When Does The COVID-19 Booster Shot Become Effective?

Getting the COVID-19 booster shot is a great way to help protect yourself and others from the virus. The shot works by providing an added layer of protection against the disease, and with the highly transmissible Omicron variant of COVID-19 circulating in the U.S., getting boosted is now more important than ever (vis Cleveland Clinic). While the primary series of the COVID-19 vaccine still provides adequate protection against the virus, this immunity appears to fade after several months.

According to a preliminary study in Israel, breakthrough cases of COVID-19 were more prevalent among those who received the vaccine at an earlier date. That's because antibody levels start to wane around six months after vaccination. "Having fewer circulating antibodies means there will be a delay in your immune response to COVID infection compared to when antibodies are circulating in greater numbers," Dr. Shruti Gohil, an associate director of epidemiology and infection prevention at University of California Irvine Health, told Good Housekeeping.

Fortunately, the COVID-19 booster shot can help your memory B cells produce more antibodies, which can significantly increase your protection against the virus (via Health). But exactly how long will it take to become fully effective? "The only way we could make an estimate is extrapolating how the immune response behaves with other viruses," Dr. Pablo Penaloza-MacMaster, PhD, an assistant professor of microbiology-immunology at Chicago's Northwestern University Feinberg School of Medicine, told Health. "We know between the first and second week you have a massive increase in protection, but there haven't been experiments looking at hours or days."

According to Pfizer's clinical trials, participants started experiencing some positive effects from the booster shot as early as one week after vaccination. Generally speaking, however, it can take up to two weeks after receiving the booster for your immune system to reach peak levels of protection. That's when the booster shot is considered fully effective.