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The level of herd immunity before and after the first 2009 pandemic season is not precisely known, and predicting the shape of the next pandemic H1N1 season is a difficult challenge. Carrat, et al. (2010) conducted a modeling study based on data on medical visits for influenza-like illness collected by the French General Practitioner Sentinel network, as well as pandemic H1N1 vaccination coverage rates, and an individual-centred model devoted to influenza.
The researchers estimated infection attack rates during the first 2009 pandemic H1N1 season in France, and the rates of pre- and post-exposure immunity. They then simulated various scenarios in which a pandemic influenza H1N1 virus would be reintroduced into a population with varying levels of protective cross-immunity, and considered the impact of extending influenza vaccination.
During the first pandemic season in France, the proportion of infected persons was 18.1 percent overall, 38.3 percent among children, 14.8 percent among younger adults and 1.6 percent among the elderly. The rates of pre-exposure immunity required to fit data collected during the first pandemic season were 36 percent in younger adults and 85 percent in the elderly. The researchers estimated that the rate of post-exposure immunity was 57.3 percent overall, 44.6 percent in children, 53.8 percent in younger adults, and 87.4 percent in the elderly. The shape of a second season would depend on the degree of persistent protective cross-immunity to descendants of the 2009 H1N1 viruses. A cross-protection rate of 70 percent would imply that only a small proportion of the population would be affected. With a cross-protection rate of 50 percent, the second season would have a disease burden similar to the first, while vaccination of 50 percent of the entire population, in addition to the population vaccinated during the first pandemic season, would halve this burden. With a cross-protection rate of 30 percent, the second season could be more substantial, and vaccination would not provide a significant benefit.
The researchers conclude that model-based findings should help to prepare for a second pandemic season, and highlight the need for studies of the different components of immune protection. Their research was published in BMC Infectious Diseases.
Reference: Carrat F, Pelat C, Levy-Bruhl D, Bonmarin I and Lapidus N. Planning for the Next Influenza H1N1 Season: A Modelling Study. BMC Infectious Diseases 2010, 10:301doi:10.1186/1471-2334-10-301.