Immunisation, Hepatitis and Blood Safety Department, Public Health England, , 61 Colindale Avenue, Colindale, London NW9 5EQ, UK, Department of Mathematics and Statistics, Strathclyde University, , 26 Richmond Street, Glasgow G1 1XH, UK, Centre for the Mathematical Modelling of Infectious Diseases, London School of Hygiene and Tropical Medicine, , Keppel Street, London WC1E 7HT, UK, Institute of Child Health, University College London, , 30 Guilford Street, London WC1N 1EH, UK, Health Protection Scotland, , 5 Cadogan Street, Glasgow G2 6QE, UK, International Prevention Research Institute, , 95 Cours Lafayette, Lyon 69006, France.
Proc Biol Sci. 2013 Oct 2;280(1771):20131939. doi: 10.1098/rspb.2013.1939. Print 2013 Nov 22.
More than 90 capsular serotypes of Streptococcus pneumoniae coexist despite competing for nasopharyngeal carriage and a gradient in fitness. The underlying mechanisms for this are poorly understood and make assessment of the likely population impact of vaccination challenging. We use an individual-based simulation model to generalize widely used deterministic models for pneumococcal competition and show that in these models short-term serotype-specific and serotype non-specific immunity could constitute the mechanism governing between-host competition and coexistence. We find that non-specific immunity induces between-host competition and that serotype-specific immunity limits a type's competitive advantage and allows stable coexistence of multiple serotypes. Serotypes carried at low prevalence show high variance in carriage levels, which would result in apparent outbreaks if they were highly pathogenic. Vaccination against few serotypes can lead to elimination of the vaccine types and induces replacement by others. However, in simulations where the elimination of the targeted types is achieved only by a combination of vaccine effects and the competitive pressure of the non-vaccine types, a universal vaccine with similar-type-specific effectiveness can fail to eliminate pneumococcal carriage and offers limited herd immunity. Hence, if vaccine effects are insufficient to control the majority of serotypes at the same time, then exploiting the competitive pressure by selective vaccination can help control the most pathogenic serotypes.
尽管肺炎链球菌（Streptococcus pneumoniae）存在竞争鼻咽携带和适应性梯度，但仍有 90 多种荚膜血清型共存。其潜在机制尚未完全了解，这使得评估疫苗接种对人群的可能影响具有挑战性。我们使用基于个体的模拟模型来推广广泛使用的肺炎球菌竞争确定性模型，并表明在这些模型中，短期的血清型特异性和非特异性免疫可能构成宿主间竞争和共存的机制。我们发现非特异性免疫会引起宿主间竞争，而血清型特异性免疫会限制一种类型的竞争优势，并允许多种血清型稳定共存。携带率低的血清型显示出携带水平的高变异性，如果它们具有高致病性，就会导致明显的爆发。针对少数血清型的疫苗接种会导致疫苗类型的消除，并引发其他类型的替代。然而，在只有疫苗效果和非疫苗类型的竞争压力共同作用才能消除目标类型的模拟中，具有类似血清型特异性效果的通用疫苗可能无法消除肺炎球菌的携带，并提供有限的群体免疫。因此，如果疫苗效果不足以同时控制大多数血清型，那么通过选择性疫苗接种利用竞争压力可以帮助控制最具致病性的血清型。
