Vaccine Safety: How Vaccines Work and How They're Tested

What vaccines are

Vaccines protect against serious, sometimes deadly, diseases. They are shots, liquids, pills or nasal sprays that teach your immune system to recognize and fight harmful viruses or bacteria.

Types

• germs that have been killed or weakened so they can't make you sick;
• only part of a germ;
• instructions for your cells to make one of the germ's proteins.

All spark an immune response, and your immune system remembers the germ and attacks it if it ever returns — immunity. Getting immunity from a vaccine is safer than getting sick, and for a few vaccines it gives a better immune response than the disease itself.

Side effects

• Usually minor: a sore arm, fatigue or a mild fever, gone in a few days — often a sign your body is building immunity.
• Serious side effects are very rare, such as a severe allergic reaction; others vary by vaccine. Talk to your provider if you're concerned after a vaccination.
• Autism: many scientific studies have found no link between childhood vaccines and autism spectrum disorder.

How vaccines are tested

Before approval — a process that often takes years:

1. Laboratory testing; the Food and Drug Administration (FDA) then decides whether to test in people.
2. Clinical trials in three phases, starting with 20 to 100 volunteers and growing to thousands. They ask: Is it safe? What dose works best? How does the immune system react? How effective is it?
3. The FDA works with the manufacturer throughout and licenses the vaccine only if it is safe and effective.
4. Experts may then add it to the CDC immunization schedule, which sets who should get which vaccines, how many doses and when.

After approval:

• The manufacturer tests every batch; the FDA reviews results and inspects the factories.
• The FDA, CDC and other agencies keep monitoring safety, with systems to track possible side effects.

Vaccines protect you — and help keep diseases from spreading to others.

Sources

Based on "Vaccine Safety," MedlinePlus, U.S. National Library of Medicine; a work of the United States government in the public domain.