Aguas Ricardo, Gonçalves Guilherme, Gomes M Gabriela M
Instituto Gulbenkian de Ciência, Oeiras, Portugal.
Lancet Infect Dis. 2006 Feb;6(2):112-7. doi: 10.1016/S1473-3099(06)70384-X.
Since the 1980s, the occurrence of pertussis cases in developed countries has increased and shifted towards older age groups. This resurgence follows 30 years of intense mass vaccination, and has been attributed primarily to three factors: (1) more effective diagnosis of the disease, (2) waning of vaccine-induced immunity, and (3) loss of vaccine efficacy due to the emergence of new Bordetella pertussis strains. Here we develop and analyse a mathematical model to assess the plausibility of these hypotheses. We consider that exposure to B pertussis through natural infection or vaccination induces an immune response that prevents severe disease but does not fully prevent mild infections. We also assume that these protective effects are temporary due to waning of immunity. These assumptions, describing the mode of action of adaptive immunity, are combined with a standard transmission model. Two distinct epidemiological scenarios are detected: under low transmission, most infections lead to severe disease; under high transmission, mild infections are frequent, boosting clinical immunity and maintaining low levels of severe disease. The two behaviours are separated by a reinfection threshold in transmission. As a result, the highest incidence of severe disease is expected to occur at intermediate transmission intensities--near the reinfection threshold--suggesting that pertussis resurgence may be induced by a reduction in transmission, independently of vaccination. The model is extended to interpret the outcomes of current control measures and explore scenarios for future interventions.
自20世纪80年代以来，发达国家百日咳病例的发生率有所上升，且发病年龄趋于老龄化。这种疫情的再度出现是在经历了30年的大规模强化疫苗接种之后，主要归因于三个因素：（1）疾病诊断更加有效；（2）疫苗诱导的免疫力逐渐减弱；（3）新的百日咳博德特氏菌菌株出现导致疫苗效力丧失。在此，我们构建并分析了一个数学模型，以评估这些假设的合理性。我们认为，通过自然感染或接种疫苗接触百日咳博德特氏菌会引发免疫反应，这种反应可预防重症疾病，但不能完全预防轻症感染。我们还假定，由于免疫力减弱，这些保护作用是暂时的。这些描述适应性免疫作用方式的假设与一个标准传播模型相结合。我们检测到两种不同的流行病学情景：在低传播率情况下，大多数感染会导致重症疾病；在高传播率情况下，轻症感染很常见，可增强临床免疫力并维持低水平的重症疾病。这两种情况由传播中的再感染阈值分隔开来。因此，预计重症疾病的最高发病率将出现在中等传播强度时——接近再感染阈值——这表明百日咳疫情的再度出现可能是由传播率降低引起的，与疫苗接种无关。该模型经过扩展，用于解释当前控制措施的效果，并探索未来干预措施的情景。
