Better estimates of A(H1N1) flu virus severity needed, say experts

14 July 2009

Accurate estimates of the severity of the new H1N1 virus, and in particular how many deaths might arise over the course of the pandemic, are central to healthcare planning over the coming months, say experts in a paper published on today.

They will also help to influence decisions on whether to implement social distancing measures such as school closures.

At first sight, the data appear to imply that this new Influenza A (H1N1) virus is relatively mild, with case fatality ratios (the total number of deaths due to the disease divided by the total number of cases) around 0.5%, similar to the upper range of that seen for seasonal influenza, and relatively low hospitalisation rates.

However, severity appears to vary substantially between countries and fatal cases have been much younger than for seasonal influenza.

So researchers at Imperial College London analysed the difficulties in assessing the severity of the new virus. They report that in most infectious diseases there is a risk of bias towards diagnosis of more severe and hospitalised cases, which overestimates the case fatality ratio.

In contrast, some deaths caused by flu might not be recognised as such, as flu infections can temporarily increase the risk of vascular death, such as heart attacks and strokes. This causes an underestimation of the case fatality ratio.

Another important source of bias arises from the time delay between disease onset and death, which in the early phase of an epidemic can lead to an underestimate of the case fatality ratio, they add.

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, say the authors. They propose study designs and statistical analysis methods to improve our ability to obtain reliable case fatality ratios.

Critically, they will ensure that any changes in the virulence of the virus are rapidly detected so that mitigation policies are applied appropriately, they conclude.

Dr Tini Garske, lead author of the study from the Medical Research Council (MRC) Centre for Outbreak Analysis & Modelling at Imperial College London, said: "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 analysed 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."

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