Decline of humoral immune responses after natural SARS-CoV-2 infection can be efficiently reversed by vaccination.

Our aim was to analyze severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific antibody level kinetics after coronavirus disease 2019 (COVID-19) infection and determine the efficiency of vaccination on SARS-CoV-2-specific antibody levels. The study included 50 SARS-CoV-2 infected and 70 uninfected cases. Levels of SARS-CoV-2-specific IgG nucleocapsid protein (IgG-NP), IgG spike protein (IgG-SP), IgM nucleocapsid protein (IgM-NP), and IgA spike protein (IgA-SP) antibodies were evaluated by an enzyme-linked immunosorbent assay in sera obtained at baseline, 1st, 3rd, and 6th month follow-up visits for infected cases and at postvaccination visits for all cases. In symptomatic cases ( = 50), IgG-SP levels were decreased in 6 months compared with baseline, while IgA-SP levels were significantly increased. IgG-NP levels were significantly decreased in symptomatic cases at the 6-month visit. After vaccination, IgG-SP levels were increased in symptomatic cases compared with prevaccination levels. Among subjects vaccinated with CoronaVac (the Sinovac COVID-19 vaccine), infected cases had approximately double the IgG-SP level of uninfected cases. SARS-CoV-2-specific antibody levels were higher at the baseline in symptomatic cases. Nevertheless, all infected cases showed significantly reduced IgG-SP levels at the 6th month. Vaccination effectively increased IgG-SP levels.