Vaccine in Adults ≥50 Years in the United States (12/15/25)
This study followed over 100,000 adults 50 and older who got both doses of the Shingrix shingles vaccine and compared them with those of more than 400,000 people who did not get the vaccine. The study followed people for up to 4.6 years.
Two doses of Shingrix cut the risk of getting shingles by nearly 74% and the risk of nerve pain that can linger for months or years after a shingles outbreak by nearly 84%. That protection held steady across all age groups, in men and women, and in people with a wide range of health problems, including diabetes, heart disease, kidney disease, and lung disease. People with weakened immune systems got somewhat lower but still meaningful protection of about 62% against shingles. Protection also did not really fade over the four years. By contrast, people who got only one dose had much less protection—about 60% against shingles and just 46% against long-lasting nerve pain. That is another reason to finish both doses and get the full benefit.
Arthritis Research & Therapy: Real-World Data on the Use of the Shingrix Vaccine Among Patients with Inflammatory Arthritis and Risk of Cardiovascular Events Following Herpes Zoster (5/17/25)
This large study looked at over 132,000 people with inflammatory arthritis (a group of diseases where the immune system mistakenly attacks the joints). The researchers wanted to see how well the Shingrix shingles vaccine works in these patients. They also wanted to know whether a case of shingles raises the risk of serious heart and blood vessel problems.
These patients get shingles more often than other people. Both the disease and the medications used to treat it can weaken the immune system. This makes it harder for the body to keep shingles from appearing. The study found that Shingrix cut the shingles rate by about half in this group. That is less than the 70–76% drop seen in the rest of the population. This is probably because the same medicines weaken how well the body responds to the vaccine.
Patients who got shingles faced more than double the risk of dangerous blood clots in the 60–90 days afterward. This shows how serious shingles can be for these patients. Even so, vaccination rates were low. Only 21.6% of patients had gotten even one Shingrix dose. Fewer than 4% of patients under age 50 had gotten the vaccine. This is true even though the American College of Rheumatology strongly recommends Shingrix for inflammatory arthritis patients on these medicines starting at age 18.
The authors say that getting more of this high-risk group vaccinated is an important, underused opportunity to prevent both shingles and the serious problems it can cause.