Postinfectious and Postvaccinal Humoral Immune Response to SARS-CoV-2 in Adults: Qualitative and Quantitative Assessment

Relevance. Serological studies play an important role in assessing the spread of SARS-CoV-2 and formation of post-infection and post-vaccination immune response.The Aims aim of the study was a comparative assessment of seroprevalence and concentration of antibodies to SARS-CoV-2 at 3–6 months after infection or vaccination.Materials & Methods. For the presence of total IgM and IgG antibodies to RBD fragment of the SARS-CoV-2 S protein, the blood sera of 1331 people aged 18-70 years were examined, of which 334 had COVID-19 3–6 months before the study, 305 did not have COVID-19 and were vaccinated (using Sputnik V, Russia, or Sinopharm, PRC) 3–6 months before the study, 692 people were not ill and were not vaccinated. A quantitative assessment of IgG antibodies to the full-size S-protein of SARS-CoV-2 was performed for 435 samples.Results. The proportion of seropositive individuals 3–6 months after COVID-19 or after vaccination did not differ: 92.5% and 93.8% of the corresponding group had antibodies. Among the non-ill and unvaccinated, 45.7% also had specific antibodies, which indicates a high level of asymptomatic infection with SARS-CoV-2. The group of vaccinated was also examined immediately before the introduction of the vaccine, and 39.7% of them already had specific antibodies due to asymptomatic infection with SARS-CoV-2. The median concentration of antibodies in the group of COVID-19 was statistically significantly higher than in asymptomatically infected (50.9 AE/ml versus 29.1 AE/ml). The largest proportion of seropositive individuals (100.0%) and the highest concentration of antibodies (110 AE/ml) were detected in the group of vaccinated who had pre-existing antibodies.Conclusion. Infection with the SARS-CoV-2 is characterized by a high frequency of subclinical course. Asymptomatic infection induced a weaker immune response compared to symptomatic COVID-19 or vaccination. Hybrid immunity caused by natural infection with SARS-CoV-2 and subsequent vaccination was the most persistent and pronounced.