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Pandemic (H1N1) 2009 Seroconversion, Singapore | CDC EID - Volume 17, Number 8–August 2011

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Volume 17, Number 8–August 2011
Risk Factors for Pandemic (H1N1) 2009 Seroconversion among Adults, Singapore, 2009

Wei-Yen Lim, Comments to Author Cynthia H.J. Chen, Yi Ma, Mark I.C. Chen, Vernon J.M. Lee, Alex R. Cook, Linda W.L. Tan, Norberto Flores Tabo Jr., Ian Barr, Lin Cui, Raymond T.P. Lin, Yee Sin Leo, and Kee Seng Chia

Author affiliations: National University of Singapore, Singapore (W.-Y. Lim, C.H.J. Chen, Y. Ma, M.I.C. Chen, V.J.M. Lee, L.W.L. Tan, N. Flores Tabo, Jr., K.S. Chia); Tan Tock Seng Hospital, Singapore (M.I.C. Chen, Y.S. Leo); Ministry of Defence, Singapore (V.J.M. Lee); National University of Singapore, Singapore (A.R. Cook); World Health Organization Collaborating Centre for Reference and Research on Influenza, Melbourne, Victoria, Australia (I. Barr); and Ministry of Health, Singapore (L. Cui, R.T.P. Lin)

Suggested citation for this article

A total of 828 community-dwelling adults were studied during the course of the pandemic (H1N1) 2009 outbreak in Singapore during June–September 2009. Baseline blood samples were obtained before the outbreak, and 2 additional samples were obtained during follow-up. Seroconversion was defined as a >4-fold increase in antibody titers to pandemic (H1N1) 2009, determined by using hemagglutination inhibition. Men were more likely than women to seroconvert (mean adjusted hazards ratio [HR] 2.23, mean 95% confidence interval [CI] 1.26–3.93); Malays were more likely than Chinese to seroconvert (HR 2.67, 95% CI 1.04–6.91). Travel outside Singapore during the study period was associated with seroconversion (HR 1.76, 95% CI 1.11–2.78) as was use of public transport (HR 1.81, 95% CI 1.05–3.09). High baseline antibody titers were associated with reduced seroconversion. This study suggests possible areas for intervention to reduce transmission during future influenza outbreaks

Each year, influenza causes large numbers of deaths (1,2) and billions of dollars in direct medical costs and indirect costs from declines in productivity and worker absenteeism (3). Influenza vaccines help prevent some costs, but the uptake of vaccination for influenza varies widely (4), and effective vaccines may not be sufficient during influenza pandemics (5). Use of antiviral drugs to mitigate the effects of seasonal and pandemic influenza is also subject to such limitations as cost considerations (6), the need for adequate and timely delivery of antiviral drugs (7), and concerns about the emergence of resistance (8).

Nonpharmaceutical measures have been proposed as adjuncts for reducing the risk for influenza infection during pandemics and seasonal epidemics (9). Studies suggest that physical interventions, such as handwashing, use of protective equipment (e.g., face masks), and social distancing measures, can effectively reduce transmission of respiratory viruses, including influenza (10,11).

A novel influenza A virus, pandemic (H1N1) 2009 virus, emerged in Mexico and the United States during 2009 and spread worldwide within months (12,13). A few studies have investigated transmission of pandemic (H1N) 2009 virus (14,15) and the public health interventions that could be used to mitigate its spread (16). However, although some data are available from studies conducted in institutions and households (17,18), little is known about population-level risk factors for pandemic (H1N1) 2009 virus infection.

We investigated risk factors for serologically detected pandemic (H1N1) 2009 virus infection during the first wave of the epidemic in Singapore in 2009. Our study population was a prospective community-dwelling cohort of adults. Singapore is a tropical city-state and global travel hub in Southeast Asia with a population of 5.0 million persons. Singapore detected its first imported cases of pandemic (H1N1) 2009 in late May 2009 and subsequently experienced an epidemic wave lasting ≈12 weeks starting in late June, peaking during the first week of August, and subsiding by early September (19–21).

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Suggested Citation for this Article

Lim W-Y, Chen CHJ, Ma Y, Chen MIC, Lee VJM, Cook AR, et al. Risk factors for pandemic (H1N1) 2009 seroconversion among adults, Singapore, 2009. Emerg Infect Dis [serial on the Internet]. 2011 Aug [date cited].

DOI: 10.3201/eid1708.101270

Comments to the Authors

Please use the form below to submit correspondence to the authors or contact them at the following address:

Wei-Yen Lim, National University of Singapore—Epidemiology and Public Health, Yong Loo Lin School of Medicine, MD3, 16 Medical Dr, Singapore 117597, Singapore; email:

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