Evaluation of direct and indirect effects of vaccines driven by an individual-based stochastic model
[Objective]Vaccination elicits a specific immune response in vaccinated individuals,resulting in direct effects such as preventing infection,reducing the severity of the disease after infection,and shortening the duration of transmission.In addition,mass vaccination can have indirect effects on unvaccinated people.In the evaluation of vaccines,randomized clinical trials can only assess the efficacy of vaccines under specific conditions and at the individual immunological level.Meanwhile,field trials can measure the effectiveness of vaccines in the real world but do not fully account for the indirect effects of vaccines on the possible impact of vaccination strategy.This study aims to enhance domestic vaccine effectiveness assessment strategies and provide a scientific basis for formulating immunization plans or selecting candidate vaccines.[Methods]This study employs an individual-based stochastic model based on the assumption of uniformly mixed populations,using the example of the Omicron variant of the SARS-CoV-2.It considers factors such as the natural history of the disease,contact patterns,vaccine coverage,and vaccine effectiveness.In each iteration,the close contacts of individualx are randomly generated,and the natural history status of the disease of all individuals is updated.The number of people in each natural history status and the number of new infections in the vaccinated and non-vaccinated groups at that time step are also calculated.The study extends the individual-level vaccine effects in preventing infection and transmission to direct and indirect effects at the population level.[Results]Simulation results discuss indexes in four scenarios,considering only the improvement of vaccine coverage(VC)or only the enhancement of vaccine effectiveness(VE).When VE is set at 0.5 and 0.9,respectively,increasing VC from 50％to 90％results in reductions of 31.1％and 72.1％in the peak daily infection rate of the population protected by indirect effects.These reductions are greater than those in scenarios focusing solely on improving VE.However,scenarios focusing solely on enhancing VE demonstrate direct effects that are superior in both cumulative and daily new infection rate compared to scenarios with only increased VC.Furthermore,overall vaccine effects are significantly influenced by direct effects.These results indicate that enhancing VE is more meaningful in terms of public health at the population level compared to increasing VC.However,the current effectiveness of SARS-CoV-2 vaccines in preventing infection and transmission is not ideal,emphasizing the continued importance of strengthening individual vaccination to increase vaccine coverage.Moreover,indirect effects increase with the rise in vaccine coverage,providing a certain level of protection for individuals within the population who are unable to receive the vaccine.Under the epidemiological parameter setting of the Omicron variant of SARS-CoV-2,even if the vaccine has a high-level effect against infection and transmission,the indirect effect of the vaccine is still not obvious.Assuming that more than 90％of susceptible individuals have received vaccinations,the vaccine's effectiveness against infection and transmission must be greater than 85％,or the effectiveness of one must be close to 90％and the other to 80％,in order to produce an indirect effect of more than 10％.[Conclusions]The unvaccinated group in the population with deployed vaccination programs showed a lower infection rate than those in which no one is vaccinated.Improving vaccine effectiveness can prevent more infections at the population level than simply increasing vaccine coverage.However,the current SARS-CoV-2 vaccine is not sufficiently effective in preventing infection and transmission,making it crucial to strengthen individual vaccination to improve vaccine coverage.The indirect effect increases with the rise in vaccine coverage,and the growth in the indirect effect will be more significant than that of the direct effect for the same proportional increase in coverage.This enhancement will provide certain protection for people in the population who cannot be vaccinated.
万方数据
维普
