Schoolchildren in the UK have started to receive the swine flu vaccination as tests begin to find out which of two different drugs performs the best.
The children, aged between six months and 12 years, will attend one of five centres across the country, including London, Oxford, Bristol, Southampton and Exeter.
One of the vaccines, Pandremix, has already been granted approval for use by European regulators. The vaccine is made by GlaxoSmithKline.
The second drug, made by Baxter, is still being considered for use by the European Medical Agency, but is expected to get approval within a week.
Professor Andrew Pollard, of the University of Oxford, said the trial on around 1,000 children will need to be completed in a 10-day period. He said: "We are sort of in a race against time because we know the flu season is already started."
Fears have been growing that a second wave of swine flu infections could be on the way over the autumn. The latest government data show the number of cases across the country almost doubled within a week from 5,000 to 9,000.
From October, the government wants to start vaccinating millions of people considered at high-risk, such as those with asthma and diabetes, and health workers.
"Don't forget that the drug companies involved have been granted immunity from prosecution if the testing goes wrong and more kids die through 'safe' innoculation" - Sheddy, Southend on Sea
"How many of the 14,000 supposed cases (5,000 then 9,000) were tested and diagnosed by a qualified GP, and how many were diagnosed by a medically unqualified, medically untrained call centre operative manning the NHS swine flu hotline? The numbers are being inflated through deception to scare people into having the vaccine I think" - J Garratt, Northamptonshire