title: Influenza A (H1N1) 2009 antibodies in residents of New South Wales, Australia, after the first pandemic wave in the 2009 southern hemisphere winter.
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.
  
  
  RESULTS
  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
  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.
authors.affiliation: Centre for Infectious Diseases and Microbiology, Institute of Clinical Pathology and Medical Research, The University of Sydney, Sydney, New South Wales, Australia. lyn.gilbert@sydney.edu.au
authors.forename: Gwendolyn L
authors.initials: GL
authors.lastname: Gilbert
authors.forename: Michelle A
authors.initials: MA
authors.lastname: Cretikos
authors.forename: Linda
authors.initials: L
authors.lastname: Hueston
authors.forename: George
authors.initials: G
authors.lastname: Doukas
authors.forename: Brian
authors.initials: B
authors.lastname: O'Toole
authors.forename: Dominic E
authors.initials: DE
authors.lastname: Dwyer
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000914:Antibodies, Viral / Q000097:blood / Q000276:immunology*; D001315:Australia / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007251:Influenza, Human / Q000097:blood / Q000453:epidemiology* / Q000276:immunology* / Q000821:virology; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D012621:Seasons; D055815:Young Adult
chemical_list: D000914:Antibodies, Viral