With the arrival of new variants of the coronavirus and the uncertainties about the duration of immunity the possibility of vaccine boosters is becoming more and more concrete. There are no conclusive studies on the subject, but finding out how long immunity lasts, both after infection and after the vaccine is essential to be able to manage the epidemic in the coming months. According to various studies, immunity could last at least 8 months; other research goes up to 12 months if not years. Two new researches reported by the New York Times find that immunity could last for years and that most of the people who have recovered from Covid-19 and who have immunized will not need boosters. Vaccinates who have never been infected with Sars-CoV-2 will most likely need the booster instead. as well as a minority of people who, despite having been infected, did not develop an adequate immune response. Under the magnifying glass the are over memory B cells, those lymphocytes that remember the pathogen-enemy with which one has had to deal with the infection or with the vaccine, and which allow the generation of neutralizing antibodies as soon as one comes into contact with the virus.
The memory B cell node
Both jobs looked at data from people who came into contact with the coronavirus about a year ago. According to the study published in the journal Nature cells persist in the bone marrow that retain a memory of the virus and are able to produce antibodies when needed. The other study published in pre-press (not yet revised) in BioRxiv concludes that memory B cells continue to mature and strengthen even 12 months after infection. The studies allay concerns that immunity to the virus is transient, as is the case with coronaviruses that cause common colds. Those coronaviruses change significantly in a few years – he told the New York Times Scott Hensley, an immunologist at the University of Pennsylvania – and the reason we get repeatedly infected with common coronaviruses may actually have to do with mutations rather than immunity.
The analysis of the bone marrow
B cells proliferate and produce antibodies in large quantities upon the first encounter with the virus. Once the acute infection resolves, a small number of cells settle in the bone marrow, constantly producing antibodies. To examine the specific B cells of the new coronavirus the researchers led by Ali Ellebedy of Washington University in St. Louis analyzed the blood of 77 people at three-month intervals, starting about one month after their coronavirus infection. Only six of the 77 had been hospitalized for Covid-19; the rest had mild symptoms. The volunteers’ antibody levels fell rapidly four months after infection and continued to decline slowly over the following months, in line with what other studies have concluded. This decrease in antibodies has been interpreted by many scientists as a sign of the decline in immunity, but not really a worrying fact, an expected biological event. Blood antibodies decrease dramatically as time passes, while memory B cells remain inactive in the bone marrow, ready to intervene when needed.
To understand the mechanism, Dr. Ellebedy’s team analyzed samples of bone marrow of 19 people seven months after infection. In fifteen of them memory B cells were detected, but the other four had very few or none at all. Not all those recovered from Covid therefore develop adequate immunity, which is why it is important that they too get vaccinated. Five of the participants donated their bone marrow a first time eight months after infection and then again four months later: the researchers found that the number of memory B cells remained stable over time. Research is very important because it is very difficult to obtain bone marrow samples for study. Based on another important work dated 2007, which discovered how B cells of various pathogens (measles, rubella, mumps) survived for decades, scientists expect long-lasting protection for Covid-19 as well.
The antibodies that evolve
The second work goes in the same direction. Second Michel Nussenzweig, an immunologist at Rockefeller University in New York who conducted the study published in BioRxiv, the memory B cells produced in response to Sars-CoV-2 infection and enhanced with vaccination are so powerful that they also counteract the variants, making booster required, as antibodies continue to evolve. The team looked at how memory B cells mature over time. The researchers analyzed the level of antibodies in the bloodstream of 63 people recovered from Covid a year ago, 26 of whom were vaccinated with a dose of Pfizer or Moderna. It was found that the neutralizing antibodies, necessary to prevent reinfection with the virus, remained unchanged between six and 12 months, while the secondary antibodies declined rapidly. The scientists observed that memory B cells continued to mature and evolve for at least a year. One year after infection, the neutralizing antibodies of those who had not been vaccinated had lost their effectiveness against the numerous variants tested, in particular the South African one. On the other hand, those who had received at least one dose of the vaccine had developed a very strong neutralizing response (up to 50 times higher) and a high level of antibodies thanks to the work of the memory B cells that are reactivated with the vaccine. The results of this research suggest that people recovered from Covid and then vaccinated will maintain high protection, even against emerging variants, without needing a booster.
According to the authors not said that the mechanism also works with the protection induced by vaccines alone because immune memory is likely to develop differently after immunization than after natural infection. therefore it is possible that those who have not had Covid and been vaccinated may need a booster dose. We will know soon concluded Michel Nussenzweig.
May 27, 2021 (change May 27, 2021 | 16:13)
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