Incidence of pandemic (H1N1) 2009 influenza infection in children and pregnant women during the 2009 influenza season in Western Australia - a seroprevalence study

Objective: To determine antibody levels and estimate incidence of infection with pandemic (H1N1) 2009 influenza in children and pregnant women during the 2009 winter in Western Australia. Design, setting and participants: Two cross-sectional serosurveys using stored specimens collected for unrelated pathology testing, from before and after (3 August to 30 November 2009) circulation of the pandemic virus, and before commencement of the pandemic vaccination program. Specimens were from three groups: children aged 1-4 years, older children and teenagers aged 5-19 years, and pregnant women aged 21-45 years. The groups were geographically representative of the WA population. Main outcome measures: Reactivity against pandemic (H1N1) 2009 and seasonal A(H1N1) influenza viruses measured using haemagglutination inhibition (HI) assays. Results: Antibody titres were determined for 648 individuals in the prepandemic period and 736 in the postpandemic period. In the prepandemic period, HI titres >= 40 against the pandemic virus were found in 0 (95% CI, 0.0%-1.6%) children aged 1-4 years, 8.3% (95% CI, 5.3%-12.7%) of older children and teenagers, and 4.5% (95% CI, 2.4%-8.3%) of pregnant women. In postpandemic specimens collected from 1 September 2009 (when influenza activity had declined to near-baseline levels), estimated infection rates (subtracting prepandemic levels) were 25.4% (95% CI for difference, 18.6%-33.4%) in 1-4-year-old children, 39.4% (95% CI, 29.8%-48.5%) in older children and teenagers, and 10.2% (95% CI, 4.1%-17.1%) in pregnant women. Conclusions: A quarter of preschool children and about 40% of school-aged children and older teenagers had serological evidence of pandemic influenza infection during winter 2009, indicating high levels of mild or asymptomatic infection. The infection rate in pregnant women was much lower. The high infection rates in children help explain the reduced impact of the pandemic virus during the 2010 winter. Augmented by vaccination, there should be sufficiently high levels of immunity in the Australian population to significantly reduce the impact of the virus in future influenza seasons.