The Seroprevalence of Pandemic Influenza H1N1 (2009) Virus in China

BackgroundMainland China experienced pandemic influenza H1N1 (2009) virus (pH1N1) with peak activity during November-December 2009. To understand the geographic extent, risk factors, and attack rate of pH1N1 infection in China we conducted a nationwide serological survey to determine the prevalence of antibodies to pH1N1.
Methodology/Principal FindingsStored serum samples (n = 2,379) collected during 2006-2008 were used to estimate baseline serum reactogenicity to pH1N1. In January 2010, we used a multistage-stratified random sampling method to select 50,111 subjects who met eligibility criteria and collected serum samples and administered a standardized questionnaire. Antibody response to pH1N1 was measured using haemagglutination inhibition (HI) assay and the weighted seroprevalence was calculated using the Taylor series linearization method. Multivariable logistic regression analyses were used to examine risk factors for pH1N1 seropositivity. Baseline seroprevalence of pH1N1 antibody (HI titer ≥40) was 1.2%. The weighted seroprevalence of pH1N1 among the Chinese population was 21.5%(vaccinated: 62.0%; unvaccinated: 17.1%). Among unvaccinated participants, those aged 6-15 years (32.9%) and 16-24 years (30.3%) had higher seroprevalence compared with participants aged 25–59 years (10.7%) and ≥60 years (9.9%, P<0.0001). Children in kindergarten and students had higher odds of seropositivity than children in family care (OR: 1.36 and 2.05, respectively). We estimated that 207.7 million individuals (15.9%) experienced pH1N1 infection in China.
Conclusions/SignificanceThe Chinese population had low pre-existing immunity to pH1N1 and experienced a relatively high attack rate in 2009 of this virus. We recommend routine control measures such as vaccination to reduce transmission and spread of seasonal and pandemic influenza viruses.

研究背景
2009年甲型H1N1流感大流行病毒（pandemic influenza H1N1 (2009) virus，pH1N1）在中国大陆的流行高峰为2009年11月至12月。为明确中国大陆地区pH1N1感染的地理分布、危险因素及攻击率，本研究开展了全国性血清学调查，以检测人群中抗pH1N1抗体的流行情况。

方法与主要研究结果
本研究首先利用2006-2008年采集的2379份留存血清样本，估算人群对pH1N1的基线血清反应原性。2010年1月，我们采用多阶段分层随机抽样方法，筛选出50111名符合纳入标准的研究对象，采集其血清样本并开展标准化问卷调查。采用血凝抑制试验（haemagglutination inhibition，HI）检测受试者针对pH1N1的抗体应答，并通过泰勒级数线性化法（Taylor series linearization method）计算加权血清阳性率。采用多变量logistic回归分析，探讨pH1N1血清学阳性的危险因素。
pH1N1抗体（HI滴度≥40）的基线血清阳性率为1.2%。我国人群中pH1N1的加权血清阳性率为21.5%，其中接种疫苗者为62.0%，未接种疫苗者为17.1%。在未接种疫苗的受试者中，6~15岁组（32.9%）和16~24岁组（30.3%）的血清阳性率显著高于25~59岁组（10.7%）及≥60岁组（9.9%，P<0.0001）。幼儿园儿童及在校学生的血清阳性优势比（odds ratio，OR）高于家庭看护儿童（优势比分别为1.36和2.05）。本研究估算我国约有2.077亿人（占总人口15.9%）曾感染pH1N1。

结论与意义
我国人群对pH1N1的既往免疫力较低，2009年该病毒的攻击率相对较高。建议通过疫苗接种等常规防控措施，降低季节性及大流行性流感病毒的传播与扩散。