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Swine flu: The US experience

Fergus Walsh | 11:47 UK time, Friday, 5 June 2009

Some interesting figures have emerged from the United States about the progress there of H1N1 swine flu.

Dr Anne Schuchat, Director of the National Center for Immunization and Respiratory Diseases gave a media update at a CDC press briefing.

The US has far more cases of H1N1 swine flu than anywhere else so they are beginning to get some more robust statistics which may help in our understanding of how this virus will affect the world. Dr Schuchat said there was evidence that flu activity was declining in the nation as a whole, with the start of the summer in North America.

Man wearing US flag mask

Young children together with young and middle age adults were those most affected by H1N1 flu. Unlike seasonal flu, the virus seems to be sparing the elderly. Here are the statistics - the US has:

• 11,468 probable and confirmed cases of H1N1 swine flu (it's worth remembering that laboratory confirmed cases in the US represent only a small fraction of the real total)
• 770 people have been hospitalised
• 19 known fatalities
• 60% of all cases are among people aged 5-24 years
• 42% hospitalisations are among people aged 5-24 years
• Next highest group are children under 5
• Very low rates of cases and hospitalisations in people over 65

Dr Schuchat said that the CDC had provided several manufacturers with candidate vaccine virus strains. This will enable them to produce pilot lots of vaccine which can be tested for safety and efficacy. That will probably take a couple of months. At that point, the WHO and CDC will have to make the difficult decision about whether to ask manufacturers to bulk produce an H1N1 pandemic vaccine.

It's a difficult call because it would mean reducing the amount of seasonal flu vaccine production. Given that manufacturers are mid-way through the production of seasonal flu vaccine for the northern hemisphere this autumn, those stocks are unlikely to be affected. It is next year's immunisation in the southern hemisphere which would probably feel the first impact of any switch to pandemic flu vaccine production.

Dr Schuchat pointed out that seasonal flu caused around 200,000 hospitalisations in the US each year and about 36,000 deaths, so this was a disease they took very seriously. Worst affected are people over 65. She said the CDC would be recommending that the usual at-risk groups get immunised.

In the US this includes children aged 6 months to 18 years (a group not targeted in the UK), pregnant women, the elderly, those with serious health problems, and anyone who wants to reduce their risk of getting flu. I think it's a safe prediction that there will be a record uptake for the jab later this year.

So no decision has been made yet about whether to push the button on creating a mass stockpile of H1N1 vaccine in the US, or indeed the world. Dr Schuchat said they would await the outcome of test vaccine studies and carefully monitor the epidemiology of the virus. A decision, she said, would be made in the early autumn.

One encouraging fact is that tests of virus samples from around the world have not shown any variations; in other words, it doesn't look like there has been any mutation or drift in its genetic structure.

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