‘I’m an Epidemiologist, and This Is How Long You’re Protected by the COVID-19 Vaccines’

Photo: Getty Images / Marko Geber
Few things have felt as hopeful in the past year as the photos currently flooding social media feeds of people getting COVID-19 vaccines. But it's still challenging to put worry aside completely, even as the public gets inoculated against the virus that's held the world hostage. Vaccines provide protection, but there are still some unknowns with respect to just how long we can breathe easy after getting the jab. Exactly how long does a COVID-19 vaccine last?

More information exists about Pfizer BioNTech and Moderna vaccines than the Johnson & Johnson vaccine, simply because trials of the former started earlier than those of the latter, explains Timothy Brewer, MD, professor of medicine and epidemiology at UCLA. And the data available is promising. Recently, Pfizer announced that its vaccine efficacy remains at 91 percent for trial participants a full six months after the second dose. "That's really encouraging news," says Dr. Brewer. "And it doesn't mean that the vaccine efficacy won't last longer, it just means that's all the data we have so far."

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Moderna also has data—albeit from a much smaller sample—that suggests lasting efficacy of its vaccine. According to Dr. Brewer, 33 Moderna trial participants maintained binding antibodies and neutralizing antibodies for up to 209 days after their second dose. "That is preliminary evidence for both Pfizer and Moderna efficacy lasting six months and probably longer," he says.

It's important to understand that when you get one of these vaccines, efficacy doesn't one day wear off overnight. Instead, what you'll experience is a slow decline over time. But the shots cause your body to produce antibodies—and memory cells, which help your body remember the infection and mount a more effective response to it in the future—at levels that are actually much higher than what you'd likely need to fight the virus. So as they diminish, you're not unprotected, you're just less protected than the very high rate at which you were originally protected. "That means you have a very long time before the levels fall far enough to actually be in a range where you could potentially become infected again, or to get serious disease if infected," Dr. Brewer says.

And there is some evidence suggesting six months is likely, as Dr. Brewer has noted, to be a conservative estimate of efficacy. One small study showed that people who were naturally infected with COVID-19 maintained antibodies for eight months thereafter, and because the vaccine creates more antibodies than natural infection does, it's likely you would be protected for even longer than that after vaccination.

Protection duration does depend to some degree, however, on how much the virus mutates. "Protection is a function of two parts. So one part is how good and durable the immune response is, and the second part is how much the virus changes," says Dr. Brewer. "One of the reasons you have to get an influenza shot every year is not because your immunity to that particular influenza virus has waned. Instead, it's because the influenza virus changes enough, over time, that when you're exposed the next year it's a slightly different virus."

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Fortunately, coronaviruses do not change or mutate at the same rate as influenza viruses, says Dr. Brewer. And if enough of us get vaccinated quickly, we should be able to slow or even halt mutations of SARS-CoV-2, the virus that causes COVID-19. "In order for the SARS CoV-2 virus to change to create what we call variants, you need to have replication and transmission," he says. "So if there's no replication and transmission, there's no formation of new variants."

Worrisome variants already exist, however, and there is some confusion and concern over whether or not the current vaccines offer protection against them.

According to Dr. Brewer, the data we have so far shows that the three vaccines currently available in the U.S. offer "very good" protection against the B.1.1.7 variant, which originated in the UK. The other two concerning variants have additional mutations which may make them more evasive; however, Pfizer trialed vaccinations on 800 people in South Africa, many of whom were exposed to the B.1.351 variant, and none of them caught COVID-19. Johnson & Johnson also trialed their vaccine in South Africa and Brazil, where the P.1 variant originated, and showed one hundred percent efficacy in preventing serious disease and death, even with exposure to the more dangerous variants. So when you hear that our current vaccines are less effective against these variants, Dr. Brewer says it's important to remember once again that you are being protected at orders of magnitude higher than you need, so lowered protection is still protection, and likely even good protection.

The best thing about new vaccine technology is that it's essentially plug-and-play, meaning that vaccines can be altered fairly easily to cover new mutations as they arise. Drug companies are already working on boosters specifically designed to offer greater protection against the more worrisome variants, though there is no news yet of when or if those will be dispatched to the public.

Overall, there's still much to learn before we known for certain how long the COVID-19 vaccines will last, says Dr. Brewer. But the takeaway for now is that you are well protected from all variants for at least six months—and probably longer. "We still have to be careful, but relaxing a little bit is reasonable advice—the vaccines are excellent, and they work very well," he says. "People should definitely get vaccinated, and still take preventive measures, but you don't need to stay up at night worrying." 

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