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Protection from Covid-19 – immunity from the background – knowledge

Even after half a year of the corona pandemic, many questions are still unanswered. It is therefore unclear why some people hardly notice a Sars-CoV-2 infection while others become seriously ill or die. Age and previous illnesses play an important role, but these factors do not explain why some athletic 50-year-olds experience severe disease while others of the same age do not even notice that they are infected.

Immunologists from the USA show in the specialist magazine Sciencethat there may be some form of background immunity that partially or completely protects some people from severe forms of Covid-19. Infection experts headed by Jose Mateus from the research institute in La Jolla and the University of San Diego suspect that the immune system of 20 to 50 percent of the population could be prepared for the pathogen. They have specific cells in the immune system that render Sars-CoV-2 harmless. Whether someone becomes seriously ill, has a mild course or even remains asymptomatic, could depend on two factors: the viral load, i.e. the amount of pathogens ingested, and the immune status – that is, how well the immune system is prepared for Sars-CoV-2.

The researchers had examined blood samples from subjects from 2019, from whom it was clear that they had no contact with the novel coronavirus. They also identified 142 fragments of the pathogen that the immune system recognizes as foreign. It was shown that so-called T cells, which make up an important part of the cellular immune response, reacted strongly to the virus components. T cells are able to attach to infected cells and destroy them. This immune response does not depend on the presence of antibodies.

The vaccine search is focused on the spike protein – but the virus offers more target

The laboratory tests showed that there are T cells in the blood of 20 to 50 percent of the test subjects that can destroy Sars-CoV-2. The scientists explain this phenomenon with the fact that some of the people previously had contact with cold pathogens from the corona virus group. During an infection with Sars-CoV-2, the T cells cross-react. Prepared by contact with these harmless cold viruses, they are able to keep the novel coronavirus in check.

Among the hundreds of viruses that can cause a flu or a cold, four well-known representatives from the Corona group that are similar to Sars-CoV-2, abbreviated as HCoV-OC43, HCoV-229E, HCoV-NL63 and HCoV, come from -HKU1. Those who have been infected with these viruses therefore have a certain background immunity. The response of protective T cells to harmless coronaviruses and to Sars-CoV-2 is similar, according to the study. “We could see that numerous T cells that react to components of Sars-CoV-2 also cross-react to the usual circulating coronaviruses that cause colds,” the authors said. “This could explain why patients with Covid-19 have such different courses.”

The analysis in Science also allows important conclusions to be drawn for vaccine development. Of the 142 fragments of the virus examined, on which the T cells rushed in their defense work, not only was the spike protein an attractive target, but also other components of the pathogen. Almost all efforts to find vaccines against Sars-CoV-2, however, have so far concentrated on the spike and thus the binding site through which the virus enters cells. Apparently there are other worthwhile targets to attack Sars-CoV-2.

The concept of background immunity through contact with cold viruses was already discussed in April by Berlin scientists around Andreas Thiel and Christian Drosten and published on a preprint server. The corresponding investigation was published in the specialist magazine at the end of July Nature published. The researchers found helpful T cells in response to infection in 83 percent of subjects with Covid-19. Among the subjects who were healthy and demonstrably had no contact with Sars-CoV-2, at least 35 percent had cross-reacting T cells, which are presumably due to a confrontation with the cold viruses from the Corona family. “These viruses make up about 20 percent of all common colds, are very common and mainly occur in winter,” writes the team around Thiel and Drosten. “Every two or three years, adults become infected with it on average. While the antibodies may disappear in the medium term, cellular immunity could remain.”

This conclusion and the findings from Nature and Science also show that it would be too easy to focus only on the antibodies in disease control. While this so-called humoral immunity is of unclear duration because it is unknown how long the antibodies circulate in the blood, the cellular immunity mediated by T cells is at least as important – and more stable in the long term. This fits an observation made by immunologists at the University of Iowa. They were able to show that after the outbreak of Sars-CoV-1 in 2002, after the outbreak of Mers from 2012 and probably also in the current case of Sars-CoV-2, the formation of antibodies fluctuated – the T-cell response however remained largely constant.

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