Publications

This research has not been peer-reviewed. It is a preliminary report that should not be regarded as conclusive, guide clinical practice or health-related behaviour, or be reported in news media as established information.

Background 

Vaccines have reduced severe disease and death from COVID-19. However, with evidence of waning efficacy coupled with continued evolution of the virus, health programmes need to evaluate the requirement for regular booster doses, considering their impact and cost-effectiveness in the face of ongoing transmission and substantial infection-induced immunity.

Methods and findings 

We developed a combined immunological-transmission model parameterised with data on transmissibility, severity, and vaccine effectiveness. We simulated SARS-CoV-2 transmission and vaccine rollout in characteristic global settings with different population age-structures, contact patterns, health system capacities, prior transmission, and vaccine uptake. We quantified the impact of future vaccine booster dose strategies with both original and variant-adapted vaccine products, in the presence of both continuing transmission of Omicron subvariants and considering the potential future emergence of new variants with modified transmission, immune escape, and severity properties. We found that regular boosting of the oldest age group (75+) is the most efficient strategy, although large numbers of hospitalisations and deaths can be averted by extending vaccination to younger age groups. In countries with low vaccine coverage and high infection-derived immunity, boosting older at-risk groups is more effective than continuing primary vaccination into younger ages. These findings hold if even if virus drift results in a gradual reduction in vaccine effectiveness over time due to immune escape. In a worst-case scenario where a new variant emerges that is 10% more transmissible, as severe as Delta, and exhibits substantial further immune escape, demand on health services could be similar to that experienced during 2020.

Conclusions 

Regular boosting of the high-risk population remains an important tool to reduce morbidity and mortality from current and future SARS-CoV-2 variants. The cost-effectiveness of boosting is difficult to assess given the ongoing uncertainty in the likelihood of future variants and their properties but focusing vaccination in the highest-risk cohorts remains the most efficient strategy to reduce morbidity and mortality.