Age dependent discrepancy between SARS-CoV-2 anti-nucleoprotein antibody and anti-RBD spike protein antibody in children reflects vaccine coverage

Background: COVID-19 has become widespread in Japanese children. However, the impact of varying immunization coverage on the seroprevalence of SARS-CoV-2 in children is unknown. Methods: We examined the SARS-CoV-2 antibody in children aged 0 to 18 who were hospitalized at a university hospital from June 2020 through May 2023. The SARS-CoV-2 anti-nucleoprotein (N) antibody and anti-RBD spike (S) protein antibody was measured. Results: A total of 586 cases were enrolled. The median age was 4 years old (interquartile range 1-9), and 362 (61.8 %) were male. The seroprevalence of anti-S antibodies gradually increased from October 2021 and reached 60 percent by early 2023. The anti-N antibody increased starting in January 2022 and reached 50 percent in May 2023. There was a discrepancy in the seroprevalence of anti-S and N antibodies in children 0 years of age or 12 years and older until the fall of 2022. This discrepancy was minimal for children 1-4 years of age and relatively small in the 5-11-year-old group. Discussion: The data suggests that approximately half of the children in our cohort had been infected with SARSCoV-2 by May 2023. The discrepancy in seropositivity between the anti-S and N antibodies corresponded to the reported vaccine uptake of each target age group, which suggested protective effects of immunization. However, this effect appeared to diminish after early 2023. Conclusion: Age dependent discrepancy between SARS-CoV-2 anti-N and anti-S antibody in children reflected differences in vaccine coverage.