Age priority in continuous coronavirus disease 2019 booster doses under China's new policy of free-will nucleic acid test: A dynamic model-based effectiveness and cost-effectiveness analysis

Background: The initial mass vaccination's effectiveness has diminished, necessitating accelerated immunization coverage scaling. China has shifted nucleic acid testing from large-scale to voluntary. This study assesses the effectiveness and cost-effectiveness of different booster vaccination strategies in China. Methods: A dynamic transmission model divided the population into three groups: 0-19, 20-59, and 60+ years. We evaluated the effectiveness and cost-effectiveness of three vaccination strategies based on previous studies and public databases. Three scenarios were modeled and compared to no-continuation-vaccination to calculate averted diseases, deaths, and net benefits. One-way sensitivity analysis and probabilistic sensitivity analysis assessed findings' stability. Results: COVID-19 vaccination had significant health benefits compared to no continuing vaccination. Strategy II (prioritizing vaccinated 20-59-year-olds, then vaccinated 60+ individuals, and finally 0-19-year-olds) was the most cost-effective. Strategy I (prioritizing unvaccinated 60+ individuals, then 20-59, and finally 0-19) prevented the most deaths. Strategy II was the most cost-effective, with a total cost of 93,995,223,462 USD and the highest net benefit of 3,054,475,908,551,960 USD. Strategy II resulted in the highest number of avoided cases across categories, including infected, asymptomatic, mild/moderate, severe, and critical cases. Each strategy's effects on preventing new cases and critical illness were comparable. Sensitivity analyses confirmed the results' reliability. Conclusion: Prioritizing vaccinated 20-59-year-olds, then vaccinated 60+ individuals, and finally 0-19-year-olds was the most effective prevention strategy. The vaccination strategy should be tailored to the pandemic situation and available medical resources for maximum health gains.