The relatively low number of new cases created by a single case of H1N1 influenza indicates that mitigation strategies such as vaccination, social distancing and the use of antiviral drugs may help to lessen the final impact of the virus, suggests an epidemiological modelling study reported in CMAJ (Canadian Medical Association Journal).
The study looked at data from laboratory-confirmed cases of H1N1 between April 13 and June 20, 2009 in Ontario, Canada and performed 1000 simulations to estimate epidemiological parameters for the virus. These findings may be useful to policy-makers in managing the pandemic.
"Because the 2009 influenza pandemic continues to evolve, these values are critical for planning and can be used to reduce some of the uncertainty around the health burden likely to be associated with this disease in the coming months," writes Dr. David Fisman of the Dalla Lana School of Public Health, University of Toronto, and coauthors.
The study found that the median incubation period for H1N1 influenza was 4 days and the duration of symptoms 7 days. Patients aged 18 years and under recovered more quickly than older patients and the risk of hospital admission among laboratory-confirmed cases (who likely represented 1-10% of total cases) was 4.5%.
People under 1 year of age and over 65 years were at higher risk of hospital admission. Adults older than 50 years made up 7% of cases but accounted for 7 of 10 initial deaths.
According to the study's estimates, the characteristics of the H1N1 virus are similar to those of seasonal influenza. "However, when combined with high attack rates in younger groups, there may be greater absolute numbers of hospital admissions and deaths than are observed in a typical influenza season," write the authors, although this may be attributed to enhanced surveillance during a pandemic.
- Ashleigh R Tuite, Amy L Greer, Michael Whelan, Anne-Luise Winter, Brenda Lee, Ping Yan, Jianhong Wu, Seyed Moghadas, David Buckeridge, Babak Pourbohloul, David N Fisman. Estimated epidemiologic parameters and morbidity associated with pandemic H1N1 influenza. Canadian Medical Association Journal, 2009 DOI: 10.1503/cmaj.091807
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