title: 2009 influenza A(H1N1) seroconversion rates and risk factors among distinct adult cohorts in Singapore.
abstract: BACKGROUND
  Singapore experienced a single epidemic wave of 2009 influenza A(H1N1) with epidemic activity starting in late June 2009 and peaking in early August before subsiding within a month.
  
  
  OBJECTIVE
  To compare the risk and factors associated with H1N1 seroconversion in different adult cohorts.
  
  
  METHODS
  A study with serial serological samples from 4 distinct cohorts: general population (n = 838), military personnel (n = 1213), staff from an acute care hospital (n = 558), and staff as well as residents from long-term care facilities (n = 300) from June 22, 2009, to October 15, 2009. Hemagglutination inhibition results of serum samples taken before, during, and after the epidemic and data from symptom questionnaires are presented.
  
  
  METHODS
  A 4-fold or greater increase in titer between any of the 3 serological samples was defined as evidence of H1N1 seroconversion.
  
  
  RESULTS
  Baseline titers of 40 or more were observed in 22 members (2.6%; 95% confidence interval [CI], 1.7%-3.9%) of the community, 114 military personnel (9.4%; 95% CI, 7.9%-11.2%), 37 hospital staff (6.6%; 95% CI, 4.8%-9.0%), and 20 participants from long-term care facilities (6.7%; 95% CI, 4.4%-10.1%). In participants with 1 or more follow-up serum samples, 312 military personnel (29.4%; 95% CI, 26.8%-32.2%) seroconverted compared with 98 community members (13.5%; 95% CI, 11.2%-16.2%), 35 hospital staff (6.5%; 95% CI, 4.7%-8.9%), and only 3 long-term care participants (1.2%; 95% CI, 0.4%-3.5%). Increased frequency of seroconversion was observed for community participants from households in which 1 other member seroconverted (adjusted odds ratio [OR], 3.32; 95% CI, 1.50-7.33), whereas older age was associated with reduced odds of seroconversion (adjusted OR, 0.77 per 10 years; 95% CI, 0.64-0.93). Higher baseline titers were associated with decreased frequency of seroconversion in community (adjusted OR for every doubling of baseline titer, 0.48; 95% CI, 0.27-0.85), military (adjusted OR, 0.71; 95% CI, 0.61-0.81), and hospital staff cohorts (adjusted OR, 0.50; 95% CI, 0.26-0.93).
  
  
  CONCLUSIONS
  Following the June-September 2009 wave of 2009 influenza A(H1N1), 13% of the community participants seroconverted, and most of the adult population likely remained susceptible.
authors.affiliation: Tan Tock Seng Hospital, Department of Clinical Epidemiology, 11 Jalan Tan Tock Seng, Singapore, 308433. mark_chen@ttsh.com.sg
authors.forename: Mark I C
authors.initials: MI
authors.lastname: Chen
authors.forename: Vernon J M
authors.initials: VJ
authors.lastname: Lee
authors.forename: Wei-Yen
authors.initials: WY
authors.lastname: Lim
authors.forename: Ian G
authors.initials: IG
authors.lastname: Barr
authors.forename: Raymond T P
authors.initials: RT
authors.lastname: Lin
authors.forename: Gerald C H
authors.initials: GC
authors.lastname: Koh
authors.forename: Jonathan
authors.initials: J
authors.lastname: Yap
authors.forename: Lin
authors.initials: L
authors.lastname: Cui
authors.forename: Alex R
authors.initials: AR
authors.lastname: Cook
authors.forename: Karen
authors.initials: K
authors.lastname: Laurie
authors.forename: Linda W L
authors.initials: LW
authors.lastname: Tan
authors.forename: Boon Huan
authors.initials: BH
authors.lastname: Tan
authors.forename: Jimmy
authors.initials: J
authors.lastname: Loh
authors.forename: Robert
authors.initials: R
authors.lastname: Shaw
authors.forename: Chris
authors.initials: C
authors.lastname: Durrant
authors.forename: Vincent T K
authors.initials: VT
authors.lastname: Chow
authors.forename: Anne
authors.initials: A
authors.lastname: Kelso
authors.forename: Kee Seng
authors.initials: KS
authors.lastname: Chia
authors.forename: Yee Sin
authors.initials: YS
authors.lastname: Leo
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000914:Antibodies, Viral; D000917:Antibody Formation; D015331:Cohort Studies; D004198:Disease Susceptibility; D005260:Female; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology / Q000276:immunology*; D008297:Male; D008875:Middle Aged; D012307:Risk Factors; D012846:Singapore / Q000453:epidemiology; D055815:Young Adult
chemical_list: D000914:Antibodies, Viral