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Warning on swine flu death predictions
15 Jul 2009The Government needs to adopt more stringent methods of monitoring swine flu to better predict the number of deaths, researchers said today.
Current methods can either overestimate or underestimate the numbers, making it difficult to plan health services for those affected, they said.
Writing in the British Medical Journal (BMJ), the researchers from Imperial College London said a main source of bias was that many people with swine flu are not visiting their GP and are looking after themselves at home.
Working out a death rate based on the numbers of people dying compared with the number of cases officially being seen fails to take into account these people, they said.
Therefore, the actual number of people dying compared with the total number of cases will be lower than current figures suggest, they said.
As swine flu spreads, only a proportion of people are being tested and recorded as having the flu, which adds extra uncertainty.
"Initially increased awareness by patients and doctors may lead to high ascertainment, but as cases increase and systems are overwhelmed, only a proportion will be tested (potentially those with links to other confirmed cases), making it difficult to understand the scale of under-reporting," the researchers said.
On the flipside, they argued, people could be dying of swine flu in hospital but their cause of death is then listed as something other than the virus.
This could mean the death toll is actually higher than is currently being reported.
So far, 17 people have died after contracting the virus, although it is unclear how many have died as a direct result of the virus and how many had underlying health problems.
Professor Azra Ghani, one of the researchers behind today's study, said the number of people who have died "could well be higher" than 17.
She added that Health Secretary Andy Burnham's declaration that 100,000 people a day could be diagnosed with swine flu by the end of August is a "reasonable" assessment.
Current estimates are that 0.5% - or one in 200 - people who are ill enough to seek medical help will die as a result of swine flu.
But the numbers appear to be varying substantially between different countries.
Fellow researcher Professor Christi Donnelly said there was "no evidence" that the flu was getting worse or mutating into a more virulent strain.
Current modelling could give the impression that things were getting worse when in fact they were not, she said.
On the issue of underlying health conditions, she said normal seasonal flu usually affects elderly people who are frail and who are not expected to live for many more years.
In contrast, some of the victims of swine flu - such as 38-year-old Jacqui Fleming, of Glasgow, who died after giving birth prematurely - would have been expected to have a long life.
The researchers said another source of bias in current modelling was that no account was taken of the "time delay" between the number of people with the flu and the numbers dying.
Crudely dividing the reported number of deaths by the number of people with the virus will result in an inaccurate ratio, they said.
"The delay between symptom onset and death means that dividing the total number of deaths by the total number of cases greatly underestimates the true case fatality ratio throughout much of the epidemic.
"This was observed during the severe acute respiratory syndrome (SARS) epidemic, causing concern that the virus was mutating to become more virulent."
Dr Tini Garske, lead author of the study, 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."
