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Influenza

The VMI has been actively engaged in the development of computational models and simulations of H5N1 and H1N1 pandemic influenza dynamics to help country governments and the World Health Organization with policy making on pandemic control strategies.

picture of thailand influenza modelsInvestigators of the VMI have a longstanding experience in the development of computational models and simulation of influenza dynamics to support public health policy making.Various models have been published on potential spread of H5N1 pandemic influenza in Thailand and the US and a range of control policies have been evaluated including school closures, antiviral medication and vaccination.

During the past year, the VMI was heavily involved in real-time analysis and modeling of the H1N1 influenza pandemic and advising on control measures (including vaccines, antivirals, face masks and school closure). The group worked closely with the WHO, the UK government, the CDC and the U.S. government more generally, the EU and the Chinese Centers for Disease Control.

Work on the H1N1 pandemic was done in collaboration with WHO and colleagues in the Mexican Ministry of Health and included the analysis of the early spread of the epidemic in Mexico and later international spread. This analysis estimated R0 to be in the range 1.2-1.7, with best estimate around 1.4. A genetic analysis dated origin of epidemic to January 2009 (range September 2008-March 2009). picture of us influenza modelThese studies demonstrated that there was at least a two-fold difference in susceptibility to infection between adults and children, and that the generation time distribution of the new pandemic virus was comparable to seasonal influenza. From the extent of international spread by the end of April 2009, it was estimated that there were at least 30,000 cases in Mexico, which allowed an upper bound on the case fatality ratio of 0.5%. An early version of this analysis informed the WHO decision to move to pandemic phase 5.

In collaboration with the UK Health Protection Agency, early stages of UK H1N1 epidemic were assessed and epidemic indicators estimated that assisted policy making on control strategies. It was found that school based transmission dominated early in the epidemic. Prompt antiviral treatment/prophylaxis significantly reduced the probability of influenza like illness in contacts of confirmed cases. An analysis of household transmission in the U.S. found an average secondary attack rate for influenza like illness in household contacts of 13% with one week of follow-up, with a strong dependence on household size. The mean serial interval was estimated to be 2.9 days. Children ≤18 yr were estimated to be two-fold susceptible to infection than 19-50 yr old adults, and adults over 50 were estimated to be less than half as susceptible.

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To assist policy making, VMI staff undertook a simulation study for the U.S. CDC to examine the extent to which reactive school closure using absenteeism based triggers could be used to mitigate the autumn wave of the pandemic. Also in collaboration with the US CDC, an analysis of very detailed data on a school-based outbreak of H1N1 influenza in Pennsylvania was conducted and another study on the generation time distribution of H1N1 influenza, pooling data from seven separate studies undertaken in different states in April-June 2009. Analyses of the H1N1 epidemic in China in collaboration with China’s CDC are ongoing.

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