Haber M, Longini I M, Halloran M E
Department of Epidemiology and Biostatistics, Emory University, Atlanta, Georgia 30329.
Int J Epidemiol. 1991 Mar;20(1):300-10. doi: 10.1093/ije/20.1.300.
Vaccine efficacy in the field is often derived from the relative attack rates in the vaccinated and unvaccinated after an outbreak. In this paper, vaccine efficacy is defined in terms of the probability that the infectious agent is transmitted from an infected to a susceptible person, and a method for estimating it from the usual attack rate data is given. We explore two mechanisms of vaccine action defined by Smith et al, but include an underlying dynamic epidemic model of an acute directly transmitted disease. We show analytically that under the model in which the vaccine mechanism reduces the probability of infection given a certain exposure, vaccine efficacy based on the relative attack rates underestimates the protective effect of the vaccine based on the relative transmission probabilities. Under the other model in which the vaccine mechanism offers complete protection to a certain proportion of those vaccinated, and no protection to the other vaccinated proportion, the vaccine efficacy based on the relative attack rates will equal that based on the transmission probabilities. Parameters for the effectiveness of a vaccination programme are defined in terms of the direct and indirect benefit to a single person as well as the total and average benefit to the entire population, and derived from the dynamic model for an outbreak of an acute directly transmitted disease. These effects can also be estimated without an actual separate unvaccinated population, independent of assumptions about the vaccine mechanism. The variation of these measures as functions of the fraction of vaccinated people in the population is explored numerically.
疫苗在实际应用中的效力通常源于疫情爆发后接种疫苗者和未接种疫苗者的相对发病率。本文根据感染源从感染者传播至易感者的概率来定义疫苗效力，并给出了一种根据常见发病率数据估算疫苗效力的方法。我们探讨了史密斯等人定义的两种疫苗作用机制，但纳入了一种急性直接传播疾病的潜在动态流行模型。我们通过分析表明，在疫苗机制降低给定暴露情况下感染概率的模型中，基于相对发病率的疫苗效力低估了基于相对传播概率的疫苗保护效果。在另一种模型中，疫苗机制为一定比例的接种者提供完全保护，而对另一部分接种者不提供保护，基于相对发病率的疫苗效力将等于基于传播概率的疫苗效力。疫苗接种计划有效性的参数是根据对个体的直接和间接益处以及对整个人口的总体和平均益处来定义的，并从急性直接传播疾病爆发的动态模型中推导得出。这些影响也可以在没有实际单独的未接种人群的情况下进行估算，且与关于疫苗机制的假设无关。我们通过数值方法探讨了这些指标随人群中接种者比例的变化情况。
