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Virus Studies

How Vaccines Work

Vaccines work by mimicking the infectious bacteria or viruses that cause disease. A vaccine stimulates the body’s immune system to build up defenses against the infectious bacteria or virus but does NOT cause the disease. After vaccination, due to these built-up defenses, the immune system is prepared to respond quickly and forcefully when the body encounters the real disease.


At present, because of vaccines, diseases that were once common around the world such as polio, measles, smallpox, tetanus, diphtheria, pertussis (whooping cough), rubella, mumps, rotavirus, and Hepatitis B can be prevented. Over the years vaccines have prevented countless cases of disease in both children and adults and saved millions of lives.

Sources: Centers for Disease Control and Prevention, National Foundation for Infectious Diseases, The National Academies of Science, Engineering, and Medicines, US Food and Drug Administration

Vaccine Safety & Efficacy

Vaccines are safe and effective in protecting you, your loved ones, and your community. 

Vaccines to prevent infectious diseases are given to millions of babies, children, adolescents and adults so they are held to extremely high safety standards. In the US, a number of safeguards are required by law to ensure that the vaccines we receive are safe and effective. For example: 

  • Before being approved for the public by the FDA, vaccines are tested in labs and in hundreds to thousands of human volunteers through the course of 4 phases of testing and continued oversight after approval. 

  • This testing process is thorough and can take more than a decade.

  • If an existing vaccine is altered, the testing process is repeated.

  • If a vaccine is meant to be given at the same time as another vaccine, additional tests are done to make sure the two vaccines are safe and effective when given together. 

Find Out More

Get Vaccinated

Advisory Committee on Immunization Practices (ACIP)

Sources: The US Department of Health & Human Services, the US Food & Drug Administration, The National Academies of Science, Engineering, and Medicines

Image by M.T ElGassier
Senior Doctor

 Vaccine Side Effects

The most common side effects of a vaccine are mild. It is important to note that these common side effects are a sign that your body is building immunity against a disease.

Common Side Effects from Vaccination:

  • Pain, swelling, or redness where the shot was given

  • Mild fever

  • Chills

  • Fatigue

  • Headache

  • Muscle and joint aches

Experiencing serious side effects from a vaccine is extremely rare. For example, if 1 million doses of a vaccine are given, 1 to 2 people may have a severe allergic reaction. 

Have you had an adverse reaction to a vaccine? Report it using the Vaccine Adverse Event Reporting System (VAERS). 

 VAERS is a "national early warning system to detect possible safety problems in U.S.-licensed vaccines. VAERS accepts and analyzes reports of adverse events (possible side effects) after a person has received a vaccination. Anyone can report an adverse event to VAERS. Healthcare professionals are required to report certain adverse events and vaccine manufacturers are required to report all adverse events that come to their attention."

Sources: Centers for Disease Control & Prevention, The US Department of Health & Human Services

Explore the Media Below for More Information About Vaccines

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