Impact of Population Immunity and Public Health Measures on the Transmission of Omicron Subvariants BA.2 and BA.5 in Hong Kong

Abstract Background The rapid evolution of SARS-CoV-2 and population-level vaccine administration significantly shift the population immunity. In Hong Kong, these shifts, coupled with the emergence of Omicron BA.5 with a strong ability of immune evasion, necessitate a deeper understanding of how population immunity and public health and social measures have shaped the epidemic dynamics across age groups within the population. Methods We developed an age-structured, multi-strain model and estimated key parameters including transmissibility for the emerging BA.5, the effects of PHSMs on transmission, and contributions of natural infection and vaccination to age-specific immunity against infection of each subvariant over time. Results We found that reactive PHSMs implemented in February 2022 decreased the time-varying effective reproductive number without the effect of immunity RtWI by 67% (95% CrI: 52–78%). However, subsequent relaxation of control measures since April 2022, alongside the enhanced transmissibility of BA.5, drove RtWI back to 3.4 (95% CrI: 2.8–4.1) by late May. Prior to the fifth wave, only 15% of the Hong Kong population had immunity and protected against BA.2 infection. Population immunity against BA.2 infection then increased significantly to 55% within 2 months given 47% cumulative infections and > 30% vaccination uptake. Subsequently, with the emergence of BA.5, population immunity against BA.5 infection was 15% lower than that against BA.2 during the end of May. Conclusions Our findings underscore the dynamic interplay between population immunity, PHSMs, and variant transmissibility and highlight the potential risks posed by immune-evasive variants in the context of waning immunity and control relaxation.