A trial looking into whether Covid jabs can be “mix and matched” is being expanded in the UK.
The major trial will now include the Moderna and Novavax vaccines, and will look at the immune responses of patients who have had doses from two different jabs.
The Com-Cov study by the University of Oxford had been probing the immune responses of people given a dose of the AstraZeneca vaccine followed by the Pfizer jab, and the other way round.
Three coronavirus vaccines are now part of the UK’s rapid vaccination programme – Moderna’s offering, the Pfizer-BioNTech jab and the Oxford-AstraZeneca vaccine.
Over 31 million people have now been vaccinated in the country.
Each vaccine that has been approved by the MHRA requires two doses to reach full immunity from the virus.
So, could you receive your first and second injections from different types of jab?
Here’s everything you need to know.
Can you mix vaccines?
Millions of people have now received the Pfizer and AstraZeneca jabs, while the UK has ordered 17 million doses of the Moderna jab - enough to inoculate 8.5 million people.
The official Joint Committee on Vaccination and Immunisation (JCVI) guidance outlines that everyone getting inoculated should receive the same vaccine for both doses.
That means, if you receive the Pfizer vaccine the first time, you should be given that specific jab during your second appointment.
The guidance states: “JCVI advises that the second vaccine dose should be with the same vaccine as for the first dose.
“Switching between vaccines or missing the second dose is not advised as this may affect the duration of protection.”
But a leading jab developer has said mixing and matching the vaccines could be an effective way of reducing disruptions from supply shortages.
Professor Robin Shattock, of Imperial College London, said it appears that this approach is safe – and could be more effective than the same jab in some cases.
“In terms of immune response it’s highly likely that mixing and matching will work and may for some vaccines be beneficial,” he said.
“I don’t think we’ll have the kinds of information we need to demonstrate that until the end of the summer. But that may be more important – not for the initial rollout but when we start thinking about boosting vaccine.”
This technique would need to be proven by trials first.
The major UK trial was launched in February by Oxford University to see if giving people different vaccines for their first and second doses works as well, or better than, the current approach of using the same type of vaccine.
The idea is to provide more flexibility with the jab rollout and to help to deal with any potential disruption to supplies.
The study will now recruit adults aged 50 plus who have received their first dose of AstraZeneca or Pfizer in the past eight to 12 weeks.
These volunteers will then be randomly allocated to receive either the same vaccine for the second jab, or a dose of the Moderna or Novavax vaccines instead.
Researchers will be looking at the immune responses of the volunteers after they take the new combinations of vaccines.
No changes will be made to the country’s current vaccine approach until the results of the study are known.
If it shows promising results, health regulators would assess the safety and efficacy of any new vaccination plan before it is given to patients.
Matthew Snape, associate professor in paediatrics and vaccinology at the University of Oxford and 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 do the vaccines differ?
The Pfizer-BioNTech vaccine, the first to be approved in the UK, was shown to be 95 per cent effective against coronavirus after two full doses in final trials.
It needs to be stored at temperatures close to -75C.
The vaccine uses mRNA technology to train the immune system to fight off the virus using a small piece of the Covid-19 genetic code.
The Oxford-AstraZeneca jab was the second to be approved in the UK and it has been found to have 62 per cent efficacy after two doses.
Later trials found efficacy rose to 90 percent when patients were given a half dose followed by a full dose.
The vaccine is an altered version of a common cold virus, known to infect chimpanzees.
It has been scientifically modified to remove the infection so it doesn’t make people ill, while carrying the coronavirus gene into human cells so the body is able to build up an immune response to the virus.
What makes it different to the Pfizer vaccine is that it can be stored at fridge temperature.
The Moderna vaccine was the latest vaccine to be approved for use in the UK.
It utilises the same mRNA technology as the Pfizer vaccine, and also has a similar efficacy at 94.5 per cent.
The Moderna vaccine is easier to store than the Pfizer vaccine, however, at temperatures around 20C.
Meanwhile, Novavax has not been given the green light for use in the UK but the company has applied for approval.
Unlike the other vaccines, which work by tricking the body's cells to manufacture the bits of the virus that then trigger the immune system, Novavax’s offering combines an engineered protein from the virus that causes Covid with a plant-based carrier to trigger the immune system directly.