With doses administered four weeks apart, research led by the University of Oxford found that results of the mixed vaccination schedule varied depending on the order in which they were administered.
An Oxford/AstraZeneca jab followed by a dose of the Pfizer/BioNTech vaccine induced 'higher antibodies and T-cell responses' than doses of the two products administered the other way around.
Vaccination in either order produced higher antibodies than a two-dose Oxford/AstraZeneca schedule, according to the study published on the Lancet pre-print server.
The highest antibody response of any combination was seen after the two-dose Pfizer/BioNTech schedule - but the highest T-cell response occurred after Oxford/AstraZeneca followed by Pfizer/BioNTech.
Chief researcher Professor Matthew Snape, associate professor in paediatrics and vaccinology at the University of Oxford said: ‘The Com-COV study has evaluated “mix and match” combinations of the Oxford and Pfizer vaccines to see to what extent these vaccines can be used interchangeably, potentially allowing flexibility in the UK and global vaccine rollout.
‘The results show that when given at a four-week interval both mixed schedules induce an immune response that is above the threshold set by the standard schedule of the Oxford/AstraZeneca vaccine.’
Professor Snape said the findings showed that mixed dose schedules could be useful options for the UK vaccination programme.
He said: ‘These results are an invaluable guide to the use of mixed-dose schedules, however the interval of four weeks studied here is shorter than the eight to 12-week schedule most commonly used for the Oxford/AstraZeneca vaccine. This longer interval is known to result in a better immune response and the results for a 12-week interval will be available shortly’.
Deputy CMO Professor Jonathan Van-Tam said: ‘Today’s data are a vital step forward, showing a mixed schedule gives people protective immunity against COVID-19 after four weeks.
‘Equally, they offer supportive evidence that the standard (non-mixed) JCVI recommendations for COVID-19 vaccination all produce highly satisfactory immune responses, for both main vaccines in use. Given the UK’s stable supply position there is no reason to change vaccine schedules at this moment in time.'
But he added: 'The results for the 12-week interval, which are yet to come, will have an instrumental role to play in decisions on the future of the UK’s vaccination programme.'