The scientists are from Oxford University and backed by the UK government.
They hope that mixing doses of the AstraZeneca/Oxford and the Pfizer/BioNTech shots could provide equivalent, or better, protection compared to using two of the same.
The conventional method for both vaccines is to give two doses, several weeks apart, to trigger a long-lasting protection against COVID-19.
According to a press release from the UK government, the new study will compare eight different configurations of doses, varying which one comes first and how far apart they are delivered.
Scientists and governments alike have been eagerly waiting for approval to start this kind of study. In late 2020, the UK government approved mixing and matching vaccines in extreme circumstances, even though the scientific evidence to support this recommendation was not yet available.
The results of this study are likely to be useful around the world, experts told reporters at a virtual briefing attended by Insider.
Professor Matthew Snape, associate professor in the Oxford Vaccine Group and chief investigator on the trial, said that, at the very least, the trial should give clues for how to increase the protection given by the vaccines.
If the vaccines are found to be interchangeable, this could offer more flexibility for vaccine delivery.
For instance, if someone turned up for a second dose of the Pfizer jab, but none were available, they could get a different one instead.
It would also allow governments to react quickly if the stock of one of the vaccines is interrupted. The study will also determine if some combinations lead to fewer side effects.
The hope is that combining the vaccines could lead to even greater protection against the virus.
Dr Peter English, a former chair of the Public Health Medicine committee of the British Medical Association, said that a combination of vaccines should work "at least as well" as using a single one.
In comments to the Science Media Centre, he said that "many vaccines work better if a different vaccine is used for boosting," citing Hepatitis B shots as an example.
The trial is working on an ambitious schedule.
It plans to start recruiting participants in February and it is hoping to provide early results by late May or early June.
AstraZeneca also announced a separate combination trial between for its vaccine and Russia's Sputnik V.
This article was originally published by Business Insider.
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