Expert Commentary

Health, Politics & Government

Childhood vaccines: What research shows about their safety and potential side effects

In this piece, we share reporting tips, explain how vaccine side effects are tracked in the U.S., and discuss research on the safety of childhood vaccines.

by | | Vaccines
childhood vaccines
(Heather Hazzan, SELF Magazine)

Dr. Sean O’Leary, chair of the Committee on Infectious Diseases at the American Academy of Pediatrics, studies vaccines and immunization for a living. And if you ask him to summarize what we know about vaccines, he’ll tell you, without hesitation, that vaccines work.

“The science behind vaccines is very clear,” says O’Leary, a professor of pediatrics and infectious diseases at the University of Colorado School of Medicine and Children’s Hospital Colorado. “The benefits outweigh the risks.”

We created this tip sheet and research-based primer on the heels of the confirmation of Robert F. Kennedy Jr., a long-time vaccine skeptic who now leads the U.S. Department of Health and Human Services.

During a time when even Sen. Bill Cassidy, a physician and lifelong advocate of vaccinations, voted for Kennedy’s confirmation, it’s important for journalists to clearly communicate what’s known about the safety of routine childhood vaccines — and dispel myths about their dangers.

We’ve gathered the following resources in this vaccine primer:

5 tips when reporting on vaccines

When public officials spread health misinformation, be quick to point it out: A tip sheet

1. When possible, avoid repeating misinformation, including the false link between autism and the Measles, Mumps and Rubella (MMR) vaccine.

“Simple myths are much more cognitively attractive to the human brain than complicated, nuanced science,” O’Leary says. “We can give a definitive statement that vaccines do not cause autism. Period. But explaining how we know that takes an additional level of nuance and complexity.”

Repeating falsehood in news stories has the potential to perpetuate myths, even if the stories include the correct information.

“People don’t remember every word of every article,” O’Leary says. “They just remember these sort of associations — vaccines, autism; vaccines, autism — ‘I keep seeing that it must be real.'”

At least 27 studies have shown that vaccines don’t cause autism. At least a dozen studies comparing thousands of children who received the MMR vaccine with thousands of children who didn’t receive it have not found a relationship between the vaccine and autism, according to the American Academy of Pediatrics.

2. Pick your words carefully. More and more, science and health communicators are moving away from using the term “misinformation” and instead using “falsehood” or “rumor.”

“A lot of people who are distrustful in the health system and institutions kind of have a knee-jerk reaction to that word,” says Dr. Kristen Panthagani, who also tries to avoid using the terms “fact-check” and “debunk” when talking to some of her patients.

“For some people those words are fine, but for others, they’ve decided that we’re suppressing all the other information and that’s just our version of reality, as opposed to scientists genuinely trying to explain what the data shows,” says Panthagani, a third-year emergency medicine resident, founder of the website youcanknowthings.com and a contributor to Your Local Epidemiologist newsletter on Substack.

3. Avoid normalizing vaccine hesitancy. There has been a growing anti-vaccination sentiment in the U.S. Recent data shows that childhood vaccination rates have fallen slightly, a downward trend that began with the onset of the COVID-19 pandemic in 2020.

Routine childhood vaccinations and changing school requirements: A primer and research roundup

Childhood vaccination rates dropped from 95% in the 2019-2020 school year to 93% for 2022-2023, according to an October 2024 study in the Morbidity and Mortality Weekly Report, the Center for Disease Control and Prevention’s scientific publication. Meanwhile, vaccine exemption rates increased during the same period, the study finds.

Even small drops in vaccination rates can increase the odds of an outbreak, especially for contagious diseases like measles.

But it’s important for journalists to also remind their audiences that the majority of U.S. children get their recommended shots every year. Most Americans favor school vaccination requirements, according to a January 2025 survey by the Annenberg Public Policy Center.

“We still have 90-plus percent uptake for the vast majority of [childhood] vaccines,” O’Leary says. “Most parents are actually pretty supportive of vaccines. They might have some questions, they might have some ambivalence, but most kids are actually getting vaccinated.”

