2009 Influenza A(H1N1) Seroconversion Rates and Risk Factors Among Distinct Adult Cohorts in Singapore | Global Health | JAMA | JAMA Network
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Original Contribution
April 14, 2010

2009 Influenza A(H1N1) Seroconversion Rates and Risk Factors Among Distinct Adult Cohorts in Singapore

Author Affiliations

Author Affiliations: Clinical Epidemiology, Tan Tock Seng Hospital, Singapore (Dr Chen); Department of Epidemiology and Public Health, National University of Singapore, Singapore (Drs Chen, Lee, Lim, Koh, and Chia and Ms Tan); Duke-NUS Graduate Medical School, Singapore (Dr Chen); Biodefence Centre, Ministry of Defence, Singapore (Drs Lee and Yap); Centre for Epidemiology and Population Health, Australian National University, Australia (Dr Lee); Centre for Health Services Research, National University of Singapore, Singapore (Drs Lee and Koh); World Health Organization Collaborating Centre for Reference and Research on Influenza, Melbourne, Australia (Drs Barr, Laurie, and Kelso, Messrs Shaw and Durrant); National Public Health Laboratory, Ministry of Health, Singapore (Drs R. T. P. Lin and C. Lin); Department of Statistics and Applied Probability, National University of Singapore, Singapore (Dr Cook); Detection and Diagnostics Laboratory, DSO National Laboratories, Singapore (Dr Tan and Mr Loh); Department of Microbiology, National University of Singapore, Singapore (Dr Chow); and Department of Infectious Diseases, Communicable Disease Centre, Tan Tock Seng Hospital, Singapore (Dr Leo).

JAMA. 2010;303(14):1383-1391. doi:10.1001/jama.2010.404

Context 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.

Design, Setting, and Participants 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.

Main Outcome Measures 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).

Conclusion 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.