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Vaccine Safety

Before anyone gets a shot at a clinic or the doctor’s office, scientists do a lot of work to make sure the vaccine is safe. The FDA and the CDC are the government offices in charge of vaccine safety. When a company makes a new vaccine, these agencies must go through many steps before it reaches to the public:

  • First, lab tests must show that there is a good chance the vaccine works to prevent a disease.
  • Then, it is tested on animals.
  • If animal tests indicate it will be safe for people, human volunteers get the vaccine in clinical trials.
  • The first round of these studies, called phase I clinical trials, tests 20-100 healthy people. They help researchers decide the best dose to give and how effective it is.
  • Phase II trials test hundreds of volunteers to pinpoint any side effects and learn more about the right dose.
  • Phase III involves hundreds or thousands of people. Scientists compare vaccinated volunteers with those who got a placebo or a different vaccine.
  • The FDA checks the results of the trials and other information on the vaccine. They also inspect the factories that make the products to ensure they will be safe and potent every time.
  • If the benefits of the vaccine are greater than the risks, the FDA approves it for the public.
  • The entire process can take 10 years or more.
  • The safety process does not stop once the FDA approves a shot. The CDC uses a few systems to watch it closely:
    • The Vaccine Adverse Events Reporting System (VAERS) is an early public health warning system. Anyone including doctors, patients, parents, and drug-makers can use it to report side effects they think are related to a shot. The CDC and FDA watch for reports of new, unexpected, or increased side effects. Any hint of a problem means the agencies will investigate.
    • Scientists use the Vaccine Safety Datalink to figure out if side effects that surface through VAERS are related to the vaccine.
    • Through the Clinical Immunization Safety Assessment network, the CDC teams up with research centers to study how vaccines cause side effects.
    • The FDA also does ongoing safety checks on drug-manufacturers’ facilities. The rules require them to test each vaccine batch they make before selling it to the public.
  • If the CDC and FDA find that a vaccine is causing a side effect, they might:
    • Change the product’s packaging.
    • Send out safety alerts.
    • Inspect manufacturer’s facilities and records.
    • Change their recommendations for who should use the vaccine and how.
    • Revoke the vaccine’s license.

Mild Side Effects

Vaccines can cause some side effects, just like any medicine. Most are mild and do not last long, like a rash, sore arm, redness, or swelling where someone gets the shot. Some people might get a slight fever. There are simple ways to ease these problems if they happen to adolescents:

  • Use a cool, wet cloth to soothe the spot where they got the shot.
  • Lower a fever with a cool sponge bath.
  • Try a non-aspirin pain reliever to help with pain or fever after a shot. Do not give them medicine before or during a shot to try to prevent pain.
  • Give them plenty of fluids. They also might not be too hungry the first day after a shot.
  • Talk to the adolescent’s doctor about a vaccine’s possible side effects so you know what symptoms to look for.

When Side Effects Get Serious

Dangerous side effects to vaccines are rare, but some adolescents do experience them. If they are allergic to eggs, rubber latex, or other vaccine ingredients, they might have a reaction. Tell the doctor about all of your adolescent’s allergies before they get a shot. Other serious side effects can include seizures, hearing loss, severe pain, or brain damage. However, these are rare. Watch your adolescent closely for a few days after they get a vaccine. Let the doctor know about any severe side effects right away.

Much of this content first appeared on WebMD, reviewed by Roy Benaroch, MD as part of a collaboration among the HHS Office on Women’s Health; the HHS Office of Adolescent Health; the HHS National Vaccine Program Office; the Centers for Disease Control and Prevention; and the HHS Center for Medicare and Medicaid services to develop web content on vaccines for preteens and teens.


** Centers for Disease Control and Prevention. (2015). Ensuring Vaccine Safety. Retrieved from http://www.cdc.gov/vaccinesafety/ensuringsafety/index.html.
** Centers for Disease Control and Prevention. (2015). FAQs about Vaccine Safety. Retrieved from http://www.cdc.gov/vaccinesafety/caregivers/faqs.html.
** Centers for Disease Control and Prevention. (2015). Vaccine Safety Monitoring. Retrieved from http://www.cdc.gov/vaccinesafety/ensuringsafety/monitoring/index.html.
** Centers for Disease Control and Prevention. (2016). Making the Vaccine Decision. Retrieved from http://www.cdc.gov/vaccines/parents/vaccine-decision/index.html.
** Centers for Disease Control and Prevention. (2016). Possible Side Effects from Vaccines. Retrieved from http://www.cdc.gov/vaccines/vac-gen/side-effects.htm.
** Centers for Disease Control and Prevention. (2016). Tips for a Less Stressful Shot Visit. Retrieved from http://www.cdc.gov/vaccines/parents/visit/index.html.
** Centers for Disease Control and Prevention. (2016). Vaccine Safety Datalink (VSD). Retrieved from http://www.cdc.gov/vaccinesafety/ensuringsafety/monitoring/vsd/index.html.
** U.S. Department of Health and Human Services. (2008). Understanding Vaccines What They Are How They Work. Bethesda, MD: U.S. Department of Health and Human Services, National Institutes of Health, National Institute of Allergy and Infectious Diseases. Retrieved from http://www.niaid.nih.gov/topics/vaccines/documents/undvacc.pdf.
** U.S. Food and Drug Administration. (2011). Ensuring the Safety of Vaccines in the United States. Retrieved from http://www.fda.gov/downloads/biologicsbloodvaccines/safetyavailability/vaccinesafety/ucm298181.pdf.
** U.S. Food and Drug Administration. (2015). Vaccines for Children – A Guide for Parents and Caregivers. Retrieved from http://www.fda.gov/biologicsbloodvaccines/resourcesforyou/consumers/ucm345587.htm.
Content created by Office of Adolescent Health
Content last reviewed on August 6, 2019