Mixing doses of the Oxford/AstraZeneca and Pfizer/BioNTech vaccines generates a robust immune response against coronavirus, research suggests.
The study found that using Pfizer followed by Oxford or vice versa induced high concentrations of antibodies against the spike protein of the virus when doses were given four weeks apart.
It also suggests people who have had two Oxford jabs could have more antibodies if they were given a different booster.
However, more antibodies do not necessarily mean more protection against the virus.
The findings could allow flexibility in the UK and global vaccine rollouts, allowing people to receive whatever jab is available, rather than waiting for a matching one.
However, given the UK’s vaccine supply position it is unlikely the schedule will change at the moment.
Professor Matthew Snape, associate professor in paediatrics and vaccinology at the University of Oxford, and chief investigator on the trial, 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.
“The investigators would like to thank the participants that made this important study possible.”
The Com-COV study, run by the University of Oxford, found that the order of vaccines made a difference.
Oxford followed by Pfizer induced higher antibodies and T cell responses than Pfizer followed by Oxford.
But both schedules induced higher antibodies than the licensed, and highly effective, standard two-dose Oxford/AstraZeneca schedule.
The highest antibody response was seen after two doses of the Pfizer jab, and the highest T cell response from Oxford followed by Pfizer.
An Oxford jab followed by a Pfizer jab generated a stronger immune response than two doses of the Oxford vaccines, the study found.
T cells play an important role in supporting and maintaining antibody production.
Researchers also found that common side effects like feeling feverish and having a headache were more common after the mixed doses rather than the standard schedule, regardless of which way the round the jabs were administered.
Prof Snape 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.”
Prof Snape said: “When it comes to thinking about the coming winter, if a third dose were to be given – a booster dose, then I think these are really important data to inform decisions about which vaccines to be using and which combinations to be using for that third dose.”
He added: “From our study you have to be thinking that if you received AZ/AZ (Oxford), then maybe there would be advantages in getting an RNA vaccine next (Pfizer or Moderna).
Deputy chief medical officer, 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 (Joint Committee on Vaccination and Immunisation) 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.
“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.
“Our non-mixed (homologous) vaccination programme has already saved tens of thousands of lives across the UK but we now know mixing doses could provide us with even greater flexibility for a booster programme, while also supporting countries who have further to go with their vaccine rollouts and who may be experiencing supply difficulties.”
Professor Andrew Ustianowski, National Institute for Health Research clinical lead for the Covid-19 vaccination programme and joint national infection specialty lead, said: “We know that the Oxford/AstraZeneca two-dose schedule is highly effective and has helped to save many lives.
“The fact we now know it works well, in terms of immune responses, when combined with the Pfizer vaccine provides researchers with the valuable knowledge that these vaccines could be used flexibly for those yet to be vaccinated in the UK and globally.”
The Com-COV study aims to evaluate the feasibility of using a different vaccine for the initial prime vaccination to the follow-up booster jab.
The paper is published on the Lancet pre-print server.
The trial recruited 830 volunteers aged 50 and above from eight National Institute for Health Research (NIHR) supported sites in England to evaluate the four different combinations of prime and booster vaccination.