The surprise however came from the blood of the subjects never exposed to the virus . In 11 of the 20 samples there was the immune response (mediated by T cells) to Sars-CoV-2. The researchers thus deduced that an important portion of the Californian population was previously exposed to one of the four weakest human coronaviruses , plus a series of subtypes (which cause colds). Having encountered other coronaviruses would have generated at least partial immunity. A sort of cross reactivity due to exposure to other coronaviruses. The blood of these subjects also contained antibodies to two of these cold coronaviruses. It is not yet clear, however, whether the observed crossreactivity provides at least a pre-existing level of immunity to Sars-CoV-2. “Given the severity of the ongoing Covid-19 pandemic, any degree of cross-reactive immunity to coronavirus could have a substantial impact on the overall course of the pandemic and is a key detail to consider for epidemiologists as they try to understand how severely Covid- 19 will affect communities in the coming months, “says Shane Cotty , one of the researchers.

Children

“The research is very partial, only 20 people, but the result is certainly interesting and needs to be studied in depth” comments Fabrizio Pregliasco , virologist and medical director of the Galeazzi Hospital in Milan. ” It is possible that those who have had recent exposure to other coronaviruses that circulate in the winter have also obtained a more adequate immune response to Sars-CoV2 . Among the hypotheses put forward as to why children seem more protectedfrom coronavirus there is precisely the fact that they more often meet the colds caused by coronaviruses producing an immune response which, being more recent than adults, can also be more effective against Sars-CoV2 ». This does not mean that those who have suffered from coronavirus colds in recent times will not fall ill with Covid-19, but it is possible that if it happens the symptoms are milder. And this could also partly explain why some get sick more severely than others, net of previous pathologies. However, it is not known how long immunity lasts to milder coronaviruses.

It does not mean that people will not develop symptoms

Biologist Enrico Bucci , researcher in Biochemistry and Molecular Biology and professor at Temple University in Philadelphia, commented on Facebook : «Excellent news. Those exposed to the virus mount a robust immune response, which remains after the infection, type T “. Bocci underlines: «A part of subjects never exposed to the virus is“ pre-immunized ”, probably due to the encounter with other common coronaviruses. Now, however, before you uncork the champagne, here are the limits of this study and some other considerations: the sample is very small; the percentage of the population that can be “pre-immunized” is therefore not necessarily 50%, but could be much smaller (or larger); the fact that there are subjects whose T cells are able to recognize the virus, even though they have never been exposed to them, does not mean that those subjects will not develop symptoms , even if they may be weaker. Potentially, if there is cross-reactivity between coronaviruses, the serological epidemiology goes to be blessed, because, in addition to T cells, there could also be cross-reactive antibodies (the meaning of the tests changes, and becomes only immunological); the study should be repeated as soon as possible (on an even wider basis).