LONDON, Nov 29 (Reuters) - Britain will offer a COVID-19 booster vaccine to all adults and give second doses to children aged between 12 and 15, the UK"s top vaccine advisers said on Monday, accelerating shots in light of concern about the spread of the Omicron variant. The Joint Committee on Vaccination and Immunisation said that all adults between 18-39 years old could receive shots, extending a programme that is already open for over 40s. The JCVI also said that the gap between second doses and boosters could be shortened to 3 months from 6 months, in response to the changing risk posed by Omicron. "Having a booster dose of the vaccine will help to increase our level of protection against the Omicron variant," said Wei Shen Lim, the JCVI"s Chair for COVID-19 immunisation.JCVI reiterated that they advised a largely age-based approach to the booster programme, with older adults as well as vulnerable people prioritised for shots. There have been nine cases of the Omicron variant reported in Britain, and scientists are concerned that it has mutations linked with higher transmissibility and a possible reduction in the effectiveness of vaccines. Prime Minister Boris Johnson on Saturday had instructed the JCVI to review the booster programme urgently given the evolving situation with Omicron after the first cases of the variant were reported. Though Johnson"s government controls health policy in England alone, JCVI has informed the rollout of COVID-19 vaccines in Scotland, Wales and Northern Ireland. The panel reiterated that Moderna (MRNA.O) and Pfizer"s vaccine were the preferred vaccines to use in booster shots.