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The body builds a protective fleet of immune cells when it becomes infected with COVID-19, and in many people, those defenses persist for more than six months after the infection is gone, according to a new study.
The immune the cells appear so stable, in fact, that immunity to the virus can last at least several years, the study authors said. “That amount of [immune] memory would likely prevent the vast majority of people from contracting hospitalized illness, serious illness, for many years, “said co-author Shane Crotty, virologist at the La Jolla Institute of Immunology in California The New York Times, who first reported on the study.
That said, making predictions about the duration of coronavirus immunity can be “tricky,” Nicolas Vabret, an assistant professor of medicine at Mount Sinai Icahn School of Medicine, who was not involved in the study, told Live Science.
“It would be amazing to see … immune cells build up in patients for more than six months and suddenly collapse after a year,” Vabret said in an email. But “the only way to know if SARS-CoV-2 immunity will last decades is to study patients over the same time period.”
In other words, we won’t know Exactly how long immunity lasts without continuing to study those who have recovered from COVID-19. However, the new study, published November 16 in the prepress database bioRxiv, provides strong clues that the protection is long-lasting, although clearly not in all people, as there have been several cases of people who have been reinfected with the coronavirus after recovery.
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Research dives into the ranks of the human immune system, evaluating how different lines of defense change after a COVID-19 infection.
These defenses include antibodies, which bind to the virus and evoke immune cells to destroy the insect or to neutralize it. Memory B cells, a kind of white blood cell, “remember” the virus after an infection has cleared up and help build the body’s defenses rapidly if the body is re-exposed. Memory T lymphocytes, another type of white blood cell, also learn to recognize the coronavirus and dispose of infected cells. Specifically, the authors looked at T cells called CD8 + and CD4 + cells.
The authors evaluated all of these immune cells and antibodies in 185 people who had recovered from COVID-19. A small number of participants never developed symptoms of the disease, but most had mild infections that did not require hospitalization. And 7% of the participants were hospitalized for a serious illness.
Most of the participants provided a blood sample, sometime between six days and eight months after their infections started. Thirty-eight participants provided different blood samples between those time points, allowing the authors to monitor their immune response over time.
Ultimately, “it could be argued that what they found is not that surprising, as the immune response dynamics they measure resemble what one would expect from the functioning of the immune system,” Vabret said.
The authors found that antibodies specific to the spike protein, a structure on the surface of the virus, remain stable for months and begin to decline six to eight months after infection. Five months after infection, almost all of the participants were still carriers of antibodies. The volume of these antibodies differed widely between people, however, with up to a 200-fold difference between individuals. Antibody counts normally drop after an acute infection, Vabret noted, so the modest drop after six to eight months wasn’t a surprise.
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By comparison, the memory T and B lymphocytes that recognize the virus appear extremely stable, the authors noted. “Essentially no decay of … memory B cells was observed between days 50 and 240,” or eight months later, Marc Jenkins, an immunologist at the University of Minnesota Medical School who was not involved in the study, he said in an email.
“Although memory T-cell decay was observed, the decay was very slow and could flatten out at some point,” added Jenkins. There is reason to believe that the number of memory T cells may stabilize sometime after infection, because T cells against a related coronavirus, SARS-CoV, have been found in patients healed up to 17 years later, according to one. study published July 15 in a journal Nature.
At the start of the pandemic, scientists expressed concern that immunity to the virus could wane in about a year; this trend can be seen with the four coronaviruses that cause the common cold, Live Science previously reported. However, studies suggest that the body’s reaction to common coronaviruses may differ from that to viruses such as SAR-CoV and SARS-CoV-2, which bounce from animals to humans.
“We don’t really know why seasonal coronaviruses don’t induce lasting protective immunity,” Vabret said. But the new studio, along with others recent test, suggests that SARS-CoV-2 immunity may be more robust, said Jason Cyster, a professor of microbiology and immunology at the University of California, San Francisco, who was not involved in the study.
That said, some participants in the new study did not activate long-lasting immune responses to the new virus. Their transient responses may come down to differences in the amount of virus they were initially exposed to, or genetics can explain the difference, Cyster said. For example, genes known as human leukocyte antigen (HLA) genes differ widely between individuals and help alert the immune system to foreign invaders, Live Science previously reported.
These inherent differences between people can help explain cases of COVID-19 reinfection, which have been relatively rare but are increasing in number. Science magazine reported.
Again, to truly understand how long COVID-19 immunity lasts, scientists must continue to study recovered patients. “Of course, we have to look six months down the road” and see if the T and B cell counts remain high, Cyster said.
If immunity is long-term, a big question is whether that duration has repercussions vaccines. But natural immunity and vaccine-generated immunity cannot be directly compared, Vabret noted.
“The mechanisms by which vaccines induce immunity are not necessarily the same as those resulting from natural infections,” Vabret said. “So the immune protection from a vaccine could last longer or shorter than that from a natural infection.”
For example, the Pfizer is Modern vaccines use a molecular messenger called mRNA to train the body to recognize and attack the coronavirus. No mRNA-based vaccine has ever been approved before, so “we basically know nothing about the duration of those responses,” Cyster said.
“I think [that’s] the great unknown to me, among many, “he said.
But while some unanswered questions remain, the main finding of the new study is that “immune memory to SARS-CoV-2 is very stable,” Jenkins said. And – fingers crossed – maybe those hopeful results will hold up well in the future as well.
Originally published on LiveScience.
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