My best wishes to those who have had, and will get, swine flu, and my profound sympathies to the families of those who have died from it. To them it is real. But to those responsible for counting cases and designing defences to protect the population, it is a statistical disease.
Before the virus was discovered in the early 1930s there was no other way to measure it. Influenza has no unique clinical features. There is no diagnostic rash. Its symptoms can be caused by a multitude of other microbes. Its activities could only be measured by looking at groups rather than individuals; studying death rates (making the assumption that a rise in the winter was due to flu), and counting the number of cases in a community with "typical" symptoms (likewise assuming that if there was a big rise, its cause was flu).
But rapid diagnostic laboratory tests came on stream in the 1960s. In the last two decades their sensitivity and precision has increased enormously. At the start of the swine flu pandemic they came into their own. Without them the identification of the virus as new would have taken much longer – so would the detection of its rapid spread in North America and the initiation of vaccine development within days thereafter.
But this month health departments in the UK gave up using them on any scale. They have reverted to the traditional method – primarily counting the consultations of patients suffering from flu-like symptoms with GPs. The system relies on reports from sentinel practices which are fed into statistical formulae to work out the total of "flu" cases across the country. This is how last week's estimate of 100,000 cases in the UK was generated.
In the quiet flu years like those we have enjoyed in recent times this GP scheme has worked, not by giving a precise estimate of the number of cases, which it can't because the statistical formulae are full of mathematical assumptions best described as guesses, but by showing believable trends. However, swine flu pandemic publicity has been so intense that any measure relying on calls to doctors from the worried "slightly unwell" (most swine flu cases are said to be mild) or from people wanting Tamiflu (a drug with a curative reputation of the kind that is the dream of every advertising executive) must be challenged, however sophisticated the mathematical models used to tweak the counts. When the chief medical officer, Sir Liam Donaldson, mentions on TV that as many as 65,000 might die it is hard to blame members of the public for an increase in fear, and it is reasonable to expect a reduction in their reluctance to have a consultation about a cold. And who knows what effect the new National Pandemic Flu Service (designed to take the pressure off GPs) will have on the surveillance system.
The best way to assess the accuracy of diagnosis in sentinel practices is to get them to send samples from patients for virology. But only 137 English patients were tested for this purpose last week; 27 were positive. As a sample supposedly covering the whole nation, this is pathetic. All that can be said in its favour is that the rest of the UK did no better; 12 tests were done in Wales (one positive) and 13 in Northern Ireland (one positive). There are 58 sentinel practices in Scotland. They sent in 50 samples; two were positive.
Why so few? GPs in the UK have never been enthusiastic users of virology tests, even the ones that can be used at the bedside to give a result in a few minutes, and it is likely that the small number of virology laboratories in the NHS are too busy working up samples from hospital patients to be shouting about systematic surveillance shortfalls. But the real deficiency is not a flu one. It is the failure over many years to take infection as seriously as it deserves. Typical are the struggles of the Health Protection Agency (which leads the laboratory response to the pandemic) to ensure that its many budget cuts over the years are as small as possible, and the long-term decline in medical microbiology teaching and research in our medical schools.
What next? The CMO said the optimistic estimate of deaths was 20 times less than his pessimistic 65,000. Such a big range means the experts don't know what is going to happen. The biggest worry is that the virus might take off on a grand scale when the schools go back – perhaps targeting the elderly, who have escaped so far. Maybe the virus will suddenly develop Tamiflu resistance. Time will tell. But panic should not rule. Put the pandemic into perspective. So far its lethality is remarkably low. In 1968-69, the mildest pandemic last century, the virus killed 1,000 in the UK in its first four months.
