ABSTRACT
Monitoring population protective immunity against SARS-CoV-2 variants is critical for risk assessment. In this serosurveillance study, older adults show much lower seropositive rates of neutralizing antibody (NAb) against ancestral virus than the younger population. The increase in NAb seopositive rate generally follows the population vaccination uptake rate, but older adults have a much lower NAb seropositive rate than vaccination uptake rate. For all age groups, the seropositive rates of NAb against Omicron variant are much lower than those against the ancestral virus. During the fifth wave of COVID-19 in Hong Kong which is dominated by Omicron sublineage BA.2, the case-fatality rate is exceptionally high in the ≥80 year-old age group (9.2%). Our study suggests that the severe BA.2 outbreak in Hong Kong can be attributed by the lack of protective immunity in the population, especially among the vulnerable older adults, and highlights the importance of continual surveillance of protective immunity against emerging variants of SARS-CoV-2.