The UK launched a trial on Thursday to test the effectiveness of mixing two different COVID-19 vaccines.
Participants will be given one vaccine dose followed by a second booster shot of a different vaccine.
The trial will combine the Astrazenca/Oxford University vaccine with the Pfizer/BioNTech one.
Scientists will assess the participants' immune response, as well as monitor for any unexpected side effects.
About 800 volunteers over the age of 50 are being recruited to take part in the trial, with results expected in June.
AstraZeneca's shot is also being tested in combination with Russia's Sputnik V vaccine.
British researchers said the study was crucial for understanding whether vaccines can be rolled out with greater flexibility around the world.
Particularly as the steady supply of vaccines is always in question.
Matthew Snape, an Oxford vaccinologist who is leading the trial, said the results could inform vaccine deployment in the second half of the year.
Britain reached the milestone on Wednesday of vaccinating more than 10 million people against coronavirus.