A trial of new swine flu vaccines has seen the first British children receive the jabs.
Two different drugs are being tested and researchers aim to use approximately 1,000 youngsters aged six months to 12 years to see which drug performs the best.
The test venues are located in Exeter, London, Southampton, Bristol and Oxford.
Both medicines have been considered for approval by European regulators, although only Pandremix, made by GlaxoSmithKline, has already been approved. The other, made by Baxter, could be approved soon.
Head researcher Professor Andrew Pollard, from Oxford University, told the BBC: “We are sort of in a race against time because we know the flu season is already started. We hope to immunise the children over the next 10 days or so.”
The most recent data from the government shows the number of swine flu cases across the country almost doubled in a week, from an estimated 5,000 to 9,000, prompting speculation a second wave of infections could be on its way.
The government is hoping to start vaccinating millions of people in high risk groups, such as those with asthma and diabetes, and health workers, next month.
Across the UK, 82 deaths have been linked to the virus, with 70 in England, nine in Scotland, one in Wales and two in Northern Ireland.