The Age-Specific Cumulative Incidence of Infection with Pandemic Influenza H1N1 2009 Was Similar in Various Countries Prior to Vaccination

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.

Estimating age‐specific cumulative incidence for the 2009 influenza pandemic: a meta‐analysis of A(H1N1)pdm09 serological studies from 19 countries

“…Our results are consistent with our estimates of H1N1pdm seroprevalence using post‐pandemic sera and with other H1N1pdm seroprevalence studies recently or not yet published from Iceland, Mexico, Chinese Taipei, India, Mongolia, Mali,53, 54, 55, 56, 57 Laos, Djibouti, and Bolivia (CoPanFlu‐International consortium unpublished data, personal communication from X. de Lamballerie), with a study from Greece with pre‐ and post‐pandemic sera that was excluded from our analyses51 but are slightly higher than the overall estimated cumulative incidence found in one analysis 17. This may be because our study includes a number of additional middle and low countries who conducted serologic studies because this analysis was published and because we excluded studies which focused on specialized populations17).…”

“…This study builds upon the findings of Kelly et al 17. by including a number of additional H1N1pdm serologic studies conducted from a number of additional countries since this publication.…”

The dynamics of infection and the persistence of immunity to A(H1N1)pdm09 virus in Israel

“…It is noteworthy that a recently published study,2 which compared many sero‐epidemiological surveys published after the 2009 pandemic, suggests that CI calculations based on the binomial distribution led to underestimation of the uncertainty associated with estimates of the cumulative incidence of infection. However, the overall conclusions remain the same.…”

“…Transmission, case‐fatality and hospitalization rates can only be evaluated when the true incidence of infection is assessed, based on sero‐epidemiological studies. A recent review concluded that exposure to the virus based on comparison between seroprevalence in samples collected before and after the pandemic was highest in children and teenagers reaching 16–43% globally 2. Pre‐pandemic seroprevalence was highest in age‐groups >60 years 1…”

Health at a Glance 2013

“…The H1N1 influenza virus (also referred to as "swine flu") infected an estimated 11% to 18% of the population (Kelly et al, 2011). Mexico was at the centre of the pandemic, being among the first countries where swine flu was detected and also where mortality rates were reportedly higher than those in many other countries (Echevarría-Zuno et al, 2010).…”