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During the 2009 H1N1 influenza epidemic, the total number of symptomatic cases was estimated by combining influenza-like illness (ILI) consultations, virological surveillance and assumptions about healthcare-seeking behavior. Ellen Brooks-Pollock of the Centre for Mathematical Modeling of Infectious Diseases at the London School of Hygiene and Tropical Medicine, and colleagues sought to improve estimates of the number of symptomatic H1N1 cases and case fatality rate (CFR) in England by quantifying healthcare-seeking behavior using an Internet-based survey carried out during the course of the 2009 H1N1 influenza epidemic.
The researchers used an online survey that ran continuously from July 2009 to March 2010 to estimate the proportion of ILI cases that sought healthcare during the 2009 H1N1v influenza epidemic. They also used dynamic age- and gender- dependent measures of healthcare-seeking behavior to re-interpret consultation numbers and estimate the true number of cases of symptomatic ILI in 2009 and CFR.
There were significant differences between age groups in healthcare usage. From the start to the end of the epidemic, the percentage of individuals with influenza-like symptoms who sought medical attention decreased from 43 percent to 32 percent (p<0.0001). Adjusting official numbers accordingly, the researchers estimate that there were 1.1 million symptomatic cases in England, over 320,000 (40 percent) more cases than previously estimated and that the autumn epidemic wave was 45 percent bigger than previously thought. Combining symptomatic case numbers with reported deaths leads to a reduced overall CFR estimate of 17 deaths per 100,000 cases, with the largest reduction in adults.
The researchers conclude in BMC Infectious Diseases that active surveillance of healthcare-seeking behavior, which can be achieved using novel data collection methods, is vital for providing accurate real-time estimates of epidemic size and disease severity. They add that the differences in healthcare-seeking between different population groups and changes over time have significant implications for estimates of total case numbers and CFR.
Reference: Brooks-Pollock E, Tilston N, Edmunds WJ and Eames KTD. Using an online survey of healthcare-seeking behaviour to estimate the magnitude and severity of the 2009 H1N1v influenza epidemic in England. BMC Infectious Diseases 2011, 11:68doi:10.1186/1471-2334-11-68.