Cold and Pre – Existing Immunity
Among us there could be people with a sort of “pre-immunity” to Sars-CoV-2 without ever having come into contact with the virus . This is probably due to the immunization generated by other lighter coronaviruses , such as that of the cold . The hypothesis was advanced by a study by a team of Californian researchers from the Center for Infectious Disease and Vaccine Research at La Jolla Institute for Immunology , published in the journal Cell. In the research, the scientists analyzed the blood of a small population of convalescent patients (20 people) and subjects never exposed to the virus (20 others). The blood of those not exposed to the virus had been collected between 2015 and 2018.
The immune response
In convalescent subjects, a robust immune response to the virus has been confirmed: the immune system is able to recognize Sars-CoV 2 in many ways and is good news for the future development of a vaccine. The researchers tested over 3,000 virus fragments to determine which ones are recognized by the human immune system and focused on a type of immune system cell called T cells , and found that 100% of the convalescents expressed T cells. These cells help B cells make antibodies. In addition, convalescents also had antibodies against many of the Sars-CoV-2 proteins, while 70% had other T cells, which intervene in the direct destruction of the cells infected with the virus.
Who has never been exposed to the virus
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.
“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).