For more, read “The Risk of Normalizing Parental Vaccine Hesitancy,” a commentary by O’Leary and Dr. David M. Higgins, published in The New England Journal of Medicine in February 2024. O’Leary and Higgins also highlight the need for surveillance of vaccine hesitancy in the U.S. in an October 2024 article published in the American Journal of Public Health.

Religious exemptions and required vaccines: Examining the research

4. Avoid false balance. When reporting on issues where there’s scientific consensus, including climate change and the effectiveness of vaccines, avoid quoting both sides in the name of balance.

“I think most journalists have gotten better about that,” O’Leary says. “But when you quote somebody like me who’s a scientist who studies vaccines, and then you have another quote from someone who is rabid anti-vax, that gives this sort of pseudo symmetry or false equivalence between these two when you read it.”

Avoiding false balance can be challenging when the person spreading falsehoods is a political leader, but it can be done.

“In cases where public officials are spreading misinformation, the journalist’s responsibility is straightforward — either don’t report it; or report it while pointing out that it’s misinformation clearly, explicitly, and early, and telling people what the truth actually is,” Ed Yong, a Pulitzer Prize-winning science journalist and book author, wrote in an email to The Journalist’s Resource in 2021. “I cannot stress enough that simply writing down what officials say is not journalism; you have to analyze, critique, and contextualize those comments, or you’re nothing more than an RSS feed with hands.”

5. Remind your audiences that vaccines save lives. “Vaccines have saved more lives in the past century than any other medical intervention. The data is clear and compelling,” wrote Dr. Katelyn Jetelina, the author and founder of “Your Local Epidemiologist” newsletter on Substack, in her Dec. 17, 2024, newsletter.

Vaccines have helped eradicate smallpox. Deaths from measles, diphtheria and tetanus are near-zero in places where there’s a high vaccine uptake. And thanks to vaccines, polio cases have dropped by 99% around the world.

Jetelina also offers a downloadable fact sheet on childhood vaccination.

Vaccine safety

Although used often, the word “safe” could be problematic, because it is often thought of as binary — something is either safe or unsafe, explains O’Leary. In reality, almost nothing is 100% safe, whether it’s medications, medical procedures, driving a car or crossing the street. But years of research and data find vaccines extraordinarily safe.

When Panthagani talks about vaccines, she avoids using the term “safe” and instead emphasizes that vaccines’ benefits outweigh their risks.

“I think some people, when they hear ‘safe’, they assume that means no risk of any side effects ever, and that’s just obviously not true,” she says.

“Like everything else, vaccines do have risks of side effects. It’s just that their risk is very small and the benefits are large,” she says.

Most research scientists, public health advocates, and national and international health organizations promote vaccines as a way to fight and in some cases eradicate infectious diseases like smallpox, measles, whooping cough, tuberculosis, polio, influenza, and, more recently, COVID-19.

The American Academy of Pediatrics “strongly recommends immunizations as the safest and most cost-effective way of preventing disease, disability and death.”

Vaccines teach the body’s immune system to recognize and respond to bacteria or viruses. Some vaccines contain an attenuated, or weakened, version of a pathogen, like the measles, mumps and rubella, rotavirus and chickenpox vaccines. Some vaccines contain the killed virus, including flu, polio, hepatitis A and rabies shots. Some vaccines, like the COVID-19 vaccine, give instructions to the immune system to make antibodies in response to the virus. Here is a list of the types of childhood vaccines.

Over the years, systematic reviews of published research on vaccine safety have found that vaccines are safe, and negative side effects from them are rare or very rare.

A 2014 systematic review by the U.S. Agency for Healthcare Research and Quality, published in the journal Pediatrics, concludes, “There is evidence that some vaccines are associated with serious adverse events; however, these events are extremely rare and must be weighed against the protective benefits that vaccines provide.” The authors updated their study in 2021 and found that “Overall, our evidence review found vaccines to be safe across populations with serious adverse events being rare, consistent with other recent systematic reviews of vaccine safety.”

The rise in non-medical vaccine exemptions is partly to blame for the current measles outbreak, which has grown from 24 unvaccinated people in Texas to nearly 100 people in Texas and New Mexico. 

