The body builds a protective fleet of immune cells when infected with COVID-19, and in many people these defenses persist for more than six months after the infection disappears, according to a new study.
The immune In fact, the cells appear so stable that immunity to the virus can last for at least several years, say the study’s authors. “This amount of [immune] memory is likely to prevent most people from getting hospitalized, a serious illness, for many years, “co-author Shane Croty, a virologist at the Institute of Immunology in La Jolla, California, told New York Times, who first reported the study.
However, predicting how long immunity to the coronavirus lasts can be “complicated,”
“It would be surprising to see how … immune cells build up in patients for six months and suddenly collapse after a year,” Vabret said in an email. But “the only way to know if immunity to SARS-CoV-2 will last for decades is to examine patients over the same period of time.”
In other words, we will not know exactly how long immunity lasts without continuing to study those who have recovered from COVID-19. The new study, published on November 16 in the preprint database bioRxiv, gives strong hints that the protection is long-lasting – although obviously not in all people, as there are several cases of individuals who have been reinfected with coronavirus after recovery.
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The study immersed itself in the ranks of the human immune system, assessing how different defense lines change after infection with COVID-19.
These protections include antibodiesthat bind to the virus and call on immune cells to destroy the bug or neutralize it themselves. Memory B cells, a type of white blood cell, ‘remember’ the virus once the infection has cleared and help to quickly boost the body’s defenses if the body is exposed again. Memory T cells, another type of white blood cell, also learn to recognize the coronavirus and discard infected cells. In particular, 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 recovered from COVID-19. A small number of participants never develop symptoms of the disease, but most often mild infections are observed that do not require hospitalization. And 7% of the participants were hospitalized for a serious illness.
The majority of participants provided a blood sample, somewhere between six days and eight months after the onset of their infections. Thirty-eight participants gave several blood samples between these time points, allowing the authors to track their immune response over time.
In the end, “it can be argued that what they found is not so surprising, because the dynamics of the immune response they measure seems to be what you would expect from a functioning immune system,” Wabre said.
The authors found that antibodies specific for the spike protein – a structure on the surface of the virus – remained stable for months and began to weaken about six to eight months after infection. Five months after infection, almost all participants still carried antibodies. However, the volume of these antibodies varies considerably between people, with up to 200 times the difference between individuals. The number of antibodies usually drops after an acute infection, Vabret noted, so moderate dropout from six to eight months was no surprise.
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In comparison, the memory of T and B cells that recognize the virus appears to be extremely stable, the authors note. “Essentially no breakdown of … memory B cells was observed between 50 and 240 days,” or eight months later, Mark Jenkins, an immunologist at the University of Minnesota School of Medicine who did not participate in the study. says in an email.
“Although there is some breakdown of T cells in memory, the breakdown is very slow and can level off at some point,” Jenkins added. There is reason to believe that the number of T cells in memory may stabilize at some time after infection, as T cells against a related coronavirus, SARS-CoV, were detected in recovered patients up to 17 years later, according to a study published July 15 in a journal Nature.
At the start of the pandemic, scientists expressed concern that immunity to the virus could disappear in about a year; this trend can be seen in the four coronaviruses that cause the common cold, Previously, Live Science reported. However, studies show that the body’s response to common coronaviruses may differ from that of viruses such as SAR-CoV and SARS-CoV-2, which jump from animals to humans.
“We don’t really know why seasonal coronaviruses don’t induce lasting protective immunity,” Vabret said. But the new study, along with others recent evidencesuggests that immunity against SARS-CoV-2 may be stronger, said Jason Sister, a professor of microbiology and immunology at the University of California, San Francisco, who was not involved in the study.
However, several participants in the new study did not receive a long-lasting immune response to the new virus. Their transient reactions may be reduced to differences in how many viruses they were originally exposed to, or genetics can explain the difference, Sister said. For example, genes known as human leukocyte antigen (HLA) genes differ significantly between individuals and help to warn the immune system of foreign invaders, Previously, Live Science reported.
These inherent differences between people may help explain the cases of reinfection of COVID-19, which were relatively rare but increasing, Science Magazine reports.
Again, to understand how long immunity to COVID-19 lasts, scientists must continue to study recovered patients. “Of course, we have to look six months down and see if the number of T and B cells remains high,” Sister said.
If the immunity is long-term, one big question is whether this durability is transmitted vaccines. But natural immunity and vaccine-generated immunity cannot be directly compared, Vabre said.
“The mechanisms by which vaccines induce immunity are not necessarily the same as those derived from a natural infection,” Wabre said. “So the immune protection derived from a vaccine can last longer or shorter than that derived from a natural infection.”
For example Pfizer and Modern vaccines use a molecular messenger called mRNA to train the body to recognize and attack the coronavirus. An mRNA-based vaccine has never been approved, so “we know virtually nothing about the durability of these responses,” Sister said.
“I think [that’s] the big unknown to me, among the many, “he said.
But while some unanswered questions remain, the main finding from the new study is that “immune memory to SARS-CoV-2 is very stable,” Jenkins said. And – thumbs up – maybe these reliable results will last in the future.
Originally published in Live Science.