COVID-19 is caused by the SARS-CoV-2 virus, and the severity of the illness it brings about varies hugely, from asymptomatic cases to very serious cases that can lead to death. The impact of this variability on the development of immune protection as the illness progresses is still largely unknown.
Studies on vaccines currently in development have shown that protection against SARS-CoV-2 correlates with high neutralizing antibody levels. On the other hand, as regards the eradication of the virus, the role of neutralizing antibodies is less clear; it seems that cellular immunity is essential. In certain cases, antibodies may cause an aggravation of the illness by facilitating entry of the virus into cells, or by increasing inflammation. Death through COVID-19 is mainly due to respiratory distress syndromes that result from hyper-inflammation. However, this fact is not sufficient to explain severe cases of infection.
A study was carried out in Boston on 113 patients infected by SARS-CoV-2 and exhibiting a large range of symptoms. Researchers first of all investigated levels of antibodies targeting the spike protein. Virus neutralization tests using serum from infected patients confirmed the effectiveness of the neutralization capacities of antibodies: the more severely ill the patients, the more antibodies they possessed against the spike protein. In serious cases high levels of IgGs and IgAs (2 families of anitbodies) were observed, with significant levels of viral particle neutralization.
So scientists were able to create an index of neutralization potential, taking into account the result of the neutralization test and the quantity of IgGs. They noticed that in critically ill patients, neutralization potential was severely diminished. Overall, these results show that antibody virus neutralization potential allows stratification of risks in infected patients: the severity of the illness correlates with high levels of antibodies targeting the spike protein, but also with a weaker neutralization potential, a factor in mortality.