This today in the NZ Herald. Swine flu more infectious than 1918 flu, study finds
Swine flu is more infectious than both seasonal influenza and the 1918 Spanish flu, a study based on New Zealand epidemic case numbers has found.
And this suggests nearly 79 per cent of an affected population will catch the virus, although not all will become ill.
The researchers, from Otago University at Wellington and Utrecht University in the Netherlands, say in a letter published in today's New Zealand Medical Journal that it was estimated early in the swine flu pandemic that the transmission rate was around 1.5. This means that every two people who are sick with the virus will infect three others.
This was calculated in Mexico, where the virus first emerged.
But Associate Professor Michael Baker, Dr Nick Wilson and Dr Hiroshi Nishiura estimate from New Zealand data that every case is infecting nearly two others - a rate of 1.96.
This is based on official notifications of swine flu between June 2 and 15. This was a period of exponential growth in the New Zealand epidemic. It began with the first case of local transmission, excludes 63 imported cases, and ends before health authorities, because so many people were catching the virus, gave up testing every potential case.
In Japan a transmission rate of between 2 and 2.6 was calculated.
The Medical Journal letter notes that a transmission rate of 1.96 is higher than published estimates for seasonal influenza in temperate-zone countries like New Zealand. It is also "slightly greater than that of Spanish influenza pandemic from 1918-19 in New Zealand", but the death rate from Spanish flu was much higher than in the current swine flu pandemic.
Drs Baker and Wilson have estimated the death rate from swine flu in developed countries could be as low as one death out of 10,000 cases.
Of course, that is only a little reassuring, since so much of our trade is with many coutnries that don't fit that criterion. A lot of oil comes out of Nigeria, food products from SE Asia, timber from Indonesia (yeah, probably dodgy) and so it goes.
If you want to get a little frisson of what it could look like, try this
I suspect there is some boost to these numbers because those figures are for consultations and in most years there will be many fewer consultations than actual infections, simply because in normal years we expect to get a flu and we expect that only a tiny proportion will need care beyond bed rest.
The high numbers of consultations certainly reflects the novelty of the virus. One sniff and we are on the horn to the Dr or turning up to the care centres. Given that it is not yet especially serious for most of us, and that we are taking it very seriously (as I believe we should) the numbers may look a lot more startling than they are in relation to actual infections in a normal year. The impact on the economy is also still invisible.
But it sure as hell will get people's attention. And like the northern hemisphere we have just had school holidays (they went back to school last Monday) so its very possible that there will be a steepening of the curve again as the kids pass it round more efficiently.
The time scale is also going to be interesting. In 5 weeks we will be into spring and temperatures will rise from averages around 12 for Auckland to about 15/16 (Celsius) and we are already seeing some beginnings of the kick up with unseasonable warmth in a few places.
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