What serosurveillance studies tell us about the 2009 influenza pandemic

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Surveillance of the impact of pandemic (H1N1) 2009 influenza during its initial seasons in both hemispheres relied on routinely available indicators, including numbers and rates of laboratory-detected cases, hospitalisations, ICU admissions and deaths, along with monitoring of influenza-like illness (ILI) from primary care sentinel surveillance systems. Estimates of the clinical attack rate and the case fatality ratio were imperfect. Understanding of the pathogenicity of the pandemic virus and prediction of the impact in subsequent seasons was hindered by a lack of information on actual infection rates in the population. Results of a number of serosurveys conducted in Australia and overseas countries have now become available, revealing that the arrival of the pandemic virus in modern urbanised and non-immune populations resulted in relatively similar infection rates in both the southern and northern hemispheres. Around 30?50% of children and teenagers were infected during the first pandemic season, with lower rates, around 10?20%, in young and middle-aged adults, and very few infections in older adults. There were significant numbers of mild or asymptomatic infections, and case fatality and hospitalisation ratios were much lower than those contemplated in pandemic plans. Many populations, including Australia, achieved a significant level of herd immunity during the first wave, and community susceptibility was further reduced by vaccination programs, although coverage was lower than expected. In the absence of significant antigenic drift or changes in virulence, the impact of the pandemic H1N1 virus should continue to decline in future influenza seasons.