Changes in Population Immunity to Omicron SARS-CoV-2 Variants and Selected Sarbecoviruses From 2020 to 2023 in Urban Colombo, Sri Lanka.

To understand how the population immunity evolved over time and possible susceptibility of the Sri Lankan population to emerging SARS-CoV-2 variants, we proceeded to evaluate the changes in antibody positivity rates to omicron variants BA.2.75 and XBB.1.5 and for selected sarbecoviruses. The haemagglutination test was carried out to determine the presence of antibodies against the RBD of the SARS-CoV-2 omicron variants XBB.1.5 and BA.2.75 and the RBD of the sarbecoviruses RaTG13, WIV1, Khosta-2, and SARS-CoV-1, in individuals aged 5-80 years of age in years 2020 (n = 381), 2022 (n = 432), and 2023 (n = 382). The highest positivity rates for BA.2.75, RaTG13, WIV1, Khosta-2, and SARS-CoV-1 were seen in 2022, with positivity rates significantly declining for many of the viruses except XBB.1.5 and Khosta-2 by 2023. The positivity rates for Khosta-2 (p < 0.001) and WIVI (p < 0.001) were significantly lower in children < 14 years age, but not for XBB.1.5, BA.2.75, and RaTG13. Children < 14 years who were SARS-CoV-2 unvaccinated had the lowest positivity rates for all tested viruses except BA.2.75. Less than 20% of individuals in all age groups had antibody titers equivalent to 1:80, which corresponds to neutralizing antibody titers by 2023. Population immunity to omicron SARS-CoV-2 variants and selected sarbecoviruses had significantly declined in Colombo, Sri Lanka by 2023. Therefore, although T cells might still offer some protection against severe disease, immunizing vulnerable individuals in the community with protective vaccine designs might be important to consider at this stage.