We investigated humoral immune responses in 222 unvaccinated Japanese people after recovery from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in 2021. Anti-spike-protein IgG antibody levels and neutralizing antibody titers were measured in serum samples obtained within 20-180 days after diagnosis. The geometric mean of antibody titers was 1555 ELU/mL (95% confidence interval (CI) = 1257-1923), and the neutralizing activity (50% inhibitory dilution) was 253 (95% CI = 204-313). The antibody titer and neutralizing activity both increased with increasing disease severity, and both values were approximately fourfold higher for hospitalized patients than for non-hospitalized patients. However, these differences were smaller in older patients. The humoral immune response, which increased with increasing disease severity, gradually decreased over time after SARS-CoV-2 infection. Most patients with mild or moderate symptoms sustained neutralizing activity for up to 180 days after the infection; the decay of the neutralizing activity in the asymptomatic patients was rather faster than in the other groups. Around 11.7% (26/222) of patients had very low neutralizing activity, and half of these were aged in their 20s. Our study's results show the importance of measuring the neutralizing activity to confirm the immune status and to estimate the timing of vaccines.
Keywords: COVID-19; SARS-CoV-2; convalescent serum; neutralizing antibody.