Healthcare planners may have been given misleading figures of the potential number of swine flu deaths, say the authors of new research published in the British Medical Journal.
The proportion of people who die due to infection during an influenza outbreak – known as the case fatality ratio – is calculated by dividing the number of deaths by the total number of cases in the same time period.
Early data from the current swine flu pandemic suggested that the new influenza A (H1N1) virus causes mild disease, with case fatality ratios of around 0.5%, or 5 deaths per 1000 people infected.
However, the researchers from the MRC Centre for Outbreak Analysis & Modelling at Imperial College London, say this ratio may not be accurate.
The researchers suggest that this inaccuracy could be because the total number of deaths is being underestimated as the cause of death is not correctly attributed to swine flu.
Other suggested reasons are that the number of cases are underestimated as people with milder symptoms may not be tested or visit a doctor at all, leaving only the most severe to be reported, or that the ’snapshot’ calculation does not take account of the time delay between infection and death, leading to the false impression that the infection is actually becoming more severe as the pandemic progresses.
Dr Tini Garske, from the MRC Centre 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.”