Development of seropositivity to SARS-CoV-2 over the course of the COVID-19 pandemic in adolescents in a longitudinal cohort study in Cebu, Philippines.

COVID-19 was first reported in the Philippines in January 2020 and the country experienced several COVID-19 waves. Little is known on the seroprevalence of SARS-CoV-2 among adolescents in a developing country such as the Philippines. In this study, we incorporated testing for acute COVID-19 and SARS-CoV-2 seropositivity in an ongoing dengue study of 9- to 14-year-old participants enrolled in 2017. Acute febrile episodes were detected through active surveillance and a nasopharyngeal swab was obtained for SARS-CoV-2 RT-PCR. Before COVID-19 vaccination was offered, we tested a subset of sera obtained in 2021 for SARS-CoV-2 receptor binding domain (RBD) antibodies. Development of SARS-CoV-2 seropositivity acquired through natural infection and vaccination was conducted by measuring antibodies pre- (2018-2019), early- (2019-2020), and late-pandemic (2022-2023) in those with matched samples. From July 2021-October 2022, we recorded 29 acute febrile illness episodes in the cohort and 17.0% were SARS-CoV-2 RT-PCR positive. In the subset of 499 samples tested for SARS-CoV-2 RBD antibodies in 2021, 30.3% were seropositive. Among these, 4.0% were seropositive in 2018-2019, and 11.0% had seroconverted by 2020. Interestingly, 74.0% experienced antibody waning between 2020 and 2021 sampling. By 2022, 345/499 children received full COVID-19 vaccination and 13/499 was partially vaccinated, with 96.6% SARS-CoV-2 seropositive by late-pandemic sample in 2022-2023. Vaccination and site were predictors of SARS-CoV-2 RBD antibodies. These findings suggest both variation in local transmission over time as well as association of vaccination with the development of SARS-CoV-2 immunity in adolescents in the Philippines.