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 we’re 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 you’ll 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.
Here’s 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 don’t clearly know if there’s a specific antibody level that would indicate someone is well protected against infection — though it’s 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 he’s seen 100-year-olds with long-lasting antibody levels — so it’s not always so black and white.
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Our T cell response is durable and longer-lasting
Protection against COVID-19 doesn’t depend only on antibodies. Where antibodies protect us from infection, the T cell response protects us against serious illness and death. And according to Monica Gandhi, an infectious diseases specialist with University of California, San Francisco, it looks like T cell immunity is going to be long-lasting, even against variants. Several studies have found that T cell responses remained nearly as effective against omicron, at six months after vaccination, as they were against earlier variants, Gandhi said.
Yang, who studies T cell responses, noted that T cells may start to wane after a few months — but this does not mean they’re gone. “About three to four months after the third shot, after the first booster, the benefit of the vaccine preventing serious illness starts to drop, but doesn’t drop precipitously,” Yang said.
If an antibody test showed that a person has no detectable antibodies then that means there are no antibodies circulating in their blood. It’s a bit more complicated when it comes to testing for T cells. T cells have a great memory, so even if a generally healthy person has low or undetectable T cells, they can reemerge, multiply very quickly and jump into action. This is most likely why real-world data shows the vaccine still protects most people from severe illness and death.
Should you time your next booster to future waves?
The timing of the booster is where it gets tricky, as doctors have different takes.
Yang recommended getting the second booster, i.e., a fourth dose, as soon as you’re eligible. Though we’re in a bit of a lull, COVID-19 is still circulating and you don’t want to have low immunity if exposed, especially if you’re at risk, he said. Doing so could also help prevent future waves.
“If you’ve tolerated three doses of the vaccine already without any serious problems, you’re going to almost certainly tolerate the fourth dose — there’s so little downside, it’s worth getting it,” Yang said.
Something else to consider, according to Yang: We need even higher levels of antibodies to fight these new variants, since they’re structurally different than the original strain that the vaccines target. “This is probably one of the reasons why boosters have been recommended so quickly,” he said.
Gandhi, on the other hand, recommends timing your boosters to future waves (which is what she’s personally advising her parents, who are older and at-risk, to do). “You want your antibodies to be plump and high, especially if you’re vulnerable, around the time of typical surges,” Gandhi said.
Officials believe COVID-19 may end up as a seasonal illness. However, seasonality varies from region to region — whereas the Northeast typically sees early fall and winter waves, Southern areas like Florida tend to experience summer surges when the hot, sticky, humid air sends people indoors.
When in doubt, talk to your doctor. Most people who have had three shots are still going to be well protected from severe outcomes like hospitalization and death. But for the elderly, immunocompromised, and people with health conditions that put them at higher risk, it’s worth hearing your doctor’s take on how the benefits of getting a fourth dose now stack up to the risks of putting it off a bit longer.
