People who recover from Covid-19 develop durable immunity to the coronavirus that can protect them from reinfection for decades, even if the infection itself was relatively mild. Two studies conducted by American scientists have come to two unexpected conclusions at the same time.
The first, by immunologists at the University of Washington in St. Louis, Missouri, was published this week in the journal Nature. The second, by a team from the Molecular Biology Laboratory at Rockefeller University in New York, has not yet been peer-reviewed and is currently published as a preprint.
Until recently, it was believed that immunity to coronavirus infection did not last long. This conclusion was reached by scientists based on the results of numerous studies showing that the concentration of specific antibodies in the blood (which provide protection against the virus) begins to decline rapidly only a few months after recovery. The majority of experts estimated the duration of effective immune protection at about 1.5-2 years, so the results of the new research came as a pleasant surprise to them. However, the independent experts interviewed by the BBC urged caution and not to get too excited about the news.
After all, several confirmed cases of reinfection with Covid-19 have already been described in the scientific literature, and the theory of lifelong immunity remains just that – a theory, albeit a very plausible one.
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Scientists from the Washington University School of Medicine in St. Louis monitored the condition and changes in the blood of 77 patients, most of whom had a mild form of Covid-19 (only six required hospitalization), for almost a year. Almost all the time, the concentration of protective antibodies that helped the patient overcome the disease continued to decline: at first very rapidly, and then (on average, four months after recovery) a little more slowly. However, as the study’s senior author, Professor of Immunology Ali Ellebedy, explained, at some point the antibody levels stopped falling and reached a plateau. This was because memory cells, which store information about the coronavirus, maintained their activity and maintained a minimum necessary concentration of protective molecules for the patient.
“We have discovered cells in patients that produce antibodies [to the Sars-Cov-2 virus] 11 months after the onset of disease symptoms,” says Professor Ellebedi. “These cells will continue to live in the patient’s body and produce the necessary antibodies as long as the patient is alive. Therefore, the immunity will last for a long time”.
In addition to regular blood tests, scientists collected bone marrow samples from two dozen patients twice for research purposes: first at 7-8 months and then at one year after infection. Specific memory T lymphocytes programmed to produce COVID antibodies were detected in 15 of 19 samples. In the analyses of the control group (i.e., people who were not infected with the virus), it was not possible to detect “trained” cells for the virus. If Covid-19 is present in a mild form in a patient, tests will no longer show the presence of the virus in the body 2-3 weeks after infection, Professor Elledi points out. So the memory cells detected cannot be a “fresh” immune response.
“In the bone marrow, they are in a kind of dormancy, continuing to produce a certain amount of antibodies,” the professor explains. “This is how they have been doing it from the moment of the patient’s recovery – and this is how they will continue to do it indefinitely.”
According to the authors of the Nature article, it is logical to assume that the immunity acquired through vaccination will be equally long-lasting. It is also possible that patients who have received Covid-19 without any symptoms can also count on long-term protection. First, however, it is necessary to determine how protected those who had a severe course of COVID-19 are. There is a version that severe patients had a hypertrophic response to the infection, essentially an autoimmune disease (known as a cytokine storm), which may have weakened the body and hindered the formation of long-term immunity.
Independent experts who were not involved in the research generally agree with the arguments of the authors of the Nature article, but caution against drawing too far-reaching conclusions. According to Professor Danny Altmann, Professor of Immunology at Imperial College London, the work of the American scientists is a good reminder that immune memory is a very fragile and complex thing.
“On the one hand, we see that the level of antibodies in the blood serum decreases rapidly – and this leads us to believe that the immune protection will not last long,” he reflects. “On the other hand, we see that the cells that have been trained to produce these antibodies remain viable for a long time and can restart this process when needed.”
“They may have the ability, they may be able to – but whether they will actually do it, we won’t know until this hypothesis is tested in practice. If the theory is correct, it could mean, for example, that vaccinations need to be updated less frequently,” he explains.
Altman’s colleague at the University of Reading’s School of Pharmacy, Professor Alexander Edwards, is less optimistic. After all, the effectiveness of immune protection depends not only on the level of antibodies or the presence of cells that produce them, but also on the virus itself. And it keeps mutating.
“Unfortunately, no measurement of immune system cells gives us the ability to accurately predict whether there will be reinfection, how soon it will start, how often it will occur, and how severe the disease will be,” Professor Edwards points out.
“Of course, the data from the immunologists is important and great news, but it does not give us an answer to the question of how the epidemic will evolve,” the professor concludes. “Only epidemiologic studies that follow recovered patients and compare them with those who have not yet had the disease can answer this question. And these studies currently indicate that reinfection is possible.”