Seroprevalence of 2009 pandemic influenza a (H1N1) virus in Australian blood donors, October - December 2009

Abstract

Assessment of the severity of disease due to the2009 pandemic influenza A (H1N1) in Australian states and territories has been hampered by the absence ofdenominator data on population exposure. We compare dantibody reactivity to the pandemic virus using haemagglutination inhibition assays performed on plasma specimens taken from healthy adult blooddonors (older than 16 years) before and after the influenza pandemic that occurred during the southernhemisphere winter. Pre-influenza season samples(April - May 2009, n=496) were taken from donation collection centres in North Queensland (in Cairns and Townsville); post-outbreak specimens (October -November 2009, n=779) were from donors at seven centres in five states. Using a threshold antibody titre of 40 as a marker of recent infection, we observedan increase in the influenza-seropositive proportion of donors from 12% to 22%, not dissimilar to recentreports of influenza A(H1N1)-specific immunity inadults from the United Kingdom. No significant differencesin seroprevalence were observed betweenAustralian states, although the ability to detect minorvariations was limited by the sample size. On the basis of these figures and national reporting data, we estimate that approximately 0.23% of all individuals in Australia exposed to the pandemic virus required hospitalization and 0.01% died. The low seroprevalence reported here suggests that some degree of priorimmunity to the virus, perhaps mediated by broadly reactive T-cell responses to conserved influenza viralantigens, limited transmission among adults and thuscons trained the pandemic in Australia.