You are reading Part 4 of 4 in this series. What are Quick Facts?

A large body of rigorous science has clearly and repeatedly shown that vaccines in use today are exceedingly safe, with benefits greatly outweighing risks for individuals in groups recommended to get them. Contrary to widely disseminated misinformation about the purported dangers of vaccines, serious adverse health effects are very rare, while public health benefits are vast.

How safe are vaccines?

  • Teams of scientists, including clinical trial experts, microbiologists, immunologists, epidemiologists, and biostatisticians develop and test vaccines through clinical trials and evaluate efficacy and the risks of side effects for various population groups, including people of different races and ethnicities, genders, and ages. For a vaccine to be authorized in the United States, the Food and Drug Administration (FDA) must judge its benefits to outweigh risks for those for whom the vaccine will be recommended.

  • As with any medical intervention, side effects sometimes occur with vaccines. The vast majority are mild, such as redness around the injection site or a sore arm muscle for a few days—evidence that the vaccine is activating the immune system. To give one example, the measles vaccine leads to a mild reaction (such as redness) at the site where it was injected for 17 to 30 out of 100 people.

  • A comprehensive 2020 study thoroughly documented the risks of side effects from vaccines routinely administered in the United States. Excluding effects seen in people at increased risk of complications due compromised immune systems, severe allergies, or other medical conditions, for whom certain immunizations are not recommended, it found:

    • In fewer than one of every 10,000 children vaccinated with the MMR vaccine, the vaccine appears to cause an immune system reaction called idiopathic thrombocytopenic purpura (ITP; sometimes known as immune thrombocytopenia)—a condition for which medical care is sometimes required but that typically resolves without treatment.

      • Context: Measles, it turns out, also sometimes causes ITP. So, although measles vaccines will very rarely cause ITP, these vaccines prevent far more cases of ITP than they cause, by preventing so many cases of measles. According to the Centers for Disease Control and Prevention (CDC), nearly 50,000 people were hospitalized annually as a result of measles infections in the decade preceding the introduction of the measles vaccine; about 1,000 per year suffered from life-threatening encephalitis (brain swelling) and 400 to 500 died. By contrast, in 2020, there were 13 cases of measles in the United States, a tribute to vaccine efficacy.

    • One to two children of every 2,500 who get the MMR or MMRV vaccinations (MMR + varicella/chickenpox) react with “febrile seizures,” which are brief (typically lasting one to two minutes) and generally harmless convulsions. Even more rarely (fewer than 1 child in 10,000), the influenza or pneumococcal conjugate vaccine (against several bacterial infections) will also cause these mostly benign reactions (1.75 in 10,000 when those two vaccines are given together).

      • Context: According to the CDC, about 5% of children get febrile seizures at some point in their first five years, even without vaccination, simply as a result of fever.

    • One to two out of every 1 million adults who get an influenza vaccine react with Guillain-Barré syndrome—an immune system disorder that usually is self-limiting but can cause nerve damage, muscle weakness, and paralysis.

      • Context: According to the CDC, hundreds of thousands of Americans are hospitalized from influenza in a typical year and tens of thousands die from it. The CDC has concluded that people are more likely to get Guillain-Barré syndrome as a result of actual influenza infections than from the influenza vaccine.

    • In adults, only one common side effect was documented for the MMR vaccine: Between 10% and 25% of adult women who receive it (typically for the rubella or “German measles” component) get mild and temporary joint pain or arthritis.

    • Importantly, the 2020 study documented 34 ailments for which vaccines have not been shown to increase risk, including epilepsy, autism, chronic fatigue syndrome, diabetes, fibromyalgia, multiple sclerosis, stroke, lupus, and sudden infant death syndrome.

  • Overall, a National Academies of Science, Engineering, and Medicine committee found that evidence for the net benefits of childhood vaccines is so overwhelming that it would be unethical to conduct a clinical trial in which some children would not get the vaccines as a point of comparison to vaccinated children.

Are vaccines safe for everyone?

  • Everyone responds to medicine a little bit differently, so no vaccine is completely safe for every person.

  • For some vaccines, there are certain groups who shouldn’t take the vaccine. Those who are pregnant, people with severe allergies, immunocompromised individuals, and other groups are instructed by the CDC to not take certain vaccines.

  • Individuals’ immune systems vary with medical conditions, age, and other factors, so vaccine recommendations sometimes differ for certain groups—not just for safety reasons but also for efficacy. For example, the FDA approved a high-dose version of the flu vaccine for adults 65 and older, which helps compensate for the fact that older individuals have weaker immune systems and often fail to mount a strong immune response to a normal-dose flu vaccine.

  • The World Health Organization produces information sheets that document the frequency of adverse reactions to specific vaccines.

What kinds of misinformation exist about vaccines?

  • One of the most widespread false beliefs about vaccination is the rumored connection between vaccinations and autism—an idea that has been thoroughly discredited through rigorous scientific research.

  • In one unfortunately influential and widely cited example, a research report published in 1998 in the medical journal Lancet, authored by Andrew Wakefield and colleagues, suggested that vaccines may be linked to gastrointestinal disease and developmental regression. That report, based on observations of 12 children, led to widespread concern about a possible link between some vaccines and autism. Britain’s General Medical Council found that the study violated research ethics rules (among other issues, one of the authors had failed to disclose funding by parties suing vaccine manufacturers). In light of these and other shortcomings in the research (some symptoms were found to have started even before vaccination), the study was retracted in 2010—as was Wakefield’s medical license.

  • Other wide-ranging myths about vaccines persist, including false claims suggesting they cause long-term severe health problems like multiple sclerosis. Rigorous research on vaccine safety has repeatedly failed to find evidence for these claims.

  • Misinformed fears about vaccine safety have been fueled in part by ongoing attention to anecdotal claims of harm by parents of children whose medical issues arose around the same time they were vaccinated. Studies carefully designed to make evidence-based distinctions between causal and coincidental correlations have repeatedly found no pattern of causal links.

  • Researchers have also noted a psychological contribution to vaccine misinformation: medical science’s successful reduction or elimination of many infectious diseases—largely by means of vaccination—has made the danger of these diseases much less immediate. With those risks now largely invisible, the very small risks of vaccination can seem intolerable—even though they are much smaller than the risk of the prevented disease or of many other risks accepted in everyday life. This complacency is dangerous, as evidenced, for example, by recent outbreaks of measles cases and deaths in places where immunization acceptance has dropped.

Vaccines in general:

COVID-19 vaccine candidates:

  • The U.S. Department of Health and Human Services gives information about Operation Warp Speed, including general plans and specific financial investments.

  • The Regulatory Affairs Professionals Society keeps a COVID-19 vaccine tracker, updated weekly, which outlines the types, sponsors, trial phases, and funding for vaccine candidates.

  • In June, the Food and Drug Administration produced guidance for industry related to the development of COVID-19 vaccines, including safety standards. The FDA also publishes plain-language news and updates on its COVID-19 Vaccines page.