Participants will be randomised on 1:1 basis to receive 2 doses of either of the H1N1 influenza vaccines being studied. These vaccines will be given 2-3 weeks apart and the blood tests will be taken at baseline and around 3 weeks after completion of the 2 dose immunisation course. If at the start of the trial there is clinical data or a recommendation from JCVI that supports the use of a half dose of either vaccine in children under 3 years of age, this will be used for this age group and the ethics committee will be informed of this change.