As with most aspects of the virus, the answer is not completely clear. Why? Because although we have been battling the pandemic for more than a year, the vaccines were granted emergency use authorization relatively recently. So experts have not had time to observe their long-term effectiveness.
However, that research is underway, and in the meantime, experts say we can make an educated guess.
How long will vaccine immunity last?
Federal health authorities have not provided a definitive answer to this question.
But based on clinical trials, experts do know that vaccine-induced protection should last a minimum of about three months. That does not mean protective immunity will expire after 90 days; that was simply the time frame participants were studied in the initial Pfizer, Moderna and Johnson & Johnson trials. As researchers continue to study the vaccines, that shelf life is expected to grow.
In the real world, the protection should last quite a bit longer, though the length of time still needs to be determined with further studies, experts said.
There are also certain factors that may influence how much protection they provide and for how long.
Chunhuei Chi, director of the Center for Global Health at Oregon State University, said immune responses vary from person to person. People who have a stronger immune response to a vaccine will produce more antibodies and memory cells — and therefore have stronger immunity, he said. But there is not currently evidence to show that a stronger immune response will increase the duration of immunity.
And it does not mean people with a stronger immune response will have more severe side effects from the shots or vice versa, according to a recent survey by the British National Health System.
Immunity could also depend on what happens with future variants. If a person were exposed to a variant capable of evading vaccine-induced antibodies, for instance, a vaccine might not be as effective as initially expected, said Lana Dbeibo, an infectious-disease expert at the Indiana University School of Medicine.
Although researchers do not yet have all the answers, previous knowledge of other coronaviruses, as well as emerging research about the current strain, may provide clues.
Looking at studies on natural immunity from the coronavirus, experts hypothesize that protective immunity from the vaccines will last at least six to eight months. And if immunity from SARS-CoV-2 ends up being similar to other seasonal coronaviruses, such as “common colds,” it is even possible the vaccines could provide protection for up to a year or two before requiring a booster, the experts said.
Can we extrapolate from what we know about natural immunity?
In fact, much of this hypothesizing comes from extrapolating data examining immune responses in people who have had covid-19 and illnesses from other coronaviruses, rather than in people who have been vaccinated, said Dbeibo, who is director of vaccine initiatives for Indiana University.
“But vaccine responses should not be less reliable than in natural infection,” she added.
Current research shows that people who have been infected with covid-19, the illness caused by the coronavirus, retained immunity that was robust after eight months. That gives researchers a starting point in predicting how long immunity may last after vaccination, Dbeibo explained.
But research also shows people who had more severe cases developed a stronger immune reaction than those with milder forms of the disease. And because vaccine-induced immunity appears to be more similar to natural immunity that is derived from severe covid-19 infections, researchers say they believe people who take a coronavirus vaccine will be protected better than most people with natural immunity, said David Topham, a professor of microbiology and immunology at the University of Rochester.
All of that said, antibodies will wane. And although it is a gradual process, once antibodies decline to a level that is no longer protective, reinfection is possible. Still, the infection is likely to be milder, experts said.
Topham, founding director of the Translational Immunology and Infectious Disease Institute at the University of Rochester, has been studying the coronavirus and the role of memory B cells — immune cells that persist for a lifetime and produce antibodies when re-exposed to a pathogen that they have been programmed to fight. He said some people who were hospitalized with severe covid-19 infections still had high frequencies of memory B cells, as well as antibodies, up to nine months after infection.
He said memory B cells can even adapt quickly to a new variant, usually within days.
“Even if antibody levels wane and you get reinfected or you get infected with a variant, the memory B cells — if you have enough of them — will respond very quickly and prevent that severe disease,” he said.
Experts are also speculating whether immunity to SARS-CoV-2 will be as durable as with seasonal coronaviruses, which people contract repeatedly throughout their lifetimes. Experts estimate that immunity from those coronaviruses lasts a couple of years, and some experts predict that, in time, that may be the case with the current virus.
“I think this coronavirus is going to become like other seasonal coronaviruses in that you will either be vaccinated or infected as a child, have a mild illness and then when you see it as an adult, you’re going to have some immunity. While it’s not going to protect you from getting infected, the worst you’ll get is a bad cold,” Topham said.
He said covid-19 is severe right now because “adults are seeing this virus for the very first time and have no preexisting immunity.”
So what now?
Experts are still trying to determine how long it will take for antibodies to decrease to the point that they are no longer protective. But once that happens, people will need boosters to remind their immune systems to make more antibodies against the disease, the experts said.
Chi, with Oregon State University, said Pfizer, Moderna and others are conducting clinical trials to determine how long a booster shot will extend protective immunity and to determine whether their vaccines can be tailored to combat new variants of the virus. And Johnson & Johnson is testing a two-dose version of its vaccine.
So far, evidence suggests the available vaccines are still effective against most variants, but that could change if the virus continues to mutate, Chi said. Chi explained that the more prevalent the virus and the longer it takes to vaccinate people against it, the higher the risk of developing mutations that will then make the vaccines less effective. That’s why, he said, it’s urgent to “vaccinate as many people as fast as possible.”
“We are running against time,” he said.