Vaccine side effects

The most common side effects from vaccines are redness, swelling and soreness at the injection site and a low-grade fever. On rare occasions, people may develop severe allergic reactions. These reactions usually appear between 30 minutes to 4 hours after vaccination. The true rate of allergic reactions to vaccines is not known because many go unreported, but researchers estimate the rate to range from 1 case in 100,000 doses to 1 in 1 million doses, depending on the vaccine.

Systematic reviews and studies over the years have found that various vaccines can very rarely lead to severe side effects, which we further break down by the type of vaccine below.

Severe side effects range from 0.1% to less than 0.01% of vaccination cases. For instance, evidence shows that 4 in 100,000 children may develop febrile seizure — a convulsion caused by fever, mostly in young children, and usually harmless — after some vaccinations. For every 1 million flu vaccinations, between one and three adults may develop Guillain­-Barré syndrome, research shows.

Over the years researchers have conducted many studies on the potential link between vaccines and side effects (“adverse events” in medical jargon), observed by patients, caregivers and health care providers. Some of these side effects were identified during the monitoring period after a vaccine was brought to the market, leading researchers to further investigate it. Several systematic reviews, which we list below, have summarized the findings of those studies.

The reviews, for instance, find no evidence that vaccines cause a slew of other side effects such as Bell’s palsy, epilepsy, fibromyalgia, hearing loss, infantile spasms, narcolepsy, miscarriage, stroke or sudden infant death syndrome.

In particular, there’s no evidence associating certain vaccines the following side effects, write O’Leary and his co-authors in a 2020 systematic review of vaccine safety evidence, including 155 papers, published in The Lancet Infectious Diseases.

“Influenza vaccines do not cause asthma, childhood vaccines do not cause autism, vaccines do not cause diabetes, vaccines given to people who are immunocompetent do not cause hepatitis, influenza vaccines do not cause multiple sclerosis in adults, and DTP (diphtheria tetanus and whole­-cell pertussis) and hepatitis B vaccines do not cause sudden infant death syndrome,” the authors write.

In comparison, the risk of severe outcomes, including death, from vaccine-preventable diseases is much higher. About 10 to 15% of people infected with the meningococcal bacteria will die. Nearly 60% of people with severe tetanus die. And about 15% to 20% of people infected with Haemophilus influenzae type b will lose their hearing. More than 28,000 people died from the flu in 2024. Roughly 76,446 people died from COVID-19 in 2023.

How vaccine side effects are tracked in the U.S.

Before the vaccines arrive on the market, they go through clinical trials. Once the U.S. Food and Drug Administration reviews a vaccine application and finds the vaccine safe and effective, it may grant the vaccine a license, also known as approving the vaccine. The Advisory Committee on Immunization Practice, or ACIP, also reviews the data to decide whether a vaccine’s benefits outweigh its risks before recommending its use, explain O’Leary and his co-authors in a 2024 study published in Pediatrics.

Although vaccine developers track common side effects during clinical trials, it’s not possible for them to design large enough studies to detect rare side effects that may affect one in 10,000 or 100,000 people.

To solve this problem, the U.S. developed robust surveillance systems in 1990. One of the most widely known programs is the Vaccine Adverse Event Reporting System or VAERS. The other is the Vaccine Safety Datalink, or VSD.

VAERS is a surveillance system managed by the Centers for Disease Control and Prevention and the FDA and acts as an early warning system for vaccine side effects. Anyone can file a report on VAERS.

“So, if you know something happens after a vaccine, and you’re not sure if it was the vaccine or not, you can report it to VAERS, and then, if it’s a potentially serious event, it gets further investigated,” O’Leary explains.

The VAERS data does not show causality. However, because its data is publicly available and downloadable, it’s been the focus of vaccine skeptics.

“I think people who are distrustful of the health care system view [VAERS] as the raw stories of people who believe they were harmed by vaccines,” Panthagani says.

“It’s information that they can collect and analyze themselves since they don’t trust the people who are putting everything together and actually do it correctly, unfortunately.”

The next layer of surveillance, the Vaccine Safety Datalink, helps researchers parse VAERS data and investigate potential links between vaccines and rare but serious side effects.

VSD is a collaboration between the CDC and 13 U.S. health care organizations. If there are strong signs of possible vaccine side effects in systems like VAERS, then researchers can use Vaccine Safety Datalink to perform more studies to see if there’s indeed an association between the shots and the side effects.

