Influenza A (H1N1) 2009 Antibodies in Residents of New South Wales, Australia, after the First Pandemic Wave in the 2009 Southern Hemisphere Winter Gwendolyn L. Gilbert 1 *, Michelle A. Cretikos 2,3 , Linda Hueston 1 , George Doukas 2 , Brian O’Toole 1 , Dominic E. Dwyer 1 1 Centre for Infectious Diseases and Microbiology, Institute of Clinical Pathology and Medical Research, The University of Sydney, Sydney, New South Wales, Australia, 2 Centre for Epidemiology and Research, New South Wales Department of Health, North Sydney, New South Wales, Australia, 3 School of Public Health, The University of Sydney, North Sydney, New South Wales, Australia Abstract Background: The first wave of pandemic influenza A(H1N1)2009 (pH1N1) reached New South Wales (NSW), Australia in May 2009, and led to high rates of influenza-related hospital admission of infants and young to middle-aged adults, but no increase in influenza-related or all-cause mortality. Methodology/Principal Findings: To assess the population rate of pH1N1 infection in NSW residents, pH1N1-specific haemagglutination inhibition (HI) antibody prevalence was measured in specimens collected opportunistically before (2007–2008; 474 specimens) and after (August–September 2009; 1247 specimens) the 2009 winter, and before the introduction of the pH1N1 monovalent vaccine. Age- and geographically-weighted population changes in seroprevalence were calculated. HI antibodies against four recent seasonal influenza A viruses were measured to assess cross-reactions. Pre- and post-pandemic pH1N1 seroprevalences were 12.8%, and 28.4%, respectively, with an estimated overall infection rate of 15.6%. pH1N1 antibody prevalence increased significantly - 20.6% overall - in people born since 1944 (26.9% in those born between 1975 and 1997) but not in those born in or before 1944. People born before 1925 had a significantly higher pH1N1 seroprevalence than any other age-group, and against any seasonal influenza A virus. Sydney residents had a significantly greater change in prevalence of antibodies against pH1N1 than other NSW residents (19.3% vs 9.6%). Conclusions/Significance: Based on increases in the pH1N1 antibody prevalence before and after the first pandemic wave, 16% of NSW residents were infected by pH1N1 in 2009; the highest infection rates (27%) were among adolescents and young adults. Past exposure to the antigenically similar influenza A/H1N1(1918) is the likely basis for a very high prevalence (49%) of prepandemic cross-reacting pH1N1 antibody and sparing from pH1N1 infection among people over 85 years. Unless pre-season vaccine uptake is high, there are likely to be at least moderate rates including some life-threatening cases of pH1N1 infection among young people during subsequent winters. Citation: Gilbert GL, Cretikos MA, Hueston L, Doukas G, O’Toole B, et al. (2010) Influenza A (H1N1) 2009 Antibodies in Residents of New South Wales, Australia, after the First Pandemic Wave in the 2009 Southern Hemisphere Winter. PLoS ONE 5(9): e12562. doi:10.1371/journal.pone.0012562 Editor: Patricia V. Aguilar, NMRCD, Peru Received May 24, 2010; Accepted August 9, 2010; Published September 7, 2010 Copyright: ß 2010 Gilbert et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study was funded by the Population Health Division of the New South Wales Department of Health Australia. The study sponsor represented by author Michelle Cretikos, had a major role in study design, analysis and interpretation of data and preparation of the manuscript. The corresponding author had full access to all the data in the study and final responsibility for the decision to submit for publication. Competing Interests: Michelle Cretikos is employed by the NSW Department of Health, which funded the study. She owns shares in CSL Ltd, which manufactures influenza vaccines. None of the other authors have any conflicts of interest. * E-mail: lyn.gilbert@sydney.edu.au Introduction The first wave of infection due to pandemic influenza A (H1N1) 2009 - pH1N1 - in Australia began in May, 2009 [1]. There was debate as to whether pH1N1 infections were significantly more prevalent or severe than during an ‘‘average’’ influenza season [2]. Most clinical infections were apparently mild and predominantly affected school children and young adults. Increased numbers of laboratory-confirmed and notified cases, compared with average influenza seasons, could be partly explained by much greater levels of public awareness, medical consultation and laboratory testing [3]. Despite a disease profile generally similar to that of seasonal influenza, pH1N1 infections led to unusually high rates of hospital and intensive care unit (ICU) admissions of relatively young patients with influenza-related illness. ICU admissions for viral pneumonitis were 15 times higher than in previous years and highest in the 25–49 year age-group; 93% of ICU patients were under 65 years [4]. In New South Wales (NSW), the most populous Australian state, syndromic surveillance of emergency department presentations showed unusually high rates of febrile respiratory illness during the 2009 winter but there was neither an increase in deaths attributable to influenza or pneumonia, nor in overall mortality [3]. It is difficult to estimate the true rates of pH1N1 infection or differences between geographic areas and age-groups from limited epidemiological data, but information about population preva- PLoS ONE | www.plosone.org 1 September 2010 | Volume 5 | Issue 9 | e12562