Medical Research Council Biostatistics Unit, Institute of Public Health, University Forvie Site, Cambridge CB2 0SR, UK.
BMJ. 2011 Sep 8;343:d5408. doi: 10.1136/bmj.d5408.
To assess the impact of the 2009 A/H1N1 influenza pandemic in England during the two waves of activity up to end of February 2010 by estimating the probabilities of cases leading to severe events and the proportion of the population infected.
A Bayesian evidence synthesis of all available relevant surveillance data in England to estimate severity of the pandemic.
All available surveillance systems relevant to the pandemic 2009 A/H1N1 influenza outbreak in England from June 2009 to February 2010. Pre-existing influenza surveillance systems, including estimated numbers of symptomatic cases based on consultations to the health service for influenza-like illness and cross sectional population serological surveys, as well as systems set up in response to the pandemic, including follow-up of laboratory confirmed cases up to end of June 2009 (FF100 and Fluzone databases), retrospective and prospective follow-up of confirmed hospitalised cases, and reported deaths associated with pandemic 2009 A/H1N1 influenza. Main outcome measures Age specific and wave specific probabilities of infection and symptomatic infection resulting in hospitalisation, intensive care admission, and death, as well as infection attack rates (both symptomatic and total). The probabilities of intensive care admission and death given hospitalisation over time are also estimated to evaluate potential changes in severity across waves.
In the summer wave of A/H1N1 influenza, 0.54% (95% credible interval 0.33% to 0.82%) of the estimated 606,100 (419,300 to 886,300) symptomatic cases were hospitalised, 0.05% (0.03% to 0.08%) entered intensive care, and 0.015% (0.010% to 0.022%) died. These correspond to 3200 (2300 to 4700) hospital admissions, 310 (200 to 480) intensive care admissions, and 90 (80 to 110) deaths in the summer wave. In the second wave, 0.55% (0.28% to 0.89%) of the 1,352,000 (829,900 to 2,806,000) estimated symptomatic cases were hospitalised, 0.10% (0.05% to 0.16%) were admitted to intensive care, and 0.025% (0.013% to 0.040%) died. These correspond to 7500 (5900 to 9700) hospitalisations, 1340 (1030 to 1790) admissions to intensive care, and 240 (310 to 380) deaths. Just over a third (35% (26% to 45%)) of infections were estimated to be symptomatic. The estimated probabilities of infections resulting in severe events were therefore 0.19% (0.12% to 0.29%), 0.02% (0.01% to 0.03%), and 0.005% (0.004% to 0.008%) in the summer wave for hospitalisation, intensive care admission, and death respectively. The corresponding second wave probabilities are 0.19% (0.10% to 0.32%), 0.03% (0.02% to 0.06%), and 0.009% (0.004% to 0.014%). An estimated 30% (20% to 43%) of hospitalisations were detected in surveillance systems in the summer, compared with 20% (15% to 25%) in the second wave. Across the two waves, a mid-estimate of 11.2% (7.4% to 18.9%) of the population of England were infected, rising to 29.5% (16.9% to 64.1%) in 5-14 year olds. Sensitivity analyses to the evidence included suggest this infection attack rate could be as low as 5.9% (4.2% to 8.7%) or as high as 28.4% (26.0% to 30.8%). In terms of the probability that an infection leads to death in the second wave, these correspond, respectively, to a high estimate of 0.017% (0.011% to 0.024%) and a low estimate of 0.0027% (0.0024% to 0.0031%).
This study suggests a mild pandemic, characterised by case and infection severity ratios increasing between waves. Results suggest low ascertainment rates, highlighting the importance of systems enabling early robust estimation of severity, to inform optimal public health responses, particularly in light of the apparent resurgence of the 2009 A/H1N1 strain in the 2010-11 influenza season.
通过估计导致严重事件的病例概率和感染人群比例，评估英格兰在 2009 年 A/H1N1 流感大流行的两个流行波次期间（截至 2010 年 2 月底）的影响。
对英格兰所有与 2009 年 A/H1N1 流感大流行相关的现有监测数据进行贝叶斯证据综合，以估计大流行的严重程度。
2009 年 6 月至 2010 年 2 月期间英格兰所有与 2009 年 A/H1N1 流感大流行相关的现有监测系统。包括基于流感样疾病向卫生服务机构咨询的症状病例估计数以及横断面人群血清学调查的现有流感监测系统，以及为应对大流行而设立的系统，包括对实验室确诊病例的随访（截至 2009 年 6 月，FF100 和 Fluzone 数据库）、确诊住院病例的回顾性和前瞻性随访，以及与 2009 年 A/H1N1 流感相关的报告死亡病例。
按年龄和流行波次划分的感染和症状性感染导致住院、重症监护入院和死亡的概率，以及感染发病率（包括症状性和总发病率）。还估计了住院后重症监护入院和死亡的概率，以评估波次之间严重程度的潜在变化。
在夏季 A/H1N1 流感波次中，估计的 606100（419300 至 886300）例症状性病例中，有 0.54%（95%可信区间为 0.33%至 0.82%）住院，0.05%（0.03%至 0.08%）进入重症监护，0.015%（0.010%至 0.022%）死亡。这对应于夏季波次中 3200（2300 至 4700）例住院治疗、310（200 至 480）例重症监护治疗和 90（80 至 110）例死亡。在第二个波次中，估计的 1352000（829900 至 2806000）例症状性病例中有 0.55%（0.28%至 0.89%）住院，0.10%（0.05%至 0.16%）进入重症监护，0.025%（0.013%至 0.040%）死亡。这对应于 7500（5900 至 9700）例住院治疗、1340（1030 至 1790）例重症监护治疗和 240（310 至 380）例死亡。超过三分之一（35%（26%至 45%））的感染被估计为症状性。因此，感染导致严重事件的估计概率分别为 0.19%（0.12%至 0.29%）、0.02%（0.01%至 0.03%）和 0.005%（0.004%至 0.008%）在夏季波次中住院、重症监护和死亡。相应的第二波概率分别为 0.19%（0.10%至 0.32%）、0.03%（0.02%至 0.06%）和 0.009%（0.004%至 0.014%）。在夏季，监测系统中检测到的住院病例约占 30%（20%至 43%），而在第二个波次中则占 20%（15%至 25%）。在两个波次中，英格兰人口中有一个中值估计的感染率为 11.2%（7.4%至 18.9%），在 5 至 14 岁的儿童中上升到 29.5%（16.9%至 64.1%）。包括证据的敏感性分析表明，这种感染发病率低至 5.9%（4.2%至 8.7%），高至 28.4%（26.0%至 30.8%）。在第二个波次中，感染导致死亡的概率为 0.017%（0.011%至 0.024%），低估计为 0.0027%（0.0024%至 0.0031%）。
本研究表明这是一次轻度大流行，其特征是病例和感染严重程度比在波次之间增加。结果表明，发现率较低，这突出表明需要建立系统，以便早期进行严重程度的稳健估计，从而为最佳公共卫生反应提供信息，特别是考虑到 2009 年 A/H1N1 病毒株在 2010-11 流感季节明显复燃。
