In a world-first, the UK is starting a government-backed trial to test the efficacy of mixing and matching coronavirus vaccines.
The vaccines currently being rolled out now require two shots weeks apart.
Participants will get one shot of the AstraZeneca vaccine followed by a dose from Pfizer, or vice versa.
Guidelines in the UK say the vaccines aren't interchangeable but can be mixed if the same kind isn't available for the second dose or if the origin of the first shot is unknown.
“This study will give us greater insight into how we can use vaccines to stay on top of this nasty disease,” said Jonathan Van Tam, the UK's deputy chief medical officer.
He said that given the challenges of immunising millions of people amid a global vaccine shortage, there would be advantages to having data that could support more “flexible” immunisation campaigns.
COVID-19 vaccines all train the body to recognize the coronavirus, mostly the spike protein that coats it. The ones from AstraZeneca and Pfizer use different technologies. AstraZeneca's uses a common cold virus to carry the spike gene into the body. Pfizer's is made by putting a piece of genetic code called mRNA — the instructions for that spike protein — inside a little ball of fat.
The British research is scheduled to run 13 months and will also test different intervals between doses - four weeks and 12 weeks apart. Initial results from the trial should be available by June.
A study published this week on the Russian-made Sputnik V vaccine showed it was about 91% effective in preventing COVID-19. Some immunologists credit the fact the vaccine uses two slightly different shots, made with similar technology to AstraZeneca's.
Dr Peter English is a consultant in communicable disease control and the outgoing chair of the British Medical Association's public health medicine committee.
He says this trial could prove one way of coping with possible vaccine shortfalls.
"The first thing is that we want to get as many people vaccinated against this disease as soon as we can and clearly the main problem at the moment is vaccine supply, " he said. "So if we know that we can safely and effectively use a different vaccine to complete the schedule, that will give more flexibility and make it easier to get more people vaccinated quickly."
Matthew Snape, the new study's leader at Oxford University, which helped develop the AstraZeneca vaccine, called for British volunteers over age 50 to sign up; scientists are hoping to enrol more than 800 people.
If the vaccines can be used interchangeably, "this will greatly increase the flexibility of vaccine delivery," he said in a statement. "(It) could provide clues as to how to increase the breadth of protection against new virus strains.”
In recent weeks, Britain, the European Union and numerous other countries have been hit with vaccine supply issues: AstraZeneca said it would dramatically reduce the expected number of doses it could deliver due to manufacturing delays and Pfizer also slowed deliveries while it upgraded its Belgian factory.