There have been no direct comparative studies between vaccines as yet. The Joint Committee on Vaccination and Immunisation (JCVI) says Phase 3 trials have indicated that “efficacy against symptomatic disease for the Pfizer-BioNTech vaccine was higher than for the AstraZeneca vaccine,” but study protocols differed. Available data indicates a high efficacy for both, “including protection against severe disease and encouraging results in older adults”.13 
While JVCI “does not advise a preference for either vaccine in any specific population”, storage and distribution conditions for the BioNTech/Pfizer vaccine may mean that “in some populations, the AstraZeneca vaccine is the only vaccine which can be deployed rapidly, and without substantial vaccine wastage”. 
In each case, the second vaccine dose should be with the same vaccineas for the first dose. However, as there is “a high priority on promoting rapid, high levels of vaccine uptake among vulnerable persons … delivery of the first dose to as many eligible individuals as possible should be initially prioritised over delivery of a second vaccine dose”. On 31 December 2020, the NHS said that people receiving their first vaccination should have an appointment for the second dose after 3 weeks (BioNTech/Pfizer) or 4 weeks (Oxford/AstraZeneca), and before 12 weeks after the first, “with most recipients to be booked in the last week of that 12-week period”.14