Seroprevalence and protective effect of SARS-CoV-2 immunity against symptomatic SARS-CoV-2 infection among children and adolescents in Nakhon Phanom province, Thailand 2024 – 2025

Abstract

Background :  The Omicron variants have altered the epidemiology of COVID-19, particularly among children and adolescent , who commonly experience mild or asymptomatic infection but may still contribute to community transmission. Understanding population-level immunity in this age group is important for guiding public health strategies.This study aimed to describe the seroprevalence of SARS-CoV-2 antibodies and to to assess the association between baseline seropositivity and the risk of symptomatic SARS-CoV-2 infection  among children and adolescents in Nakhon Phanom province, Thailand. Methods : A population-based prospective cohort study was conducted aming children and adolescents aged 6 months to 17 years using stratified random sampling by age group. A total  of 999  participants were enrolled.  Blood samples were collected at enrollement, 6 months, and 12 months to measure anti-S and anti-N SARS-CoV-2 IgG antibodies using ELISA.. Participants were followed weekly for COVID-19–like illness, and symptomatic infections were confirmed by multiplex reverse transcription polymerase chain reaction (multiplex RT-PCR). Cox proportional hazards regression was used to evaluate the association between baseline anti-S IgG seropositivity and the risk of symptomatic SARS-CoV-2 infection, adjusting for relevant confounders. Results :  The mean age of participants was 8.1 years and 53.3% had received at least one dose of COVID-19 vaccine. Anti-S IgG seropositivity exceeded 80% in all age throughout the 12-month follow-up. During follow-up, 439 episodes of COVID-like illness were reported, of which 32 were confirmed SARS-CoV-2 infections. Baseline anti-S IgG seropositivity was associated with a lower risk of symptomatic SARS-CoV-2 infection (Hazard Ratio: 0.64; 95% confidence interval 0.23–1.81), corresponding to a protective effect of 36%, although the association was notstatistical significance. Conclusions  :   Children and adolescents in Nakhon Phanom province exhibited a high seroprevalence of SARS-CoV-2 antibodies throughout the study period. Baseline seropositivity showed a non-significant trend toward reduced risk ofsymptomatic SARS-CoV-2 infection. These findings highlight the importance ofcontinued surveillance and targeted vaccination strategies focusing on high-risk pediatric and adolescentspopulations.