MRC Centre for Outbreak Analysis and Modelling, Department of Infectious Disease Epidemiology, School of Public Health, Imperial College London, London, United Kingdom.
PLoS Pathog. 2011 Sep;7(9):e1002225. doi: 10.1371/journal.ppat.1002225. Epub 2011 Sep 1.
While in Northern hemisphere countries, the pandemic H1N1 virus (H1N1pdm) was introduced outside of the typical influenza season, Southern hemisphere countries experienced a single wave of transmission during their 2009 winter season. This provides a unique opportunity to compare the spread of a single virus in different countries and study the factors influencing its transmission. Here, we estimate and compare transmission characteristics of H1N1pdm for eight Southern hemisphere countries/states: Argentina, Australia, Bolivia, Brazil, Chile, New Zealand, South Africa and Victoria (Australia). Weekly incidence of cases and age-distribution of cumulative cases were extracted from public reports of countries' surveillance systems. Estimates of the reproduction numbers, R(0), empirically derived from the country-epidemics' early exponential phase, were positively associated with the proportion of children in the populations (p = 0.004). To explore the role of demography in explaining differences in transmission intensity, we then fitted a dynamic age-structured model of influenza transmission to available incidence data for each country independently, and for all the countries simultaneously. Posterior median estimates of R₀ ranged 1.2-1.8 for the country-specific fits, and 1.29-1.47 for the global fits. Corresponding estimates for overall attack-rate were in the range 20-50%. All model fits indicated a significant decrease in susceptibility to infection with age. These results confirm the transmissibility of the 2009 H1N1 pandemic virus was relatively low compared with past pandemics. The pattern of age-dependent susceptibility found confirms that older populations had substantial--though partial--pre-existing immunity, presumably due to exposure to heterologous influenza strains. Our analysis indicates that between-country-differences in transmission were at least partly due to differences in population demography.
在北半球国家，大流行性 H1N1 病毒（H1N1pdm）在典型流感季节之外传入，而南半球国家在 2009 年冬季经历了一波单一的传播。这为比较不同国家中单一病毒的传播并研究影响其传播的因素提供了独特的机会。在这里，我们估计并比较了 8 个南半球国家/地区（阿根廷、澳大利亚、玻利维亚、巴西、智利、新西兰、南非和维多利亚州（澳大利亚））的 H1N1pdm 传播特征。从各国监测系统的公共报告中提取了病例的每周发病率和累计病例的年龄分布。从国家疫情的早期指数阶段得出的经验繁殖数 R（0）估计值与人口中儿童的比例呈正相关（p=0.004）。为了探索人口统计学在解释传播强度差异方面的作用，我们随后针对每个国家独立地拟合了流感传播的动态年龄结构模型，并针对所有国家同时进行了拟合。国家特定拟合的 R₀后验中位数估计值范围为 1.2-1.8，全球拟合的 R₀后验中位数估计值范围为 1.29-1.47。总体攻击率的相应估计值在 20-50%之间。所有模型拟合均表明，随着年龄的增长，感染的易感性显著下降。这些结果证实，与过去的大流行相比，2009 年 H1N1 大流行性病毒的传播能力相对较低。发现的年龄依赖性易感性模式证实，老年人群具有大量（尽管是部分）先前存在的异源流感株免疫力，这可能是由于接触了异源流感株。我们的分析表明，国家之间传播的差异至少部分是由于人口统计学差异造成的。
