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

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.

Pandemic (H1N1) 2009 influenza virus seroconversion rates in HIV-infected individuals.

The impact of pandemic (H1N1) 2009 influenza in HIV-infected individuals is unknown. Determining the prevalence of pandemic influenza in this at-risk group will guide vaccination programs. After the first pandemic wave, the seroprevalence rate of pandemic influenza in HIV-infected individuals in western Sydney, New South Wales, Australia, was 34.