UK children receive swine flu jab
Doctors at five sites in England are comparing vaccines from two manufacturers to see which works best and has the fewest side-effects.
Over the next two weeks 1,000 children aged six months to 12 years are being recruited in Oxford, Bristol, Southampton, Exeter and London.
Both vaccines are expected to be licensed in the coming weeks.
One has already been recommended for use.
Professor Andrew Pollard, from the University of Oxford, says vaccines are the best way to protect against flu and any side effects should be limited to sore arms or sometimes a fever.
This clinical trial comes just a month before more than 11 million people considered to be most at risk from flu will be offered the swine flu vaccine.
It will also be made available to more than two million front-line health workers.