Virus-specific humoral immune response in Bulgarian COVID-19 patients with varying disease severity

Our study aimed to analyze virus-specific humoral immune responses in COVID-19 patients with varying disease severity.

A total of 109 serum samples from 87 patients, symptomatic for COVID-19 were studied using anti-SARS-CoV-2 immunoassays detecting different classes of immunoglobulins.

Clinical samples were divided into 2 groups - collected up to and more than 2 weeks post-onset of symptoms (PoS). In the first group, the highest percentage of positive samples was found for IgA class virus-specific antibodies (78.1%), followed by IgM (71.9%/59.4%) and IgG (56.3%/53.1%). In the second group, samples positive for virus-specific IgA class antibodies were also the most (97.7%) along with those positive for IgG. A total of 72 IgA and/or IgM and/or IgG positive samples were further tested for SARS-CoV-2 neutralizing antibodies (NAbs) - 89.1% and 100% of samples obtained up to and after 2 weeks PoS, respectively were positive. Serological test results were also analyzed depending on the severity of the disease - SARS-CoV-2 positive samples in mild forms of COVID-19 were fewer than in moderate and severe forms but this difference was not statistically significant.

SARS-CoV-2 specific antibodies and a high virus neutralization capacity of these antibodies appear early PoS; Immunoglobulins of IgA class are of most significant diagnostic value for detection of SARS-CoV-2 infection; Timing of testing is the most important factor for positivity rate.