Immunity and Covid: Why the Cross Immunity Thesis Resurfaces

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Two studies recently reported cross immunity to the coronavirus. but how does it work? Does it allow faster access to collective immunity?

Are we all on the way to being immunized with the coronavirus? If the evolution of the epidemic still casts many shadows for scientists, recent studies show an optimistic picture of the upcoming collective immunity.

This collective immunity, achieved when 65% of the population contracted Covid-19, would allow the epidemic to end. However, according to the models of the Pasteur Institute, only 4.4% of the French were infected with SARS-CoV-2. Therefore, this chimera of collective immunity currently appears to be out of reach.

Cross immunity: everything protected?

A recent Californian study, published in the journal Cell, evokes the hypothesis of cross immunity. This term designates an "acquired immunity against an infectious agent, which makes it possible to protect against another agent (virus or bacteria)", specifies the Futura science site. "Cross-immunity is linked to the phenomenon of cross-reaction. [...] Antibodies sometimes bind to nearby antigens" and protect against diseases similar to the first infection. In fact, a patient previously infected with a coronavirus, the virus responsible for 15 to 20% of colds, could be immunized against SARS-CoV-2. The authors estimated that between 40 and 60% of the population would be protected against Covid-19. "A significant part of the population may not be sensitive to the coronavirus, because non-specific antibodies to this virus can stop it," epidemiologist Laurent Toubiana told AFP. Another Berlin experiment, previously published on MedRxiv, had shown that 34% of study participants had cross immunity against covid-19.

These works are only in his sketches and an article available on the Vidal site suspects the scope of this protection. "Immunity against cold coronaviruses lasts for approximately 2 years and would be sufficient if the cross immunity observed by the Berlin and California teams is due to only one of the 4 SARS-CoV-2-favorable" windows of opportunity "cold coronaviruses that appear regularly "is described in the document. The authors of the article concluded: "If the two studies give reasons to expect 'natural' protection for part of the population, they in no way show that this cross-immunity can play a role in controlling the Covid-19 pandemic."

Is it protective immunity?

In addition to this cross-immunity problem, questions remain about a possible post-infectious immunity. The teams at the University Hospital of Strasbourg and the Pasteur Institute carried out a study to specify the immune response of people suffering from minor forms of infection with SARS-CoV-2. The researchers reported their findings in a previous publication available on the MedRxiv website. The methodology applied is as follows: the scientists carried out "immunodiagnostic" tests and rapid tests on 160 volunteers, members of the hospital staff of the two sites of the University Hospital of Strasbourg, infected with Covid-19 (and diagnosed positive by PCR ). The study found that the "rapid immunodiagnostic test detected antibodies in 153 (95.6%) of the samples and the S-Flow test in 159 (99.4%)". Furthermore, "neutralizing antibodies (NAb) were detected in 79%, 92% and 98% of the samples taken, respectively 13-20, 21-27 and 28-41 days after the onset of symptoms." "Our study shows that antibody levels are, in most cases, consistent with protection against reinfection with SARS-CoV-2, at least up to 40 days after the onset of signs. The goal now is to assess long-term persistence of the antibody response and its associated neutralization capacity in these healthcare personnel, "Timothée Bruel and Olivier Schwartz, respectively researcher and head of the Virus and Immunity Unit at the Pasteur Institute, said in a press release.

Previous experiments with rhesus macaques have shown that apes are protected from disease after a first infection. In Le Monde, Olivier Schwartz commented, "This does not invalidate reinfection, but the researchers noted a rapid increase in the level of antibodies to SARS-CoV-2, a faster clearance of the virus. These antibodies promote a benign and attenuated disease form. It is protective but not sterilizing immunity."

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