The seroprevalence of pandemic influenza H1N1 (2009) virus in China

Background

Mainland China experienced pandemic influenza H1N1 (2009) virus (pH1N1) withpeak activity during November-December 2009. To understand the geographicextent, risk factors, and attack rate of pH1N1 infection in China weconducted a nationwide serological survey to determine the prevalence ofantibodies to pH1N1.

Methodology/Principal Findings

Stored serum samples (n = 2,379) collected during2006-2008 were used to estimate baseline serum reactogenicity to pH1N1. InJanuary 2010, we used a multistage-stratified random sampling method toselect 50,111 subjects who met eligibility criteria and collected serumsamples and administered a standardized questionnaire. Antibody response topH1N1 was measured using haemagglutination inhibition (HI) assay and theweighted seroprevalence was calculated using the Taylor series linearizationmethod. Multivariable logistic regression analyses were used to examine riskfactors for pH1N1 seropositivity. Baseline seroprevalence of pH1N1 antibody(HI titer ≥40) was 1.2%. The weighted seroprevalence of pH1N1among the Chinese population was 21.5%(vaccinated: 62.0%;unvaccinated: 17.1%). Among unvaccinated participants, those aged6-15 years (32.9%) and 16-24 years (30.3%) had higherseroprevalence compared with participants aged 25–59 years(10.7%) and ≥60 years (9.9%, P<0.0001). Children inkindergarten and students had higher odds of seropositivity than children infamily care (OR: 1.36 and 2.05, respectively). We estimated that 207.7million individuals (15.9%) experienced pH1N1 infection in China.

Conclusions/Significance

The Chinese population had low pre-existing immunity to pH1N1 and experienceda relatively high attack rate in 2009 of this virus. We recommend routinecontrol measures such as vaccination to reduce transmission and spread ofseasonal and pandemic influenza viruses.