A fourth COVID-19 shot has been authorized for people over the age of 50 along with those under 50 who are immunocompromised. The authorization comes as BA.2, a subvariant of omicron, spreads across the globe and escalates in parts of the United States , sparking concerns about how long immunity lasts after a booster.

The latest evidence suggests antibody levels, which protect us from infection, wane about four months after a booster shot. T cells, the part of our immune system that protects us against severe illness, see a slight dip at the four-month mark but remain robust. This waning does not mean were no longer protected; real-world data shows that the vast majority of people who have received three doses are incredibly well protected from hospitalization (a 90% risk reduction ) several months after their latest dose.

In three to four months, it will start to drop, but youll still be in pretty good shape, said Otto Yang , a professor of medicine in infectious diseases at the David Geffen School of Medicine at UCLA.

Heres how quickly antibody levels decline, and what that means for your next booster:

How long do antibodies last?

The data on antibody levels is limited, and not very well understood, but it appears that antibody levels wane, on average, about four to six months after a booster shot.

A recent study from the Centers for Disease Control and Prevention suggests vaccine effectiveness starts to wane about four months after a booster dose (which is, presumably, due to declining antibody levels). Among people who had received three doses during the delta wave, vaccine effectiveness against urgent care and emergency room visits was 97% within two months of vaccination; that dropped to 89% after four months.

During the omicron wave, vaccine effectiveness against hospitalization in people who had three doses was 91% within two months after their latest dose and 78% after four months. A paper from the United Kingdom published in The Lancet last July found that antibody levels after two doses waned at the two-to-three-month mark, though antibody levels were still pretty high at that point with the Pfizer vaccine (higher than with the AstraZeneca shots). A report published in The New England Journal of Medicine in January 2022 found that while antibody levels declined six months after a Moderna booster, the remaining antibodies were still able to successfully fend off omicron.

At this point, scientists dont clearly know if theres a specific antibody level that would indicate someone is well protected against infection though its clear that higher levels of antibodies generally equal greater protection against infection. What we do know is that even when waning occurs, most people who have had booster shots continue to be safe from severe illness, hospitalization and death.

Research shows that the pace at which antibody levels decline is somewhat influenced by age, gender (antibodies decline more rapidly in men), and immunosuppressive health conditions. But Yang, who studies our immune response against COVID-19, said hes seen 100-year-olds with long-lasting antibody levels so its not always so black and white.