Covid-19.The two vaccines currently being injected among the country's priority groups, Pfizer/BioNTech and Oxford/AstraZeneca, both require a booster shot a few weeks later.Initial advice for the second dose to be delivered within 21 days was modified by the UK government scientists to up to 12 weeks apart, with the aim of vaccinating larger numbers with at least one jab to inject some level of immediate protection.The government's Joint Committee on Vaccination and Immunisation (JCVI) claims unpublished data suggest that the Oxford-AstraZeneca vaccine is still effective with doses 12 weeks apart.But American pharma major Pfizer has said it has tested its vaccine's efficacy only when the two doses were given up to 21 days apart, creating.