Time course of the antibodies response in COVID-19

Background The role of the immune response to SARSCoV-2 infection is not yet well known, in particular about the persistence of circulating antibodies The aim of the study is to assess the time course of the IgM and IgG response Methods We conducted a study on 123 specimens of 62 subjects with disease confirmed by laboratory tests (time from the onset of symptoms from 3 to 110 days) Both IgM and IgG were measured with a chemiluminescence immunoassay (MAGLUMI 2019-nCoV IgM and MAGLUMI 2019-nCoV IgG respectively) on the Maglumi 800 Analyzer (Snibe, Shenzen, China) according to the manufacturer's instructions Results The case study was subdivided into 8 groups (≤10 days from the onset of symptoms, 11-13 days, 14-17, 18-21, 22-28, 30-43, 46-71 and 72-110) The clinical sensitivity of IgG rose from 52 9% to 100% after 18 days, and then remained constant The sensitivity of IgM rose from 35 3% up to about 78 6% at 20 days, then fell to about 27% after 80 days Considering the concentrations in the samples, after a rapid increase up to about 20 days, we can see a subsequent decrease of the mean concentrations of both IgM and IgG levels The quantitative comparison of the different classes showed significantly higher IgG concentrations in the days from 14 to 28 compared to both the first two groups and the groups after 46 days (Kruskall-Wallis test p=0 000001) As for the IgM, the trend is similar to that found for IgG, but the differences between groups were lower (Kruskall-Wallis test p=0 0022) and the absolute concentrations were about one order of magnitude smaller Discussion The persistence of antibodies against SARS-CoV-2 is not known Studies on the immune response to other coronaviruses showed that the concentrations of IgG decline after few months from the onset of symptoms, although the positivity rate remained relatively stable over a longer period In the present study, the results seems to confirm this trend In fact, although the positivity rates did not fall below 100%, in the class with specimens from 72 to 110 days after the onset of symptoms the median IgG concentrations were less than 10% compared to the levels found after 15-20 days Conclusion The antibodies titre in patients exposed to COVID-19 may decrease already after 6-7 weeks after the symptoms' onset