LONDON, Dec 22 (Reuters) - Britain on Wednesday said it would start vaccinating vulnerable children aged five to 11 against COVID-19 after the country"s medicines regulator approved the use of a lower dose of the Pfizer-BioNTech shot in that age group. The children will receive two 10-microgram doses of the Pfizer-BioNTech vaccine - a third of the adult dose - with an interval of eight weeks between the first and second doses, the Joint Committee on Vaccination and Immunisation (JCVI) said. A decision on whether to offer vaccination in general to younger children would be taken after additional data on the rapidly spreading Omicron variant and the effect of immunising young children could be considered, the JCVI said. "The majority of children aged 5 to 11 are at very low risk of serious illness due to COVID-19," the JCVI"s chair of COVID-19 immunisation, Wei Shen Lim, said. "However, some 5 to 11 year-olds have underlying health conditions that put them at higher risk, and we advise these children to be vaccinated in the first instance." The JCVI also advised that booster shots be offered to 16 and 17 year-olds and those aged between 12 and 15 who were in a clinical risk group in response to the Omicron variant.