VSD has helped identify rare side effects, including the link between the MMR vaccine and immune thrombocytopenic purpura, a bleeding disorder where the immune system destroys platelets, which can occur in one in 40,000 vaccinations. It has also shown an association between MMR and varicella vaccines and febrile seizures, which can occur in 4.3 of 10,000 vaccinations.

“Perhaps more importantly, the VSD has been able to demonstrate the lack of association of numerous vaccines with purported vaccine adverse events,” O’Leary and his co-authors write in their 2024 study.

There are two other systems tracking vaccine safety: the FDA’s Biologics Effectiveness and Safety (BEST) system, which uses real-world data, including electronic health records, insurance claims and patients registries to assess the safety and effectiveness of vaccines that are already on the market; and the Clinical Immunization Safety Assessment Project, a collaboration between CDC and medical research centers.

List of childhood vaccines, their safety and possible side effects

To help journalists report on vaccine safety, we’ve created a list of childhood vaccines along with what’s known about their safety and side effects. The majority of these vaccines have been on the market for more than two decades. They have all been found to be safe and effective.

We haven’t included the vaccine schedule — when and how many doses of each vaccine are recommended for children — because you can find that information on several websites, including the CDC and the American Academy of Pediatrics.

We used the following systematic reviews and resources, among others, for this section. We avoided relying on the findings of single studies. An analysis of a large body of research carries more scientific weight than a single study.

Also, as we mention above, certain side effects are listed because researchers have investigated their potential link with certain vaccines after they were brought up by patients, caregivers and health care providers. Some were identified during the monitoring period after a vaccine was brought to the market.

COVID-19 vaccine

The COVID-19 vaccine protects against COVID-19, a disease caused by the SARS-CoV-2 virus. It is very contagious and spreads quickly. The vaccines are designed to protect people from severe illness, which can lead to hospitalization or death.

Effectiveness: A February 2024 study published in the MMWR finds that people who received the updated COVID-19 vaccines were 54% less likely to get COVID-19 between September 2023 and January 2024.

Rare side effects:

  • Five cases per million vaccine doses can result in a severe allergic reaction, according to the CDC.
  • In rare cases, some patients — mostly adolescent and young adult men — can develop inflammation of the heart, which resolves after treatment with medication. The rates are estimated to be 22.4 cases per million after the second dose of the Pfizer-BioNTech vaccine and 31.2 cases per million after the second dose of the Moderna vaccine.
  • The J&J/Janssen COVID-19 vaccine has been associated with an increased risk of Guillain-Barré syndrome. Data from VSD showed that the rate of the condition was 21 times higher after receiving the J&J/Janssen COVID-19 vaccine, compared with the Pfizer-BioNTech and Moderna vaccines. Based on this data, the vaccine is no longer available in the U.S. as of May 2023.

How we know it works: From December 2020 to November 2022, COVID-19 vaccination programs in the U.S. prevented more than 18 million hospitalizations and 3.2 million deaths, according to a 2022 analysis by The Commonwealth Fund.

Diphtheria, Tetanus, and Pertussis (DTaP) vaccine

The DTaP vaccine protects against three serious infections: diphtheria, a bacterial infection affecting the mucous membranes and causing significant complications; tetanus, a bacterial infection causing muscle stiffness and spasms; and pertussis, also known as whooping cough, a highly contagious respiratory disease.

Effectiveness: While diphtheria and tetanus numbers are at historic lows in the U.S., no one has studied the efficacy of the DTaP vaccine in a clinical trial, according to the CDC. Instead, experts estimate how well the vaccines work by looking at the amount of protective antibodies in people. The complete DTaP vaccine series is 100% protective against tetanus and 97% against diphtheria.

The effectiveness of the pertussis vaccine might wane over time. The vaccine is given in five doses, starting at 2 months old to 4 to 6 years old. The vaccine fully protects 98% of children within the first year of the last dose. Its effectiveness drops to 71% five years after the last dose of DTaP.

