Why have some people never contracted Covid?

    Why have some people never contracted Covid?

    The reason would lie in the different immune responses, so it could be possible that people never exposed to the virus or not vaccinated are in any case protected thanks to an antecedent immunity

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    Many of us in these two long periods have received at least once a diagnosis of positivity to the new coronavirus (and in some cases even more than once over several months), many others, however, seem to be completely immune to it. How come?





    If, at present, it can be thought that a large part of the population has reached a certain level of immunity thanks to the doses of the vaccine or a previous infection (and for this not contracting Covid finds a quick explanation in the fact that the protection can derive from the acquired immune shield), many studies conducted before vaccinations have however highlighted various phenomena related to the immune response, so it is possible that people never exposed to the virus and not vaccinated are in any case protected.

    Read also: Coronavirus: there are those who are "immune" even if they have never been infected. The Californian study

    In short, some subjects would already seem immune or pre-immune to the coronavirus, although they have never been infected, and therefore may never get sick.

    Studies

    A research conducted by scholars from Imperial College London analyzed the so-called "cross-reactive immunity", Due to the response of our immune system to viral agents similar to Sars-Cov-2, (such as HCoV-OC43 and HCoV-HKU1, the coronaviruses responsible for the common seasonal cold). According to other studies, it is also possible that the infection is cleared during the early stages, before the viral load is able to reach levels where symptoms appear.

    An example is the study conducted during the first wave of the pandemic, when researchers at University College London monitored health workers who were regularly exposed to infected patients, but who never tested positive or developed antibodies against Sars-Cov-2. . Their blood tests then revealed about 15% had cross-reactive memory T cells, a type of lymphocyte that could remember the pathogen they had already encountered and recognize multiple strains of the same pathogen, as well as other markers of infection. viral. It could therefore be that memory T lymphocytes from previous seasonal coronavirus infections have cross-reacted with the new coronavirus and protected from Covid.



    On the other hand, Sweden seems to deserve a separate discussion, which has never registered very high numbers of infections. Here, Cecilia Söderberg-Nauclér, immunologist at the Karolinska Institute in Stockholm, and Marcus Carlsson of Lund University, showed that the incidence of Covid cases in Sweden could only be explained in the presence of a large percentage of people with a sort of protective immunity, for which the research team scanned existing virus protein sequence databases, looking for small segments (peptides) similar to those of the new coronavirus, to which antibodies are likely to bind.

    Also Read: Antibodies to SARS-CoV-2 Can Last for Life. The new study

    The results revealed the identification of a six amino acid peptide in an H1N1 influenza protein that corresponds to a crucial part of the coronavirus Spike protein, suggesting that immune responses triggered by the H1N1 influenza virus (responsible for the outbreak of 2009 swine flu) and possibly related subsequent strains, may provide people with partial, if not complete, protection against Covid-19. Antibodies against this peptide were detected in 68% of blood donors in Stockholm, supporting the (yet to be revised) thesis of partial cross-protective immunity.

    All of this does not rule out that a small percentage of people might as well be genetically resistant to Covid-19, as some researchers say who are looking for people who have not contracted the infection despite having been in contact with infected people in the hopes of identifying protective genes.

    Non-infection, however, is not new. There are rare phenomena of resistance to infections for other diseases, including HIV, malaria and norovirus. In these cases, the presence of a genetic defect means that some people do not have a receptor that the pathogen uses to enter cells, so they cannot be infected.



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