Mixing Pfizer and AstraZeneca vaccines creates stronger immune response against Covid

Mixing vaccines can generate high levels of antibodies which can protect against Covid (Photo: Getty Images)

People who have been vaccinated against Covid-19 with the AstraZeneca vaccine could have a stronger immune response if they are given the Pfizer jab as a booster, research suggests.

The Com-COV study, run by the University of Oxford, found that a ‘mix and match’ approach can generate high levels of antibodies which could provide additional protection against the virus.

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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.

What did researchers find?

Researchers found that using the Pfizer vaccine followed by AstraZeneca, or vice versa, induced high concentrations of antibodies against the spike protein of Covid-19 when doses were administered four weeks apart.

The study also suggested that people who have received two doses of the AstraZeneca vaccine could have more antibodies if they were given a different jab as a booster.

Administering the AstraZeneca vaccine followed by Pfizer induced higher antibodies and T cell responses than when Pfizer was followed by AstraZeneca. T cells play an important role in supporting and maintaining antibody production.

The highest antibody response was seen after two doses of the Pfizer jab, while the highest T cell response was from AstraZeneca followed by Pfizer.

An AstraZeneca jab followed by a Pfizer jab generated a stronger immune response than two doses of the AstraZeneca vaccine alone.

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.”

Flexibility for the future

The findings of the study could allow greater flexibility in the UK and global vaccine rollouts, meaning people would be able to receive whichever jab is available, rather than waiting for a second dose of the same one.

However, given the UK’s vaccine supply position it is unlikely the schedule will change at the moment.

Researchers also found that common side effects, including feeling feverish and having a headache, were more common after the mixed doses rather than when people are given the same one, regardless of which way around the jabs were administered.

Prof Snape added: “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.”

“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.”

“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 the data from the study marks a vital step forward in understanding the effects of mixing doses, but stressed the current vaccine schedule will not be changed in the UK at the moment.

Prof Van-Tam said: “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.”