LONDON (Reuters) - Prime Minister Boris Johnson urged Britons on Wednesday to celebrate Christmas with “extreme caution”, saying that although the vaccination programme had got off to a very good start, asymptomatic transmission of COVID-19 still posed a threat. “(He) is absolutely right to stress the importance of people taking care because although some things are unquestionably going well ... and I am very pleased to tell the House (of Commons) that we’ve had a very good start with the roll out of the vaccination programme ... we should exercise extreme caution in the way we celebrate Christmas,” he told parliament when asked whether people should be cautious in their celebrations.