A world-first study to find out if different types of Covid vaccines can be mixed has been expanded.
Medics at Liverpool School of Tropical Medicine are testing whether combining doses from different manufacturers may give longer-lasting immunity against the virus and any new variants.
It is also hoped that it will allow sites to offer a greater flexibility when it comes to the vaccine rollout.
Scientists have already begun giving volunteers shots of the Oxford/AstraZeneca and Pfizer jabs several weeks apart, and are now expanding the study to include Moderna and Novavax.
Dr Helen Hill, Co-Prinicpal investigator of the study at the Liverpool School of Tropical Medicine, said: "What we are trying to do is look at whether or not, if somebody had either AstraZeneca or Pfizer in February this year, and we need to bonus them with a different vaccine such as Moderna, or a newer one called Novavax, or the same vaccine again we will look to see what those comparisons are so in future this could be a method they used in the roll out."
The study began in February but has now been expanded to include the newest vaccines.
The researchers will study any adverse reactions and the immune system responses to these new combinations of vaccines.
The study is designed as a so-called non-inferiority study, which means its intent 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.
Researchers are now appealing for more volunteers.
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The study is looking for those who:
Are aged over 50
Have received their first vaccination of Pfizer or AstraZeneca between the last week of January and the first week of March
Not yet had their booster
The Liverpool School of Tropical Medicine is one of several sites across the country carrying out the tests.
Nurses carrying out the vaccinations are part of the school's Well Travelled Clinic - who have swapped their regular work vaccinating people for travel abroad, and are now working as part of the wider team's coronavirus research.
The first results from the 13-month-long study are expected by the summer.
But, if the research shows promise, medical regulators would still need to approve 'mix and match' vaccinations.
Dr Hill said her team was trying to provide reassurance and work with teams who are going out to provide more information to those who are hesitant about the vaccines.
She added: "I have spoken to local GPs who say, the uptake is still very good - I think the key message is you are at risk of blood clots because of Covid illness as well and that risk is much great, so having a vaccine considerably reduces your risk of clots.
"However we do need to keep monitoring this, and monitoring it to try and find any risk factors that are particular to the people who have had a problem so that e can then tailor vaccine delivery more towards people who may not be at risk of any clots and look at vaccines that are more suitable to different populations.
"That will evolve over time the more we learn from research."
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