Thanks to a phenomenon called “cross-immunity”, some people are partially protected against SARS-CoV-2 by defenses acquired by fighting other infections. These defenses rest on two pillars: B lymphocytes (which produce antibodies) and T lymphocytes, including CD4 (“warlords”) and CD8 (“killer soldiers”). After each infection, the body “stores” for a longer or shorter period part of this arsenal: it will be reactivated in the event of a new attack by the same infectious agent, or by another presenting certain similarities.
“The best example of cross immunity is vaccinia and smallpox”, remarks Hervé Fleury, professor emeritus at the University of Bordeaux (CNRS UMR5234). Vaccine is cow’s pox and, unlike human smallpox, it is a mild disease. “In the XVIIIe century, an English doctor, Edward Jenner, voluntarily contaminates a man with the vaccine. Then exposed to the smallpox virus, the “vaccinated” man does not develop the disease. The immunity acquired against a virus similar to that of smallpox effectively protects against the latter “, says the virologist.
During the influenza A (H1N1) pandemic in 2009-2010, the elderly were less prone to a severe form of the disease, unlike the young. They are believed to have retained an acquired CD8 cytotoxic response against other strains of the H1N1 influenza virus that had circulated a few decades earlier.
Part of the population may also benefit from some protection against SARS-CoV-2. In the absence of a vaccine, it would partially compensate for the low level of immunity in the population, even if it is too early to know the impact.
“Genetic distance”
Thus, a study, published in the prestigious journal Cell by a Californian team, shows that 100% of patients infected with SARS-CoV-2 have CD4 reacting to the virus, but this is also the case for 50% of people who have never met it. These figures are 70% and 20% respectively for CD8.
“This cross-immunity could play a role in people with little or no symptoms, especially children”, underlines virologist Etienne Decroly, director of research at CNRS, recalling that“At the start of the epidemic it was thought that the genetic distance between SARS-CoV-2 and the other coronaviruses was too far for it to work”. Since then, knowledge has evolved.
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