What are the chances of dying from swine flu?
The recent deaths of people with H1N1 swine flu especially that of a six-year-old girl in London, has led many to perceive that the virus may be getting more deadly. NHS Direct say the number of calls they've received on swine flu has increased by 50% to 9,000 a day.
The reality is that there are no indications that the virus is mutating or getting more virulent. Laboratory testing around the world indicates the H1N1 virus appears to be behaving in the same manner that it was three months ago.
The rise in the death rate is due to the huge increase in the number of cases. Even seasonal flu can, rarely, kill young children. Every winter, flu contributes to thousands of deaths, mostly of the frail elderly. The difference with H1N1 swine flu is that it is tending to affect those under 65.
Many people want to know what their chances are of dying from the virus. The simple answer is that it's very, very low. The UK has probably had tens of thousands of cases (we don't know how many - more on this in a moment) and there have been 17 deaths.
Researchers at Imperial College London have warned that accurate predictions about the death rate are not yet possible.
The researchers say the best estimate so far is that about 0.5% of those who get swine flu bad enough to seek medical help may die from it. This is what they call the case fatality rate.
Professor Azra Ghani, an epidemiologist at Imperial, explained that this could be very wide of the mark:
"Our best estimate, based on the cases that come to the health system is around 1 in 200 (deaths) at a maxiumum which is very similar to the estimate you would see for seasonal influenza but that doesn't take into account many of those milder infections where individuals may stay off for a few days or not display any symptoms at all."
It's worth reiterating that this case fatality rate doesn't include a huge group - people who get sick and never seek medical help, nor those who get infected but are without symptoms.
If there is a sudden surge in the numbers seeking medical help because they perceive the virus is more dangerous, it would change the case fatality rate.
The researchers also point out that some deaths where swine flu was a contributory factor may be missed because it is not always tested for.
They want more accurate mapping of the actual numbers getting the virus. This would involve community testing of representative households and combining this with hospital reports of sickness.
For those of you who like the raw data, I'm attaching one of the tables presented by the disease mappers at Imperial. I'm indebted to them and to the British Medical Journal for permission to reproduce it here. The crude and adjusted case fatality ratios show that estimates in the worst affected countries vary very widely.