疫苗环形接种阻断猴痘传播的可行性建模

Modeling on availability of blocking monkeypox transmission by ring vaccination strategy

[目的]探讨在男-男性行为(men sex with men,MSM)人群中开展环形接种从而阻断猴痘传播的可行性.[方法]将人群接触区分为固定与非固定接触两部分,建立描述环形接种的常微分方程猴痘传播数学模型.进而基于该模型的数值模拟,评估并探讨环形接种在群体层面的防传播效果,以及接种过程中涉及的若干环节实施力度的影响.[结果]模拟显示,在基线场景的200 d传播模拟中,仅追踪接种80%和90%密接的环形接种方案可以分别使人群中平均每7.00和9.18 d产生一个病例,二者均大于病例的实际传染期,意味着发生传播阻断.密接追踪比例α1≥0.5时,继续提高α1可以减少疫苗消耗,提高有限疫苗资源的利用率.额外针对次密接的追踪接种将消耗与大规模接种类似的大量疫苗资源,效益较低,仅适用于疫情传播早期病例数极少的情形.[结论]对于猴痘这种自限性疾病,仅针对密切接触者的追踪接种可以很好地控制猴痘传播,同时对局部暴发具有较好的控制效果.然而为进一步减少聚集性暴发带来的额外负担,仍应预先提高MSM人群的疫苗覆盖率.

[Objective]Ring vaccination is a temporary additional vaccination strategy targeted on individuals who contacted a patient(close contact)and individuals who contacted a close contact(sub-close contact),playing a significant role in responding to emerging infectious diseases.In the current monkey pox(Mpox)transmission,ring vaccination within the high-risk population of men sex with men(MSM)is more widely accepted than massive vaccination,potentially offering better effectiveness and cost-efficiency.This study aims to clarify the differences between fixed and unfixed contact,assess the availability of block Mpox transmission through ring vaccination,and evaluate the influences of the intensity of contact tracing.[Methods]The entire set of MSM population is divided into subsets of 4 stages of the disease times 2 vaccination statuses.A basic framework of ordinary differential equations(ODEs)is used to model Mpox transmission,based on the Mpox natural history of susceptible-exposed-infectious-removed(SEIR),from a population average perspective.The individual contact frequency is divided into two fixed and unfixed components.Several terms corresponding to vaccination through tracing of close contact,sub-close contact,and infection,are modeled for both fixed and unfixed contacts,and incorporated into the ODEs.The developed differential-integral equations are numerically solved using the Euler discretization schema with a constant step size of 0.1 d.[Results]Simulations of the baseline scenario without other interventions show that tracing and vaccinating close contacts at 80%and 90%effectiveness produce an average incidence rate of one case per 7.00 d and 9.18 d,respectively.With both durations of time longer than the actual infectious period,it reflects the block of transmission.If the proportion of close contact tracingα1≥0.5,increasingα1 further reduces the consumption of vaccination doses,thus leading to greater effectiveness in the use of finite vaccine resources.Further tracing and vaccinating the sub-close contacts require massive vaccine resources similar to those needed for massive vaccination strategy,and is ineffective at reducing transmission,suggesting its inefficiency and limited utility in early stage of outbreaks.The numerical algorithm is stable,as evidenced by the convergence of cumulative cases when reducing the time-step size.All parameters show reasonable sensitivity against four objective indices:the cumulative number of infectious individuals,the maximum incidence speed,the maximum number of real-time cases,and the expenditure of vaccine doses.The combination and synergy of parameters show that all objective indices,except for the vaccine doses,are more sensitive to close contact vaccination parameters than to those for sub-close vaccination.The individual vaccine efficacy in reducing infectious is three times more sensitive than the efficacy in reducing susceptibility across all objective indices.The utilization of vaccine doses increase significantly when sub-close contact vaccination is further introduced in the basic close contact vaccination,while the cumulative infection decreased slightly.[Conclusions]Fixed and unfixed contact play different roles in transmission and intervention,requiring distinct approaches when evaluating interventions involving contact tracing.Taking advantage of a long infectious period and a low number of reproductions,which results in a relatively slow transmission speed,the strategy of vaccinating only in close contacts is theoretically sufficient to contain Mpox transmission and manage local outbreaks effectively.To further reduce the burden of the disease in outbreaks involving group gatherings,increasing vaccine coverage in advance remains valuable.The modeled differential equations provide a simple and quick assessment of ring vaccination,considering types of contact and enabling the establishment of optimization problems for specific demands.