Claims like this are circulating as memes on social media and they are false. Vaccines “work” by reducing the risk or severity of illness. They train your immune system to fight illness. The seasonal flu vaccine causes your body to create protective proteins against different flu viruses—this can reduce your risk of illness by 40 to 60%. Two doses of the measles, mumps, and rubella (MMR) vaccine are 97% effective in preventing measles and 88% in preventing mumps.

The immune system protects the body from harmful germs, like toxins and bacteria. Once the immune system detects a harmful germ, it attempts to attack the germs and keep you safe.

There are two types of immunity—innate immunity and acquired immunity. Innate immunity, including physical barriers like the skin, is the body’s first line of "general" defense against germs. Acquired immunity, which is what vaccines help build, works with the innate immune system to target specific, harmful germs.

Vaccines work by triggering the innate immune system. Vaccines generally contain inactivated, weakened, or manufactured forms of harmful germs. This modified germ in a vaccine does not cause illness, but instead trains the body to fight the “real” germ. If the real germ appears later, your immune system will work quickly to prevent you from getting sick.

The seasonal flu shot is an inactivated vaccine that can help prevent the flu and severe illness. In 2019-2020, the CDC estimated that flu vaccines prevented 7.5 million flu illnesses and 105,000 flu-related hospital stays.

The measles, mumps, and rubella (MMR) vaccine is a weakened vaccine that is also very effective. Two doses of the vaccine are 88% effective in preventing mumps and 97% effective in preventing measles.

International travel makes it easier to spread diseases from other places in the world, where they are more common, to another like the U.S. If we didn’t continue to vaccinate for measles and mumps, the small numbers of current cases in the U.S. could quickly grow.

Vaccines are continually studied and monitored for safety and effectiveness, even after their approval for public use.

New organism (e.g., virus) variants can sometimes evade vaccine protection, requiring regular updates to vaccines for ongoing effectiveness.