title: Factors influencing infection by pandemic influenza A(H1N1)pdm09 over three epidemic waves in Singapore.
abstract: BACKGROUND
  Previous influenza pandemics had second and on occasion third waves in many countries that were at times more severe than the initial pandemic waves.
  
  
  OBJECTIVE
  This study aims to determine the seroepidemiology of successive waves of H1N1pdm09 infections in Singapore and the overall risks of infection.
  
  
  METHODS
  We performed a cohort study amongst 838 adults, with blood samples provided upon recruitment and at 5 points from 2009 to 2011 and tested by haemagglutination inhibition (HI) with A/California/7/2009 (H1N1pdm09). Surveys on key demographic and clinical information were conducted at regular intervals, and associations between seroconversion and these variables were investigated.
  
  
  RESULTS
  After the initial wave from June to September 2009, second and third waves occurred from November 2009 to February 2010 and April to June 2010, respectively. Seroconversion was 13·5% during the first wave and decreased to 6·2% and 6·8% in subsequent waves. Across the three waves, the elderly and those with higher starting HI titres were at lower risk of seroconversion, while those with larger households were at greater risk. Those with higher starting HI titres were also less likely to have an acute respiratory infection.
  
  
  CONCLUSIONS
  The second and third waves in Singapore had lower serological attack rates than the first wave. The elderly and those with higher HI titres had lower risk, while those in larger households had higher risk of seroconversion.
authors.affiliation: Saw Swee Hock School of Public Health, National University of Singapore and National University Health System, Singapore; Institute of Infectious Diseases and Epidemiology, Tan Tock Seng Hospital, Singapore.
authors.forename: Mark I C
authors.initials: MI
authors.lastname: Chen
authors.forename: Alex R
authors.initials: AR
authors.lastname: Cook
authors.forename: Wei Yen
authors.initials: WY
authors.lastname: Lim
authors.forename: Raymond
authors.initials: R
authors.lastname: Lin
authors.forename: Lin
authors.initials: L
authors.lastname: Cui
authors.forename: Ian G
authors.initials: IG
authors.lastname: Barr
authors.forename: Anne
authors.initials: A
authors.lastname: Kelso
authors.forename: Vincent T
authors.initials: VT
authors.lastname: Chow
authors.forename: Yee Sin
authors.initials: YS
authors.lastname: Leo
authors.forename: Jung Pu
authors.initials: JP
authors.lastname: Hsu
authors.forename: Rob
authors.initials: R
authors.lastname: Shaw
authors.forename: Serene
authors.initials: S
authors.lastname: Chew
authors.forename: Joe Kwan
authors.initials: JK
authors.lastname: Yap
authors.forename: Meng Chee
authors.initials: MC
authors.lastname: Phoon
authors.forename: Hiromi W L
authors.initials: HW
authors.lastname: Koh
authors.forename: Huili
authors.initials: H
authors.lastname: Zheng
authors.forename: Linda
authors.initials: L
authors.lastname: Tan
authors.forename: Vernon J
authors.initials: VJ
authors.lastname: Lee
mesh_terms: D000328:Adult; D000368:Aged; D000914:Antibodies, Viral / Q000097:blood; D015331:Cohort Studies; D005260:Female; D006385:Hemagglutination Inhibition Tests; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000821:virology*; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D012307:Risk Factors; D016036:Seroepidemiologic Studies; D012846:Singapore / Q000453:epidemiology; D055815:Young Adult
chemical_list: D000914:Antibodies, Viral
keywords: Epidemic waves; H1N1pdm09; haemagglutination inhibition; risk factors; seroconversion; seroepidemiology