* AstraZeneca says on track to deliver 2 million weekly shots * Industry bodies cautious about changing dose regimes LONDON, Jan 13 (Reuters) - Britain is targeting a 24-hour, 7-day a week vaccination programme as soon as possible, Prime Minister Boris Johnson said on Wednesday, as he bids to step up the pace of the rollout and give shots to at-risk groups by mid-February. AstraZeneca executives said the company was on track to deliver 2 million doses a week before mid-February, and Johnson said the availability of more shots would be crucial to scaling up to a round-the-clock service. “Weâ...