Two swine flu vaccines were compared for effectiveness
A trial of swine flu vaccines given to children in Southampton and Oxford found they provided "good protection" against the virus.
The Health Protection Agency conducted the study with the Universities of Bristol, Oxford, Southampton, Exeter, and St George's in London.
More than 900 children aged between six months and 12 years from the five cities were volunteered for the study.
Researchers said the children responded well and most side effects were minor.
The trial compared two types of vaccine.
Researchers said 98% of children under three responded well to two doses of the "adjuvanted" vaccine, which contained immunity boosting agents.
The adjuvanted vaccine, also called the split virus vaccine, was the most commonly used on children during the pandemic but was also the most likely to cause side effects such as fever.
Only 80% of under threes responded to two doses of the "whole virus" vaccine.
The difference between the vaccines in children over three was less pronounced, with 99% responding well to the adjuvanted version and 95% for the whole virus jab.
Dr Matthew Snape of the Oxford Vaccine Group at The University of Oxford said: "Most children receiving either vaccine had no more than minor reactions, and this study provides reassuring evidence that both vaccines were well tolerated and likely to provide good protection against swine flu."
He added: "Traditionally the under threes don't tend to respond well to flu vaccines.
"This could show us the way to improve seasonal flu vaccines and help make a vaccine that generates a better response in young children."
The study, published in the British Medical Journal today, is the first to be carried out comparing the immune response rates to the two vaccines in children aged between six months and 12 years.