SARS-CoV-2 Ancestral and Omicron variant immunity in Australian children in 2023, a seroprevalence study

The emergence of the SARS-CoV-2 Omicron variant in late 2021 led to widespread infection in Australian children, many unvaccinated. The breadth and magnitude of antibody responses to SARS-CoV-2 variant infections and vaccination in children remain incompletely understood. We aimed to estimate seroprevalence against SARS-CoV-2 spike (S) and nucleocapsid (N) antigens and explore antibody kinetics, focusing on Omicron subvariants.

Between 1 November and 7 December 2023, blood samples were collected from children aged 0-16 years undergoing anaesthetic procedures at eight tertiary paediatric hospitals. SARS-CoV-2 antibodies to ancestral spike and nucleocapsid protein and eight Omicron spike proteins were tested. Crude and adjusted seroprevalence estimates derived using multilevel regression and poststratification were obtained. Geometric mean concentrations (GMCs) were calculated for S- and N- antibodies and analysed by age and vaccination status.

Of 1065 children tested, adjusted seroprevalence to ancestral S-antibody was 93.1% (95% CrI, 91.4-94.5) and N-antibody was 79.8% (77.1-82.2). Seroprevalence was high across jurisdictions, socioeconomic quintiles, and remoteness categories, and lowest in infants aged 6-11 months (S-antibody, 69.5% [95% CrI: 54.7-81.8] and N-antibody 35.2% [95% CrI: 21.1-49.1]). GMCs to nine distinct SARS-CoV-2 spike variants were uniformly high and increased with age and vaccination. GMC to ancestral S-antibody was highest in two-dose vaccinated children while unvaccinated children aged >1 year had highest GMC to Omicron BA.5.

By late 2023, nearly all participants had serological evidence of SARS-CoV-2 infection, with highest concentrations to ancestral strain and Omicron BA.5 spike proteins, suggesting variant-specific immune imprinting.