Seroepidemiologic Effects of Influenza A(H1N1)pdm09 in Australia, New Zealand, and Singapore

James M. Trauer

, Don Bandaranayake, Robert Booy, Mark I. Chen, Michelle Cretikos, Gary K. Dowse, Dominic E. Dwyer, Michael E. Greenberg, Q. Sue Huang, Gulam Khandaker, Jen Kok, Karen L. Laurie, Vernon J. Lee, Jodie McVernon, Scott Walter, Peter G. Markey, and for the Australia, New Zealand and Singapore Pandemic Serosurveillance Study Group

Author affiliations: Author affiliations: Melbourne Sleep Disorders Centre, East Melbourne, Victoria, Australia (J.M. Trauer); Environmental Science and Research, Wallaceville, New Zealand (D. Bandaranayake, Q.S. Huang); National Centre for Immunisation Research and Surveillance, Westmead, New South Wales, Australia (R. Booy, G. Khandaker); National University Health System, Singapore (M.I. Chen); University of Sydney, Sydney, New South Wales, Australia (M. Cretikos); Communicable Disease Control Directorate, Shenton Park, Western Australia, Australia (G.K. Dowse); Centre for Infectious Diseases and Microbiology, Westmead (D.E. Dwyer, J. Kok); CSL Limited, Parkville, Victoria, Australia (M.E. Greenberg); World Health Organization Collaborating Centre for Reference and Research in Influenza, North Melbourne, Victoria, Australia (K.L. Laurie); World Health Organization, Geneva, Switzerland (V.J. Lee); Melbourne School of Population Health, Parkville (J. McVernon); Centre for Epidemiology and Research, North Sydney, New South Wales, Australia (S. Walter; Centre for Disease Control, Tiwi, Northern Territory, Australia (P.G. Markey)

Abstract

To estimate population attack rates of influenza A(H1N1)pdm2009 in the Southern Hemisphere during June–August 2009, we conducted several serologic studies. We pooled individual-level data from studies using hemagglutination inhibition assays performed in Australia, New Zealand, and Singapore. We determined seropositive proportions (titer >40) for each study region by age-group and sex in pre- and postpandemic phases, as defined by jurisdictional notification data. After exclusions, the pooled database consisted of, 4,414 prepandemic assays and 7,715 postpandemic assays. In the prepandemic phase, older age groups showed greater seropositive proportions, with age-standardized, community-based proportions ranging from 3.5% in Singapore to 11.9% in New Zealand. In the postpandemic phase, seropositive proportions ranged from 17.5% in Singapore to 30.8% in New Zealand, with highest proportions seen in school-aged children. Pregnancy and residential care were associated with lower postpandemic seropositivity, whereas Aboriginal and Torres Strait Islander Australians and Pacific Peoples of New Zealand had greater postpandemic seropositivity.

Australia, New Zealand (NZ), and Singapore all experience regular influenza seasons that coincide with winter in the Southern Hemisphere. After pandemic influenza A(H1N1) 2009 (A[H1N1]pdm09) emerged during spring in North America (1), influenza notifications and other markers of influenza activity peaked in Australia, NZ, and Singapore during July 2009 (2–4). The 3 countries continued to experience the circulation of an influenza strain closely related to the original virus until at least the following winter (5).
Most influenza surveillance systems are passive, laboratory-based systems that capture only symptomatic patients who seek medical advice and are then appropriately tested and case notifications sent. Therefore, these systems are likely to underestimate the true attack rate. Measurement of antibodies against A(H1N1)pdm09 can be used to assess the extent of population exposure to the virus (6). The emergence of a novel influenza virus provided a unique opportunity to study the behavior of influenza viruses to better understand their differential effects across various population groups.
Standardization of epidemiologic and serologic techniques across our region enabled more direct comparison of the effects of pandemic influenza on the different populations studied. Three of the countries in our region performed such studies, with publications originating from Australia (7–15), NZ (16), and Singapore (17). We pooled individual-level serologic data from studies that used the hemagglutination inhibition (HI) assay to describe the effects of the 2009 winter influenza pandemic in the Southern Hemisphere.