How Vaccines Work

COVID-19 vaccines teach our immune systems how to recognize and fight the virus that causes COVID-19. It typically takes a few weeks after vaccination for the body to build protection (immunity) against the virus. That means it is possible a person could still get COVID-19 just after vaccination, because the vaccine has not had enough time to build immunity. If the vaccine you got requires two shots, be sure to get both doses so it can work fully.

Vaccine safety

COVID-19 vaccines authorized by the FDA have been shown to be safe and effective in clinical trials. These vaccines were authorized only after it was found that they make it substantially less likely you’ll get COVID-19.

The authorized vaccines are up to 95% effective against a person becoming ill with COVID-19.

The U.S. vaccine safety system ensures that all vaccines are as safe as possible. Learn how the federal government is working to ensure the safety of COVID-19 vaccines.

Safety monitoring after vaccination

Millions of people in the United States have received COVID-19 vaccines. These vaccines have undergone the most intensive safety monitoring in U.S. history, using both established and new safety monitoring systems. These vaccines cannot give you COVID-19. Learn more facts about COVID-19 vaccines.

Results from monitoring efforts are reassuring. Many people have reported only mild side effects after COVID-19 vaccination. Some people have no side effects.

You cannot get COVID-19 from the vaccine

The vaccines do not contain coronavirus and cannot give you COVID-19.