According to the CDC, “different types of vaccines work in different ways to offer protection. But with all types of vaccines, the body is left with a supply of ‘memory’ T-lymphocytes as well as B-lymphocytes that will remember how to fight that virus in the future. It typically takes a few weeks after vaccination for the body to produce T-lymphocytes and B-lymphocytes. Therefore, it is possible that a person could be infected with the virus that causes COVID-19 just before or just after vaccination and then get sick because the vaccine did not have enough time to provide protection.”

Specifically, the COVID-19 mRNA vaccine teaches our cells how to make a protein—or even just a piece of a protein—that triggers an immune response inside our bodies. The benefit of mRNA vaccines, like all vaccines, is those who are vaccinated gain protection without ever having to risk the serious consequences of getting sick with COVID-19. Learn more about mRNA vaccines.

You may have some side effects after taking the vaccine, which are normal signs that your body is building immunity. These side effects may affect your ability to do daily activities, but they should go away in a few days. Some people have no side effects. See the CDC for common side effects and tips to relieve side effects.
People who have taken the vaccine may still become infected, but they will be much less likely to become seriously ill or hospitalized with the disease because their body has the memory to be able to better fight the infection.

Learn more about vaccines.