A 'mix and match' vaccine trial has been launched in the UK to see whether doses of the AstraZeneca and Pfizer vaccines can be combined.
The trial - said to be the world's first - began on Thursday and could help understand whether two different types of jabs could be rolled out around the world.
Patients will be given an initial jab of the Pfizer vaccine followed by a booster of AstraZeneca's and vice versa with intervals of four and 12 weeks.
Recruitment for the study began today, with more than 800 participants expected to take part across eight different sites including London, Birmingham and Liverpool.
Matthew Snape, an Oxford vaccinologist who is leading the trial, said initial results could inform vaccine deployment in the second half of the year.
If successful, it could be rolled out to the rest of the UK, including Scotland.
He said: "We will get some results through, we expect, by June or thereabouts that will inform the use of booster doses in the general population."
Minister for Covid-19 Vaccine Deployment, Nadhim Zahawi, said: "This is a hugely important clinical trial that will provide us with more vital evidence on the safety of these vaccines when used in different ways.
"Nothing will be approved for use more widely than the study, or as part of our vaccine deployment programme, until researchers and the regulator are absolutely confident the approach is safe and effective."
Scientists are looking to recruit people over the age of 50 who may be at higher risk than younger people and have not been vaccinated already.
The Government-backed study, which has received £7 million in funding from the Vaccine Taskforce, aims to establish whether a mixed-dose vaccine regimen is better than, or a good alternative to, using two doses of the same Covid-19 jab.
The trial will not assess the overall efficacy of the shot combinations, but researchers will measure antibody and T-cell responses, as well as monitor for any unexpected side effects.