Rare side effects:

  • Serious but rare side effects include persistent crying for three hours or more, high fever, or swelling of the entire arm or leg.
  • The vaccine may cause severe allergic reactions among those who are allergic to the tetanus toxoid.
  • The preponderance of the evidence suggests there is no link between the vaccine and type 1 diabetes, asthma, encephalitis, fibromyalgia, sudden infant death syndrome, Bell’s Palsy, multiple sclerosis or meningitis.

How we know it works:

  • For diphtheria, there were 100,000-200,000 cases and 13,000-15,000 deaths reported annually in the 1920s in the U.S. before the vaccine was introduced in the 1940s. From 1996 to 2018, 14 cases and one death were reported in the U.S., according to the CDC.
  • For pertussis, there were more than 200,000 cases each year before the vaccine arrived on the market. Since then, the number of new cases has dropped by 75%, according to the CDC.
  • For tetanus, there were as many as 600 cases each year before the arrival of the vaccine. In 2018, 23 cases were reported with no deaths, according to the CDC.

Haemophilus influenzae type b (Hib) vaccine

The Hib vaccine prevents infections caused by Haemophilus influenzae type b bacteria, which can lead to severe illnesses like meningitis, pneumonia, and epiglottitis, primarily in children under 5 years old.

Effectiveness: The vaccine is 95% effective in infants who receive the vaccine series of 2 or 3 doses.

Rare side effects: Studies haven’t found any serious side effects associated with this vaccine.

How we know it works: There were about 20,000 cases of Hib each year, mostly in children younger than 5 years old, before a vaccine was approved in the U.S. in 1987. The rate has dropped by 99% since, according to the CDC.

Hepatitis A vaccine

The hepatitis A vaccine protects against the hepatitis A virus, which causes liver disease and can lead to symptoms like fatigue, nausea and jaundice.

Effectiveness: At least 94% of people who get the recommended two doses of the vaccine become immune to the hepatitis A virus. Protection from the vaccine lasts 14 to 20 years in children.

Rare side effects: There’s some evidence that in rare cases the vaccine may increase the risk of idiopathic thrombocytopenic purpura among children aged 7 to 17 years, a bleeding disorder where the immune system mistakenly destroys platelets, leading to easy bruising and bleeding. (Source)

How we know it works: Hepatitis A vaccines were first licensed in the U.S. in 1995. Before 2004, hepatitis A was the most frequently reported type of hepatitis in the U.S. From 1996 and 2011, hepatitis A cases dropped by 95%, but re-emerged in 2016 because of widespread outbreaks among people reporting drug use and people who were unhoused, according to the CDC.

Hepatitis B vaccine

The hepatitis B vaccine protects against the hepatitis B virus, which can lead to chronic liver infection, liver failure and liver cancer. The vaccine was first licensed in the U.S. in the 1980s.

Effectiveness: The vaccine is more than 90% effective in preventing infection in infants, children and adults.

Rare side effects: There’s an increased risk of allergic reaction in patients with allergies to yeast in about 1.1 cases per million doses given. (Source)

The preponderance of the evidence suggests there is no link between the vaccine and a higher risk of asthma, autoimmune disease, heart attack, stroke, diabetes or multiple sclerosis.

How we know it works: Before 1982, an estimated 200,000 to 300,000 people in the United States were infected annually with HBV, including approximately 20,000 children, according to a 2002 CDC study. Thanks to nationwide vaccination efforts, including laws requiring the vaccination of school children, hepatitis B rates among children up to 9 years old dropped by 80% between 1986 and 2000.

Human Papillomavirus (HPV) vaccine

The HPV vaccine protects against human papillomavirus, a sexually transmitted disease, which can cause six types of cancer: cervical, vaginal, vulvar, anal, penis and throat cancers. It can also cause genital warts.

Effectiveness: The HPV vaccine is nearly 100% effective in preventing HPV infections, according to the CDC.

Rare side effects: There’s a one per million chance of severe allergic reaction.

The preponderance of the evidence suggests there is no link between the vaccine and an increased risk of multiple sclerosis, Guillain-Barré syndrome, birth defects, miscarriage or seizure.

How we know it works: Nearly 79 million people in the U.S. were infected with HPV before the vaccines arrived on the market. In the decade after their introduction, the prevalence of HPV types 6, 11, 16 and 18 has decreased by 86% among girls between 14 and 19 years old and 71% among women 20 to 24 years old, according to the CDC.

