Cross-reacting antibodies against the pandemic (H1N1) 2009 XXXX virus in older Australians.

@entity1 @entity683 @entity9598
OBJECTIVE: To assess background pre-pandemic cross-reacting antibodies to the pandemic (H1N1) 2009 virus in older populations in Australia. DESIGN, SETTING AND @entity1 : Data were opportunistically generated from three cross-sectional pre-pandemic studies involving @entity1 aged 60 years or older: a 3-year (2006-2008) study of @entity683 outbreaks in aged care facilities (ACFs) in Sydney; an investigation of a respiratory virus outbreak in an @entity9598 in rural New South Wales in June 2009; and a non- @entity683 serosurvey undertaken in NSW in 2007 and 2008. MAIN OUTCOME MEASURE: Prevalence of pandemic (H1N1) 2009 haemagglutination inhibition (HAI) antibody titres >=  1:40 (putative protective level) in pre-pandemic sera. RESULTS: In total, 259 serum samples from individuals aged 60 years or older (range, 60-101 years) were tested. More than half of the individuals tested were @entity1 (151/259; 58.3%). About a third of individuals (37.5%) had cross-reacting HAI antibody titres >=  1:40. The prevalence of cross-reacting antibodies was highest in the oldest age groups (>=  85 years), with more than 60% of these @entity1 having HAI antibody titres >=  1:40. The proportion of subjects with HAI antibody titres >=  1:40 decreased significantly and successively in younger groups to only 12% of those aged 60-64 years. CONCLUSIONS: Our study suggests a pre-existing @entity683 A antibody reserve in most of the oldest group of @entity1 that was cross-reactive to the new pandemic (H1N1) 2009 virus; this is likely to be lifelong and to have provided them with clinical protection against the first wave of the pandemic. Pandemic @entity683 control measures need to focus more on younger adults naive to the pandemic virus and at increased risk of severe disease.
