Three Covid vaccines are now part of the UK’s rapid vaccination programme – Moderna’s offering, the Pfizer-BioNTech jab and the Oxford-AstraZeneca vaccine.
Each vaccine that has been approved by the MHRA requires two doses to reach full immunity from the virus.
Some countries have been looking at mixing vaccines in the face of supply shortages and even to improve protection from the virus.
So, can you receive your first and second injections from different types of jab in the UK?
Here’s everything you need to know.
Can you mix Covid vaccines?
Millions of people have now received a first or second dose of the Covid jab.
The current 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 major study was launched in February 2021 by the University of Oxford to probe the immune responses of people given a dose of the AstraZeneca vaccine followed by the Pfizer jab, and the other way round.
Scientists want to find out if giving people different vaccines for their first and second doses works just as well, or better than, the current approach of using the same type of vaccine.
The idea is to provide more flexibility with the jabs rollout and to help to deal with any potential disruption to supplies.
The study has since found that adults are more likely to report mild and moderate side effects after they mix vaccine doses, according to preliminary data published in medical journal the Lancet.
Researchers discovered that people reported chills, headaches and muscle pain more frequently after mixing vaccines.
One in 10 volunteers who were given two AstraZeneca jabs four weeks apart reported feverishness.
But when they were given one AstraZeneca dose and one Pfizer, in any order, that number rose to around 34 per cent.
Any adverse reactions were short lived, and there were no other safety concerns.
"It's a really intriguing finding and not something we were necessarily expecting," Prof Matthew Snape, from the Oxford Vaccine Group said.
He added: “The same real differences applied for other symptoms such as chills, fatigue, headache, malaise and muscle ache.”
The trial, which has recruited 830 volunteers aged over 50, is expected to publish its first full results in June 2021.
It was expanded in April to add an extra 1,050 volunteers to test combinations of the Modern and Novavax jabs, as well as the AstraZeneca and Pfizer.
Researchers are 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.
Prof Snape 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 has also been approved for use in the UK and some people are now being inoculated with it.
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.