En español | After half a million deaths and more than a year of lockdowns, quarantines, masking and social distancing, the U.S. is in the midst of a vaccination campaign that aims to put an end to the worst of the COVID-19 pandemic. Months of rigorous testing and clinical trials have shown that the vaccines are safe and highly effective at preventing COVID-19 and will likely fend off serious illness or hospitalization even if you do get sick. What's still unclear is just how long that protection will last, or whether we'll eventually need boosters or follow-up shots to protect against new variants of SARS-CoV-2, the virus that causes COVID-19.
A piece of the answer came today from Pfizer, which announced that its vaccine, after the second dose, was shown to be highly effective for up to six months — even against one of the well-known virus variants.
That real-world data helps fill in some blanks left by a shorter-then-usual clinical study period for the vaccine. Under normal circumstances, phase 3 vaccine studies would have continued for another few years, showing how long immunity lasts, before the vaccine was distributed to the general public, says Clare Rock, associate professor of medicine in the Division of Infectious Diseases at Johns Hopkins School of Medicine. Because of the extreme emergency related to the pandemic, that process was expedited. “We can look at neutralizing antibodies in people's blood and do some laboratory testing of the vaccine, but it's extremely difficult in the lab to say how long that's going to last for,” says Rock, an infectious disease physician and hospital epidemiologist. “A lot of it is a matter of time passing and seeing what happens.”
What existing viruses tell us about a time frame
So what is likely to happen? Other viruses provide some clues. Those that are classified as stable, like measles or mumps, rarely mutate, and an infection or vaccination will generate lifelong immunity. “If you get measles or get vaccinated against measles, you won't get ill for the rest of your life,” explains William Schaffner, M.D., professor of infectious diseases at Vanderbilt University Medical Center. “The duration of protection is very long, even though measles is very contagious, likely even more contagious than COVID-19."
Coronaviruses, on the other hand, tend to produce only short-lived immunity. (Other coronaviruses already circulate widely in humans and cause common colds, which we can get again and again.) However, experts are encouraged by research showing that immunity to SARS and MERS viruses, which are also related to SARS-CoV-2, is more durable after infection.
Vaccine immunity vs. immunity from COVID-19
Another reason to cheer: COVID-19 vaccines actually induce higher levels of antibodies than natural infection, says John Wherry, director of the Institute for Immunology at the University of Pennsylvania.
"Early on, looking at people who had recovered from COVID, there were studies that showed antibodies may wane quickly after acute infection,” he says. “What we eventually realized is that antibodies naturally go up, they come down, and then they settle into a steady state.” More recent studies indicate that people who've had COVID-19 have good antibody and other immune memory (which translates into protection) for at least eight to nine months, which was as long as could be analyzed this far into the pandemic.
Since people who've been vaccinated mount an even better immune response, Wherry says he thinks immunity from the COVID-19 vaccines will likely last several years, if not longer. He notes that scientists will need to be on alert for “canaries in the coal mine” that indicate immunity may be waning, particularly in certain populations, like those over 65 or people with compromised immune systems.
"We have to monitor these things and plan accordingly, and be ready to make decisions if we start to see emergence of new infections,” he says.
How vaccine immunity may stand up to variants
A revaccination program may also be necessary if variants emerge that are significantly different from the original virus that the vaccines were developed against. RNA viruses like the COVID-19-causing SARS-CoV-2, which use RNA rather than DNA as genetic material, are known for evolving and mutating rapidly. Copying RNA is more error-prone than copying DNA, so mutations occur frequently during replication. If a random mutation turns out to be beneficial for the virus — helping it infect new species or evade the host's immune system, for example — it will take over. Already, there is concern that the vaccines on the market may not provide as robust an immune response against variants like B.1.351, which was first identified in South Africa.
However, Wherry offers this reassurance: “While the South African variant reduces efficiency by two- to fivefold, the vaccine induces fifty-fold more protection than we need, so most people will still be protected.” That is, even if the variant saps some of the vaccine's power, it will still have plenty of gas in the tank to keep us protected against serious illness.
Still, experts note, we're still racing against the virus's propensity for mutation as it continues to spread throughout the globe. The faster we get everyone vaccinated, the smaller the chances that a more cunning variant will emerge that threatens our vaccine-induced immunity.
"Viruses only mutate when they multiply. They only multiply when they spread from person to person,” Schaffner says. The more people get infected, the more likely it is that new, problematic variants will appear, he adds. “To diminish the likelihood of that happening, we need to vaccinate as many people as possible, not only here in the U.S. but in every country around the world."