A major UK study looking into whether Covid vaccines can be mixed has been extended to include Moderna and Novavax, which is still waiting for UK approval.
Using different vaccines for the first and second dose could allow more people to be fully immunised more quickly and protect against shortfall in supplies.
The Com-Cov trial is looking for volunteers to take part, and adults over the age of 50 who have had their first dose of the Pfizer or AstraZeneca jab can now apply to take part.
This is what you need to know about the trial - and how to volunteer.
What is the purpose of the Com-Cov study?
The Com-Cov study has been investigating the immune responses of people given alternate doses of the Oxford and Pfizer jabs.
The trial has been designed as a ‘non-inferiority’ study, which means its purpose is to demonstrate that mixing is not substantially worse than not mixing.
It will compare the immune system responses to the gold-standard responses reported in previous clinical trials of each vaccine.
If the study shows promising results, the vaccine watchdogs would assess the safety of any new vaccination before it is rolled out to patients.
Matthew Snape from the University of Oxford, who is chief investigator on the trial, said: “The focus of both this and the original Com-Cov study is to explore whether the multiple Covid-19 vaccines, that are available, can be used more flexibly - with different vaccines being used for the first and second dose.
“If we can show that these mixed schedules generate an immune response that is as good as the standard schedules, and without a significant increase in the vaccine reactions, this will potentially allow more people to complete their Covid-19 immunisation course more rapidly.
“This would also create resilience within the system in the event of a shortfall in availability of any of the vaccines in use.”
How can I volunteer to take part in the trial?
Led by the University of Oxford, the extra study will seek to recruit adults aged over 50 who have received their first vaccination in the past eight to 12 weeks.
People interested in volunteering can sign up via the NHS Vaccine Research Registry – meaning researchers, with permission, can reach out to ask questions about a study.
The volunteers, who will have received either the Oxford/AstraZeneca, or Pfizer/BioNTech vaccine, will be randomly allocated to receive either the same vaccine for their second dose, or a dose of the jabs produced by Moderna or Novavax.
The six new arms of the trial will each recruit 175 candidates, adding 1,050 recruits into this programme.
Previous clinical trials have seen volunteers paid hundreds and sometimes even thousands of pounds to take part.
When will the results of the trial be available?
Professor Snape said he hoped the results of the second part of the study would be available in June or July, with the first part expected to report results next month.
He told a press briefing: “What I’m hoping is that we won’t rule out any combinations.
“That’s how we need to look at it – are there any combinations we shouldn’t be giving, because they don’t generate a good immune response and I’m hoping that won’t be the case.
“And that will give us lots of flexibility, not just in the UK, not just in Europe where we’re looking about restricting uses of some vaccines for some age groups, but across the world, where we have, perhaps, a little bit more intermittent supply of vaccines, not as reliable.
“But let’s hope that we can actually use this to get two doses of vaccine to as many people as possible.”