Immunity to the coronavirus lasts at least a year, possibly a lifetime, and improves over time, especially after vaccination, according to two new studies. The results could help dispel lingering fears that protection from the virus will be short-lived.
Taken together, the studies suggest that most people who have recovered from Covid-19 and were later immunized don’t need boosters. However, vaccinated people, who most likely never got infected, need the shots, as do a minority who were infected but did not evoke a robust immune response.
Both reports looked at people who had been exposed to the coronavirus about a year earlier. Cells that retain a memory of the virus remain in the bone marrow and can produce antibodies when needed, according to one of the studies published in Nature on Monday.
The other study, published online at BioRxiv, a biological research website, found that these so-called memory B cells continue to mature and strengthen at least 12 months after the initial infection.
“The publications are consistent with the growing body of literature suggesting that immunity induced by infection and vaccination against SARS-CoV-2 appears to be long-lasting,” said Scott Hensley, an immunologist at the University of Pennsylvania, who was not involved in the research.
The studies could allay fears that immunity to the virus is temporary, as is the case with coronaviruses, which cause colds. But these viruses change significantly every few years, said Dr. Hensley. “The reason we become repeatedly infected with frequent coronaviruses over the course of life could have a lot more to do with the variation in these viruses than with immunity,” he said.
In fact, memory B cells, which were produced in response to SARS-CoV-2 infection and boosted by vaccination, are so effective that they even thwart variants of the virus and nullify the need for boosters, according to Michel Nussenzweig, immunologist at Rockefeller University in New York, who led the study on memory maturation.
“People who have been infected and vaccinated really have a great response, a great set of antibodies, because they keep developing their antibodies,” said Dr. Nut branch. “I assume they will last a long time.”
The result may not only apply to vaccine protection, as immune memory is likely to be organized differently after immunization than after natural infection.
That means people who haven’t had Covid-19 and have been vaccinated may need a booster shot, said Dr. Nut branch. “We’ll know something like that very, very soon,” he said.
When a virus first appears, B cells multiply quickly and produce antibodies in large quantities. Once the acute infection has subsided, a small number of cells take their place in the bone marrow and steadily pump out modest amounts of antibodies.
To study the memory B cells specific to the new coronavirus, researchers led by Ali Ellebedy of Washington University in St. Louis analyzed the blood of 77 people at three-month intervals, starting about a month after they were infected the coronavirus. Only six of the 77 had been hospitalized for Covid-19; The rest had mild symptoms.
Antibody levels in these individuals fell rapidly four months after infection and continued to decline slowly for months afterward – results that are consistent with other studies.
Some scientists have interpreted this drop as a sign of waning immunity, but it’s exactly what is expected, other experts said. If blood contained large amounts of antibodies to every pathogen the body had ever encountered, it would quickly turn into thick mud.
May 26, 2021, 5:08 p.m. ET
Instead, blood levels of antibodies drop sharply after an acute infection, while memory B cells in the bone marrow remain calm and ready to take action if necessary.
Dr. Ellebedy received bone marrow samples from 19 people approximately seven months after infection. Fifteen had detectable storage B cells but four did not, suggesting that some people may have very few cells or no cells at all.
“It tells me that even if you got infected, it doesn’t mean you have a super immune response,” said Dr. Ellebedy. The results confirm the idea that people who have recovered from Covid-19 should be vaccinated, he said.
Five of the participants in Dr. Ellebedy’s study donated bone marrow samples seven or eight months after the initial infection and again four months later. He and his colleagues found that the number of storage B cells remained stable over this time.
The results are especially noteworthy given that bone marrow samples are difficult to obtain, said Jennifer Gommerman, an immunologist at the University of Toronto who was not involved in the work.
A landmark 2007 study showed that antibodies can theoretically survive for decades, perhaps well beyond the average lifespan, suggesting the long-term existence of memory B cells. But the new study offered rare evidence of its existence, said Dr. Gommerman.
Dr. Nussenzweig studied how memory B cells mature over time. The researchers analyzed the blood of 63 people who had recovered from Covid-19 about a year earlier. The vast majority of participants had mild symptoms and 26 had also received at least one dose of the Moderna or Pfizer BioNTech vaccine.
So-called neutralizing antibodies, which were needed to prevent re-infection with the virus, remained unchanged between six and twelve months, while related but less important antibodies slowly disappeared, the team found.
As memory B cells evolved, the antibodies they produced developed the ability to neutralize an even wider group of variants. This continued maturation may be due to a small piece of the virus being bound by the immune system – for target practice, so to speak.
One year after infection, the neutralizing activity was lower in the non-vaccinated participants compared to all forms of the virus, with the greatest loss being recorded compared to the variant first identified in South Africa.
The vaccination significantly increased antibody levels and confirmed the results of other studies. The shots also increased the body’s ability to neutralize by 50 times.
Kentucky Republican Senator Rand Paul said Sunday he would not receive a coronavirus vaccine because he was infected last March and was therefore immune.
However, there is no guarantee that such immunity will be strong enough to protect him for years, especially given the emergence of variants of the coronavirus that can partially bypass the body’s defenses.
The results of the study by Dr. Nussenzweig suggest that people who have recovered from Covid-19 and were later vaccinated will continue to have extremely high levels of protection against emerging variants, even without receiving a vaccine booster later.
“It looks exactly what we’d hope a good memory B-cell response would look like,” said Marion Pepper, an immunologist at the University of Washington in Seattle who was not involved in the new research.
All experts agreed that immunity in people who have never had Covid-19 is likely to vary widely. Fighting a live virus is different from responding to a single viral protein introduced by a vaccine. And in those who had Covid-19, the initial immune response had time to mature over six to 12 months before being challenged by the vaccine.
“These kinetics are different from someone who has been immunized and re-immunized three weeks later,” said Dr. Pepper. “That doesn’t mean they might not have that broad answer, but it could be very different.”