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

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.

Post-Pandemic Seroprevalence of 2009 Pandemic Influenza A (H1N1) Virus in Shandong Province, China

“…Individuals of all professions, except for teachers, had significantly lower OR of A(H1N1)pdm09 seropositivity than children in family care; this was discordant with the report of Xu et al (12). In addition, the symptoms (cold, fever, and cough) were not significantly associated with the seropositive rate.…”

“…We did not have baseline (pre-pandemic) data for accurately inferring the attack rates among the population in Shandong Province. However, the baseline serum samples collected from individuals in China prior to May 2009 had shown a very low positive rate (1.2z) against A(H1N1)pdm09 (12). Third, the HI assay may not be the most sensitive assay for detecting low levels of A(H1N1)pdm09 (e.g., when compared to microneutralization); thus, we may have underestimated seropositivity in the serological survey samples.…”

Human infection with avian influenza A H7N9 virus: an assessment of clinical severity

“…Under method 1 for estimating the number of symptomatic A(H7N9) virus infections, nationwide serosurvey and ILI surveillance of A(H1N1)pdm09 in China during June 2009 to January 2010 indicated that around 0.75% of symptomatic A(H1N1)pdm09 virus infections sought medical care at ILI sentinels. 23 Assuming that a similar proportion of symptomatic A(H7N9) virus infections attended ILI sentinels, we estimated that approximately 3,020 (95% credible interval, CrI: 900–7,800) and 5,310 (95% CrI: 880–17,300) symptomatic A(H7N9) virus infections had occurred in Shanghai and Nanjing, respectively (Appendix). With 40 hospitalised cases occurring in Shanghai and Nanjing, pro-rata extrapolation based on 123 total hospitalisations to date gives a total of 27,000 (95% CrI: 9,530–65,000) symptomatic A(H7N9) virus infections.…”

A new approach to characterising infectious disease transmission dynamics from sentinel surveillance: Application to the Italian 2009–2010 A/H1N1 influenza pandemic

“…It is likely both susceptibility and reporting rates were higher in children and lower in adults, but the exact age-dependence of each is difficult to resolve without additional data. Post-pandemic serology data now available from a number of countries [49, 50, 51, 52] shows age-specific seroconversion rates more compatible with estimates from the Baseline model than from the other two models (which produce much higher infection attack rate estimates in adults), suggesting that variation in susceptibility to infection with the H1N1pdm virus was more extreme than suggested from serological testing of pre-pandemic samples.…”