Seroprevalence of severe acute respiratory syndrome coronavirus 2 N antibodies between December 2021 and march 2023 in Japan.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged in 2019 in China and rapidly spread worldwide, leading to a pandemic. The threat of SARS-CoV-2 is subsiding as most people have acquired sufficient antibodies through vaccination and/or infection to prevent severe COVID-19. After the emergence of the omicron variants, the seroprevalence of antibodies against the N protein elicited by SARS-CoV-2 infection ranged from 44.4% to 80.2% in countries other than Japan. Here, we assessed the seroprevalence in Japan before and after the appearance of omicron variants. Serosurveillance of antibodies against N was conducted between December 2021 and March 2023 in Japan. In total, 7604 and 3354 residual serum or plasma samples were collected in the Tokyo metropolitan area and Sapporo, respectively. We found that the seroprevalence in representative regions of Japan increased approximately 3% to 23% after the emergence of the omicron variants. We also found higher seroprevalence among the young compared with the elderly. Our findings indicate that unlike other countries, most of the Japanese population has not been infected, raising the possibility of future SARS-CoV-2 epidemics in Japan.

Cellular and humoral immunogenicity against SARS-CoV-2 vaccination or infection is associated with the memory phenotype of T- and B-lymphocytes in adult allogeneic hematopoietic cell transplant recipients.

We conducted a cross-sectional study to evaluate cellular and humoral immunogenicity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination or infection and examine how lymphocyte subpopulations in peripheral blood correlate with cellular and humoral immunogenicity in adult allogeneic hematopoietic cell transplantation (HCT) recipients. The median period from SARS-CoV-2 vaccination or infection to sample collection was 110.5 days (range, 6-345 days). The median SARS-CoV-2 spike-specific antibody level was 1761 binding antibody units (BAU)/ml (range, 0 to > 11,360 BAU/ml). Enzyme-linked immunosorbent spot (ELISpot) assay of T cells stimulated with SARS-CoV-2 spike antigens showed that interferon-gamma (IFN-γ)-, interleukin-2 (IL-2)-, and IFN-γ + IL-2-producing T cells were present in 68.9%, 62.0%, and 56.8% of patients, respectively. The antibody level was significantly correlated with frequency of IL-2-producing T cells (P = 0.001) and IFN-γ + IL-2-producing T cells (P = 0.006) but not IFN-γ-producing T cells (P = 0.970). Absolute counts of CD8+ and CD4+ central memory T cells were higher in both IL-2- and IFN-γ + IL-2-producing cellular responders compared with non-responders. These data suggest that cellular and humoral immunogenicity against SARS-CoV-2 vaccination or infection is associated with the memory phenotype of T cells and B cells in adult allogeneic HCT recipients.