Inactivated Poliovirus (IPV) vaccine (Polio vaccine)

IPV protects against poliomyelitis — polio — a viral disease that can cause paralysis and is potentially fatal. The vaccine contains an inactivated — killed — virus, which cannot cause disease.

Effectiveness: Two doses of the vaccine are 90% effective. Three doses are almost 100% effective, according to the CDC.

The preponderance of the evidence suggests there is no link between the vaccine and negative side effects, such as childhood-onset leukemia.

How we know it works: Polio infections peaked in the U.S. in 1952, with more than 21,000 paralytic cases, according to the CDC. Polio vaccines arrived on the market in the U.S. in 1955. The last case of polio acquired in the U.S. was in 1979.

Influenza vaccine

The influenza vaccine is designed to protect against the influenza virus, which causes the flu, a contagious respiratory illness that can lead to severe health complications, especially in young children, the elderly, pregnant individuals and those with certain chronic health conditions. Annual vaccination is recommended due to the virus’s frequent changes, which means the vaccine’s composition is updated each year.

Effectiveness: The flu vaccine is updated each year, and its effectiveness varies by year, but overall, it reduces the risk of flu by 40% to 60%. A 2014 study in The Journal of Infectious Diseases found that the vaccine during the 2011 and 2012 flu seasons reduced the risk of life-threatening flu illness in children by three-quarters.

Rare side effects:

The preponderance of the evidence suggests there is no link between the vaccine and a higher risk of asthma, autoimmune disease, Bell’s palsy, heart attack or stroke. Flu vaccines don’t cause multiple sclerosis in adults.

Measles, Mumps, and Rubella (MMR) vaccine

The MMR vaccine is a combined vaccine that protects against measles, mumps, and rubella — three contagious viral diseases that can lead to serious health complications.

Effectiveness: The vaccine is 96% effective against measles, about 86% effective against mumps, and about 89% effective against rubella. Immunity against mumps may decrease over time in some people, according to the CDC. A 2024 study, based on mathematical modeling, estimates that following vaccination, immunity against measles may wane slowly at a rate of about 0.04% per year.

Rare side effects:

  • The vaccine may cause a severe allergic reaction in children with allergies. It may also lead to high-fever seizures. There’s no evidence of an increased risk of asthma.
  • One to three in 100,000 children can develop immune thrombocytopenic purpura, a bleeding disorder where the immune system destroys platelets, especially in people with a history of the condition.
  • Some children between ages 10 and 12 may develop temporary joint pain.

There’s no evidence that the vaccine causes autism. This has been shown in at least 27 studies. There’s also no evidence that the vaccine causes epilepsy or fibromyalgia.

The preponderance of the evidence suggests there is no link between the vaccine and chronic fatigue syndrome, encephalitis or other neurological conditions, hearing loss, type 1 diabetes or epilepsy.

How we know it works:

  • Before a vaccine became available in 1963, measles was nearly universal during childhood, according to the CDC. There were nearly 500 deaths each year. Although the number of measles infections has dropped by more than 95% in the U.S., outbreaks occur, mostly in communities where people are not vaccinated. The MMR vaccine was licensed in the U.S. in 1971.
  • Mumps was one of the most common causes of aseptic meningitis and sensorineural hearing loss in childhood in the United States until the introduction of a vaccine in 1967, according to the CDC. There were about 152,000 cases of mumps in 1968. That number dropped to fewer than 300 cases in 2004.
  • Rubella was endemic before the arrival of the vaccine. By 2004, there were fewer than 10 cases per year. Since 2012, all reported rubella cases have been imported from other countries.

Meningococcal vaccine

The meningococcal vaccine protects against infections caused by Neisseria meningitidis, a bacterium responsible for serious diseases such as meningitis (inflammation of the protective membranes covering the brain and spinal cord) and septicemia (blood poisoning). There are multiple strains of N. meningitidis, with strains A, B, C, W, and Y being the most common causes of disease.

Effectiveness: The effectiveness of the vaccine ranges from 69% to 88%, depending on the vaccine and bacteria strain, according to the HHS.

Rare side effects: The vaccine may cause severe allergic reactions in children with allergies.

