Objective: To assess background pre-pandemic cross-reacting antibodies to the pandemic (H1N1) 2009 virus in older populations in Australia.
Design, setting and participants: Data were opportunistically generated from three cross-sectional pre-pandemic studies involving people aged 60 years or older: a 3-year (2006-2008) study of influenza outbreaks in aged care facilities (ACFs) in Sydney; an investigation of a respiratory virus outbreak in an ACF in rural New South Wales in June 2009; and a non-influenza 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 women (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 people 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 influenza A antibody reserve in most of the oldest group of people 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 influenza control measures need to focus more on younger adults naive to the pandemic virus and at increased risk of severe disease.