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 are vaccines tested?

  • Before a vaccine is tested in people, it goes through laboratory testing (in cells or tissues) as well as in animals (such as mice or monkeys, which in some tests are exposed to the targeted disease after vaccination). Depending on test results, researchers may alter aspects of the experimental vaccine or decide not to pursue human clinical trials.

  • For a vaccine candidate to get tested in people, the Food and Drug Administration (FDA) must approve an Investigational New Drug Application. This application includes results from animal testing and suggests a design for human clinical trials.

  • Clinical trials are designed in three phases to find out whether a vaccine is safe for people, what dose is most effective, and how the human immune system responds to the vaccine. These trials generally take several years, although the first COVID-19 vaccines were created and authorized in under a year.

    • Phase I trials usually test up to 100 people and are aimed primarily at assessing safety at various doses.

    • Phase II trials often enroll hundreds of people. Safety remains a focus, but these trials also aim to determine immune responses to the vaccine.

    • Phase III trials may test thousands or tens of thousands of people with the primary goal to determine if the vaccine works to prevent disease. These studies generally include volunteers randomly selected to receive an inactive placebo instead of the vaccine to help determine if evidence of protection—or any emerging safety issue—is attributable to the vaccine. These studies are larger and longer than the other trials and so can reveal rarer “adverse events” such as fever or a seizure following vaccination—and can help settle whether those issues are causally connected to the vaccine. Adverse events during a clinical trial may require a pause for assessment (the FDA may stop trials or request more research at any point in the approval process ). In one example, the phase III trial for a COVID-19 vaccine candidate was temporarily paused in September 2020 when one subject came down with an illness after getting the vaccine. The trial resumed about a week later after analysis convinced regulators it was safe to do so.

  • In considering whether to authorize or approve a vaccine for marketing, the FDA reviews safety and efficacy data from clinical trials along with other information—including the severity of the disease and whether other protective strategies or medical treatments are available—to determine whether the vaccine’s benefits outweigh its risks for those who will be deemed eligible to take it.

Is vaccine safety tracked after a vaccine is approved?

  • As vaccines are produced at scale for marketing, samples are tested for purity and for batch-to-batch consistency.

  • The U.S. Centers for Disease Control and Prevention (CDC) and the FDA monitor reports of adverse events, to identify any side effects that may not have been apparent during clinical trials.


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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.


Creative Commons LicenseThis page is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License. License applies to text and video only. Journalists are free to use any text or video on this page with or without attribution to SciLine.