The preponderance of the evidence suggests there is no link between the vaccine and an increased risk of heart attacks, diabetes, febrile seizures, idiopathic thrombocytopenic purpura, Kawasaki disease, seizure, asthma, autoimmune disease, encephalitis/encephalopathy, meningitis, multiple sclerosis, or transverse myelitis.

How we know it works: The disease peaked in the U.S. in the late 1990s. The first vaccine was introduced in 1999. In 2018, there were 329 cases reported in the U.S., equal to 0.1 cases per 100,000 people, according to the CDC.

Pneumococcal conjugate vaccine

The vaccine protects against 13 types of pneumococcal bacteria, which can cause serious infections like pneumonia, meningitis and bloodstream infections.

Effectiveness: The vaccine is 60% to 70% effective in preventing serious pneumococcal disease, according to the CDC.

Rare side effects: A rare side effect is febrile seizures, with a risk of 3.92 cases per 100,000 vaccinations in children between one and five months old.  

The preponderance of the evidence suggests there is no link between the vaccine and an increased risk of asthma, childhood-onset leukemia, heart attack, meningitis, Kawasaki disease, seizures or death.

How we know it works: Before the routine use of the vaccine in 2000, the infection affected more than 17,000 children under 5 each year, with nearly 200 dying each year. There has been a more than 90% drop in the number of new cases since the vaccine’s arrival, according to the CDC.

Rotavirus vaccine

The rotavirus vaccine protects against rotavirus infection, a leading cause of severe diarrhea and dehydration in infants and young children.

Effectiveness: The vaccine provides 85% to 96% protection against severe illness.

Rare side effects: There’s a small risk of bowel blockage that may need hospital treatment and surgery. This may happen in 1 in 20,000 to 1 in 100,000 U.S. infants, according to the CDC.

The preponderance of the evidence suggests there is no link between the vaccine and an increased risk of diabetes, asthma, autoimmune disease, encephalitis/encephalopathy, febrile seizures, idiopathic thrombocytopenic purpura, seizures, stroke, autoimmune thyroiditis (Hashimoto’s disease), Kawasaki disease or meningitis.

How we know it works: Nearly all children in the U.S. were infected with the virus by age 5, and as many as 40% developed severe illness before a vaccine arrived on the market in the U.S. in 2006. More than 200,000 visited the emergency department as a result of illness and as many as 70,000 were hospitalized. The vaccine has helped reduce 45,000 hospitalizations and 62,000 emergency department visits between 2007 and 2011, according to the CDC.

RSV immunization for infants

RSV, or respiratory syncytial virus, is a common respiratory virus that is typically seasonal. It is most serious in infants, older adults and people with compromised immune systems. Adults receive the RSV vaccine, while infants and young children receive RSV immunization, which contains the RSV antibody and provides immunity for up to a year. The antibodies fight the disease, rather than teach the body how to create antibodies, as most vaccines do.

Effectiveness: The immunization is 90% effective in preventing RSV-associated hospitalization in infants during the RSV season, according to a March 2024 study in MMWR.

The RSV immunization arrived on the market in the U.S. in late 2023.

Varicella (Chickenpox) vaccine

The varicella vaccine protects against chickenpox, a contagious disease caused by the varicella-zoster virus, which can cause an itchy rash and fever.

Effectiveness: One dose of the vaccine is effective in preventing any form of varicella infection by 82%. Two doses of the vaccine is 92% effective, according to the CDC.

Rare side effects:

  • The vaccine can cause severe allergic reactions, in 1 per million vaccine doses.
  • It can also reactivate the varicella-zoster virus, causing shingles, meningitis or encephalitis, especially among people with a weakened immune system.
  • There’s some evidence that it may lead to immune thrombocytopenic purpura, a bleeding disorder where the immune system destroys platelets, among children 11 to 17 years.

How we know it works: Before the vaccine was licensed for use in the U.S. in 1995, varicella affected most people in the U.S., infecting an estimated 4 million people each year. The number of new cases has dropped by 97% since the vaccine’s arrival, according to the CDC. In 2005, a combination measles, mumps, rubella and varicella (MMRW) vaccine was licensed for 12-month to 12-year-old children.

Additional reading

Reporting resources