A paper published on bmj.com reports that there is a need for precise estimates of the severity of the new H1N1 virus. Experts need to assess in particular how many deaths might arise over the course of the pandemic. This will be the focus of healthcare planning over the upcoming months.
Those forecasts will be helpful in deciding whether to put into practice social distancing measures, such as school closures.
Initially, information suggests that this new Influenza A (H1N1) virus is relatively mild. The case fatality ratios which are the total number of deaths due to the disease divided by the total number of cases are around 0.5 percent. This is comparable to the upper range of that seen for seasonal influenza, and it shows relatively low hospitalization rates.
On the other hand, severity appears to fluctuate considerably among countries. Fatal cases have been much younger than for seasonal influenza.
Researchers at the Imperial College London studied the complexity of assessing the severity of the new virus. They indicate that in most infectious diseases there is a risk of predisposition that leads towards diagnosis of more severe and hospitalized cases. This results in overestimation of the case fatality ratio.
However, some deaths caused by flu might not be recognized as such. Flu infections can momentarily amplify the risk of vascular death, such as heart attacks and strokes. As a result, there can be an underestimation of the case fatality ratio.
The time delay between disease onset and death is another main cause of bias. The authors explain this can lead to an underestimate of the case fatality ratio, especially in the early phase of an epidemic.
The authors mention: “In order to get a clear picture of the severity of the H1N1 virus, it is vital to take these and other factors into account.” They recommend study designs and statistical analysis methods to develop our capability in obtaining reliable case fatality ratios.
They say in closing that this is crucial and that they will guarantee that any changes in the virulence of the virus are rapidly detected so that mitigation policies are applied correctly.
Dr Tini Garske, lead author of the study from the Medical Research Council (MRC) Centre for Outbreak Analysis & Modelling at Imperial College London, wrote: “Accurately predicting the severity of this swine flu pandemic is a very tricky business, and our research shows that this can only be achieved if data is collected according to well designed study protocols and analyzed in a more sophisticated way than is frequently being performed at present.”
“If we fail to get an accurate prediction of severity, we will not be providing healthcare planners, doctors and nurses, with the information that they need to ensure they are best prepared to fight the pandemic as we head into the flu season this autumn.”
“Assessing the severity of the novel influenza A (H1N1) pandemic”
Tini Garske, Judith Legrand, Christl A Donnelly, Helen Ward, Simon Gauchemez, Cristophe Fraser, Neil M Ferguson, Azra C Ghani
DOI 10.1136/bmj.b2840
bmj.com
Written by Stephanie Brunner (B.A.)