SARS-CoV-2 infection- induced seroprevalence among children and associated risk factors during pre- and omicron-dominant wave, from January 2021 through November 2022, Thailand: Longitudinal study

Background Severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) infection can be asymptomatic in young children. Therefore, the true rate of infection is likely underestimated. Few data are available on the rate of infections in young children, and studies on the SARS-CoV-2 seroprevalence among children during omicron wave are limited. Our study aims to assess the SARS-CoV-2 infection-induced seroprevalence among children and estimated the associated risk factors for seropositivity. Methods A longitudinal serological survey was conducted from January 2021 through November 2022. Samples were tested for anti-nucleocapsid (N) IgG, anti-receptor binding domain (RBD) IgG using a chemiluminescent microparticle immunoassay (CMIA) and detected anti-RBD Immunoglobulin (Ig) using an electrochemiluminescence immunoassay (ECLIA). The vaccination and SARS-CoV-2 infection history were collected. Results A total of 452 serum samples were obtained from 249 children aged 5-7 years old who were annually followed-up in the longitudinal serological survey. Of these, 191 participants provided samples at two serial time points, including during the pre-and omicron dominant wave. Overall, seroprevalence induced by SARS-CoV-2 infection was increased from 9.1% (95%CI: 0.6-12.6%) during the pre-omicron wave to 49.7% (95%CI: 35.9-66.8%) during the omicron wave. Amongst seropositive individuals, the infection-induced seroprevalence was lower in vaccinated participants than those with no vaccination (40.4% vs. 57.4%; risk ratio, 0.71; 95%CI: 0.52-0.95). Nevertheless, the ratio of seropositive cases per recalled infection was 1.56 during the omicron dominant wave. In addition, overall seroprevalence induced by infection, vaccination and hybrid immunity was 76.6% (151/197; 95%CI: 54.6-97.9%) between January and November 2022. Conclusions our study reports an increase in infection-induced seroprevalence among children during the omicron wave. These findings highlight that estimating seroprevalence is crucial to monitor SARS-CoV-2 exposure, particularly in asymptomatic infection, and help to optimize public health policies and determine the effect of immunization in the pediatric population.