Abstract
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.
Highlights
On June 11, 2009, the World Health Organization (WHO) declared the first influenza pandemic of the21st century caused by a novel swine-origin influenza A H1N1 virus [1], which contains gene segments derived from classical swine H1N1 virus, human seasonal influenza H3N2 virus, avian influenza H1N1 virus and Eurasian swine H1N1 influenza virus [2].Studies on the extent of infection with pandemic influenza H1N1 (2009) virus (pH1N1) are essential for pandemic severity assessment and for the development of response and vaccination strategies
Using sampling weight constructed based on the multi-stage random sampling design to adjust for oversampling of certain age groups and community settings, the weighted proportion of the Chinese population estimated to have received pH1N1 vaccine was 9.7%
We aimed to estimate the adjusted seroprevalence of antibodies to pH1N1 among Chinese adults and children, to estimate the total number of persons infected in China, and to understand risk factors for infection among this population
Summary
On June 11, 2009, the World Health Organization (WHO) declared the first influenza pandemic of the21st century caused by a novel swine-origin influenza A H1N1 virus [1], which contains gene segments derived from classical swine H1N1 virus, human seasonal influenza H3N2 virus, avian influenza H1N1 virus and Eurasian swine H1N1 influenza virus [2].Studies on the extent of infection with pH1N1 are essential for pandemic severity assessment and for the development of response and vaccination strategies. Modeling methods have been used to estimate the incidence of infection during the pandemic period, using clinical surveillance data in which only patients with influenza-like illness who seek care are captured, while those who do not seek care or have asymptomatic infections are excluded [3,4,5]. These estimates provide useful and timely information, but may lead to an underestimation of the actual number of infections. 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
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