title: Seroprevalence and severity of 2009 pandemic influenza A H1N1 in Taiwan.
abstract: BACKGROUND
  This study is to determine the seroprevalence of the pandemic influenza A H1N1 virus (pH1N1) in Taiwan before and after the 2009 pandemic, and to estimate the relative severity of pH1N1 infections among different age groups.
  
  
  RESULTS
  A total of 1544 and 1558 random serum samples were collected from the general population in Taiwan in 2007 and 2010, respectively. Seropositivity was defined by a hemagglutination inhibition titer to pH1N1 (A/Taiwan/126/09) ≥1:40. The seropositivity rate of pH1N1 among the unvaccinated subjects and national surveillance data were used to compare the proportion of infections that led to severe diseases and fatalities among different age groups. The overall seroprevalence of pH1N1 was 0.91% (95% confidence interval [CI] 0.43-1.38) in 2007 and significantly increased to 29.9% (95% CI 27.6-32.2) in 2010 (p<0.0001), with the peak attack rate (55.4%) in 10-17 year-old adolescents, the lowest in elderly ≥65 years (14.1%). The overall attack rates were 20.6% (188/912) in unvaccinated subjects. Among the unvaccinated but infected populations, the estimated attack rates of severe cases per 100,000 infections were significantly higher in children aged 0-5 years (54.9 cases, odds ratio [OR] 4.23, 95% CI 3.04-5.90) and elderly ≥ 65 years (22.4 cases, OR 2.76, 95% CI 1.99-3.83) compared to adolescents aged 10-17 years (13.0 cases). The overall case-fatality rate was 0.98 per 100,000 infections without a significant difference in different age groups.
  
  
  CONCLUSIONS
  Pre-existing immunity against pH1N1 was rarely identified in Taiwanese at any age in 2007. Young children and elderly--the two most lower seroprotection groups showed the greatest vulnerability to clinical severity after the pH1N1 infections. These results imply that both age groups should have higher priority for immunization in the coming flu season.
authors: Division of Pediatric Infectious Diseases, Department of Pediatrics, Chang Gung Children's Hospital and Chang Gung Memorial Hospital, Taoyuan, Taiwan.
authors: Chih-Jung
authors: CJ
authors: Chen
authors: Ping-Ing
authors: PI
authors: Lee
authors: Shih-Cheng
authors: SC
authors: Chang
authors: Yhu-Chering
authors: YC
authors: Huang
authors: Cheng-Hsun
authors: CH
authors: Chiu
authors: Yu-Chia
authors: YC
authors: Hsieh
authors: Shang-Chwen
authors: SC
authors: Chang
authors: Feng-Yee
authors: FY
authors: Chang
authors: Jen-Jyh
authors: JJ
authors: Lee
authors: Shey-Chiang
authors: SC
authors: Su
authors: Gwan-Han
authors: GH
authors: Shen
authors: Yin-Ching
authors: YC
authors: Chuang
authors: Yao-Shen
authors: YS
authors: Chen
authors: Jien-Wei
authors: JW
authors: Liu
authors: Tzou-Yien
authors: TY
authors: Lin
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D003710:Demography; D005260:Female; D006385:Hemagglutination Inhibition Tests; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000502:physiology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000276:immunology / Q000517:prevention & control / Q000821:virology*; D008297:Male; D008875:Middle Aged; D058873:Pandemics / Q000706:statistics & numerical data*; D011159:Population Surveillance; D016036:Seroepidemiologic Studies; D012720:Severity of Illness Index; D013624:Taiwan / Q000453:epidemiology; D014611:Vaccination / Q000706:statistics & numerical data; D055815:Young Adult