CDC guidance says it typically takes a few weeks for the body to build immunity after getting a shot.

We are still learning about the length of time both natural immunity (immunity the body develops after an illness) and vaccine-induced immunity for COVID-19 will last. It is important to continue to wear your mask over your nose and mouth, practice social distancing, and continue to wash your hands and use hand sanitizer often. You should also wear a mask because although the vaccine (shot) will protect you from getting sick, it will not keep you from spreading COVID-19 to others.