Publication date: May 29, 2025

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. 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. 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. 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.

Semantics

Original Article