Abstract
BackgroundDuring the influenza pandemic of 2009 estimates of symptomatic and asymptomatic infection were needed to guide vaccination policies and inform other control measures. Serological studies are the most reliable way to measure influenza infection independent of symptoms. We reviewed all published serological studies that estimated the cumulative incidence of infection with pandemic influenza H1N1 2009 prior to the initiation of population-based vaccination against the pandemic strain.Methodology and Principal FindingsWe searched for studies that estimated the cumulative incidence of pandemic influenza infection in the wider community. We excluded studies that did not include both pre- and post-pandemic serological sampling and studies that included response to vaccination. We identified 47 potentially eligible studies and included 12 of them in the review. Where there had been a significant first wave, the cumulative incidence of pandemic influenza infection was reported in the range 16%–28% in pre-school aged children, 34%–43% in school aged children and 12%–15% in young adults. Only 2%–3% of older adults were infected. The proportion of the entire population infected ranged from 11%–18%. We re-estimated the cumulative incidence to account for the small proportion of infections that may not have been detected by serology, and performed direct age-standardisation to the study population. For those countries where it could be calculated, this suggested a population cumulative incidence in the range 11%–21%.Conclusions and SignificanceAround the world, the cumulative incidence of infection (which is higher than the cumulative incidence of clinical disease) was below that anticipated prior to the pandemic. Serological studies need to be routine in order to be sufficiently timely to provide support for decisions about vaccination.
Highlights
A novel pandemic influenza virus, influenza A H1N1 2009, was identified in North America in April 2009 and rapidly spread around the world [1]
Around the world, the cumulative incidence of infection was below that anticipated prior to the pandemic
An early serological study in the US suggested that most of the population would likely be susceptible to infection with the new virus, but that older people may have had some protection against pH1N1 [2]
Summary
A novel pandemic influenza virus, influenza A H1N1 2009 (pH1N1), was identified in North America in April 2009 and rapidly spread around the world [1]. When anticipating the arrival of pandemic specific vaccines later in 2009, a key public health priority at that time was to identify the optimal target groups for vaccination This depended on the course of the pandemic, on the proportion of children that had been infected, as well as the groups in the population considered to be most at risk of an adverse outcome. An early serological study in the US suggested that most of the population would likely be susceptible to infection with the new virus, but that older people may have had some protection against pH1N1 [2]. Consistent with this suggestion were the early epidemiologic observations of higher clinical attack rates in children [3]. We reviewed all published serological studies that estimated the cumulative incidence of infection with pandemic influenza H1N1 2009 prior to the initiation of population-based vaccination against the pandemic strain
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