New evidence in infective and cardiovascular medicine

Almost 100,000 cases had been reported worldwide (most deaths in the American continents). The case fatality ratio (around 0.5%, similar to the upper range of that seen for seasonal influenza) vary substantially between countries and deaths have occurred in younger patient than is the case for seasonal flu. A natural definition for the case fatality ratio is the ratio of the total number of deaths from a disease divided by the total number of cases. In a fully ascertained (and complete) epidemic, this simple method works perfectly. However, people who are asymptomatic or have mild infection will be less likely to present to health care, and if they do present they will be less likely to be tested and confirmed. It is therefore likely that there will be a bias towards diagnosis of more severe cases, with the result that the case fatality ratio and other measures of severity are overestimated. A second important source of bias arises from the delay between disease onset and knowledge of the final outcome in severe cases. This effect, known statistically as censoring, means that a case fatality ratio estimated crudely by dividing the cumulative number of reported deaths by the cumulative number of reported cases will be too low and will grow as the epidemic unfolds. This was observed during the severe acute respiratory syndrome (SARS) epidemic, causing concern that the virus was mutating to become more virulent. Strategies to account for these biases based on analysis of the SARS epidemic are presented. Quantitative estimates of the severity of the new influenza A/H1N1 virus are central to healthcare planning over the coming months.