Retrospective assessment of vaccination policy optimizations during an Omicron BA.2 epidemic in Hong Kong: A...

Suboptimal COVID-19 vaccine distribution hampers their potential disease protection impact. The 2022 BA.2 epidemic in Hong Kong, occurring over a year into vaccination efforts, resulted in one of the highest daily mortality rates of the pandemic.

We developed an age- and dose-structured SEIR model to describe the transmission dynamics in this epidemic. Then we fitted the model to severe disease data to estimate the transmission dynamics of the epidemics by Bayesian inference. After that, we conducted extensive counterfactual simulations based on the fitted transmission dynamics and considered varying age-specific vaccination coverages and timing to estimate the impact on severe disease across scenarios.

If individuals over 80 had primary series coverage similar to the adult population, 27.6 % (95 % posterior predictive interval [PPI]: 24.7 %, 31.1 %) of deaths could have been averted. High primary series coverage in 40-49-year-olds averted 32.6 % (95 % PPI: 30.0 %, 35.5 %) more hospitalisations. Advancing the adult or senior vaccination campaign by two weeks significantly reduced deaths by 498 (95 % PPI: 309, 688) and 601 (95 % PPI: 412, 792), respectively. Earlier vaccination of children had no significant impact on population-wide deaths.

Low elderly primary series coverage led to the most preventable deaths, while high coverage in middle-aged groups significantly averted excess hospitalisations. Our study underscores the importance of targeting COVID-19 vaccinations in high-risk and transmission-driving groups, as well as vaccination time-sensitivity, where each week difference, averts or abets morbidity and mortality.