Analysis of neutralization antibodies in patients with mild COVID-19 infection after 100 days using microneutralization test

Neutralizing antibodies play a critical role in blocking viral infections and in viral clearance during acute infection. The microneutralization assay and enzyme-linked immunosorbent assay (ELISA) targeting the receptor binding domain were performed for 30 patients with mild coronavirus disease (COVID)-19 infections. The elapsed number of days between sample collection and diagnosis was 115 days, and real-time polymerase chain reaction (PCR) cycle threshold (Ct) values at diagnosis were recorded. Clinical characteristics and Ct values were compared between neutralization antibody-positive and -negative patients as measured by the microneutralization assay. Neutralization antibody-positive patients (n = 9) were likely to be older, have low Ct values, have more pneumonia during admission, and have a higher optical density in ELISA than the neutralization antibody-negative patients (n = 21). Elderly people seemed to have a higher viral load causing more pneumonia and to produce more neutralization antibodies. Neutralization antibodies persisted in only 30% of patients as detected by microneutralization test after 100 days of diagnosis.