Assessment of age-dependent effects during the transmission of Omicron and the outcomes of booster campaign vaccination strategies

At the end of 2021, the Omicron variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) took over from the preceding Delta variant as the dominant strain, causing a huge wave of infection and death due to its high transmissibility and immune escape. The Omicron wave was the fifth of the pandemic in Hong Kong Special Administrative Region, China. Given the huge variability in the mortality and vaccination rates associated with coronavirus disease (COVID-19) across age groups in the fifth wave, we constructed a two-age-group transmission model with time-varying transmission, vaccination, and death rates to assess the dynamics of the Omicron variant in two age groups (a younger group aged 0-59 years and a group of older adults aged ≥ 60 years). We calibrated the model to eight sets of epidemic data from Hong Kong using the Markov Chain Monte Carlo algorithm to disentangle the transmission degree and disease severity for the two age groups. The calibrated model was then used to explore counterfactual scenarios in which the booster vaccination campaign was modified to assess the direct and indirect effects of booster doses in the two age groups. We also performed sensitivity analyses by varying key model parameters so as to evaluate the uncertainty of some parameters on the epidemic trend. The results indicated a high degree of assortativity (intra-group contact) in the two age groups, and the intra-group transmission rate was much higher than the inter-group transmission rate. Younger individuals showed a slightly higher transmission advantage in the fifth of the pandemic in Hong Kong. Moreover, the average mortality rate in the older group was 87 times higher than that in the younger group. We confirmed that administering booster doses of vaccines offered substantial protection to the target population, particularly against death. There are vast infection-suppression benefits of vaccinating younger individuals that play an important role in transmission. Meanwhile, vaccination in the older individuals has vast benefits in reducing deaths. Vaccination campaigns targeting a particular age group are far less effective than boosting wider segments of the population. This study provides a retrospective understanding of Hong Kong's fifth wave of the pandemic, which is crucial for future projections and the planning of interventions.