Swine flu in the UK – how reliable are the numbers?

So how exactly do we know that there are – as the Health Protection Agency says -around  55,000 new cases of H1N1 “swine” flu each week – especially now that we are no longer swabbing patients before prescribing for it?
Extrapolation. The HPA does give a range, between 30,000 and 85,000 cases. There are about 100 general practices in England that work with the HPA to track how many consultations about “flu-like” illnesses there are each week. These “surveillance” practices were set up long before the current outbreak, and have been quietly providing flu data for many years. HPA weekly pandemic flu update

The population covered by these surveillance practices is about 900,000. This means that, for the English population of 61 million, the sample is about 1.5 per cent of the total.
In Wales, between 30 and 70 per cent of practices contribute data on how many patients with flu-like illness they have seen, but they also have a group of practices who are formally involved in surveillance: GP Surveillance of Infections in Wales Report. The sample size is about 10%.

In Scotland, the number of practices involved in surveillance has just increased – with 55 practices taking part –  Weekly Situation Report Influenza A H1N1v – this means that 6% of the population there are sampled. 

While these practices are recording how many consultations have been about flu-like illnesses, this doesn’t tell us how many of these cases are actually H1N1 flu. Currently, we are in the “treat” stage, meaning that swabbing is not done prior to presumptive treatment with antivirals, should they be needed. The flip side of this is that we have less information about how much H1N1 is going about.
A few weeks ago in my area of Scotland, when we were swabbing everyone who had symptoms of flu, the pick up rate of H1N1 was about 8%. We are not swabbing routinely now, so I don’t know if this has changed. Only a minority of English surveillance practices are continuing to swab routinely.

If you look at page 6 of the HPA Weekly National Influenza Report, week 30 you can see from table 3, in week 28, there were 271 swabs done, with 24.7 per cent positive for pandemic flu, and in week 29, 137 swabs, with 19.7 per cent testing H1N1 positive. Extrapolating up from small sets of figures is fraught with difficulty.  
I don’t want it to seem that I don’t think that H1N1 is important. It is, but perspective, especially about the uncertainty inherent in managing it, is important. And bear in mind that this is only one infection of many. This year, the WHO  have reported 4,276 deaths from cholera in Zimbabwe, 1,513 deaths mainly in northern Nigeria and the Niger area from meningococcal meningitis, and children still die of measles: an estimated 197,000 a year.

The difference is, of course, that while we wait for a vaccine for H1N1, which will almost certainly be available to high-income countries only. In developing countries, despite current efforts, people are still dying of preventable illnesses

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