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People who took part in trials of Covid vaccines which haven't been approved yet will be offered two doses of another vaccine from next week, health officials say.
This means more than 15,000 volunteers who received Novavax or other jabs will finally be able to travel abroad.
An eight-week gap is needed between the third and fourth doses.
The NHS Covid app will also be altered to permit travel after the extra two doses.
The move, announced by the Department of Health in England, has been developed with the help of the UK's vaccine committee and investigators who worked on the clinical trials.
Although people who signed up to Covid vaccine trials are recognised as fully-vaccinated in the UK - even if the vaccine has not been approved by the UK regulator - many other countries are not allowing them to enter.
And many volunteers had complained they were at a disadvantage through no fault of their own.
'I'm delighted - we have a trip planned with friends'
Darren Green, aged 51 and his wife Linda, 59, who live in south Wales, were volunteers in the Novavax trial.
Linda was given a placebo rather than the vaccine and has since received two doses of an approved Covid jab, meaning she is free to travel.
Darren, however, is not, because he received Novavax in the trial.
He says it is a relief to know he can now get the travel vaccines.
"I felt a bit emotional when I heard the news. It's a big weight off my mind.
"I'll be able to go out and do what other vaccinated people can. We have a trip planned with friends. I thought I was going to have to cancel."
On the prospect of needing more doses, Darren said: "Apart from feeling a bit like a pin cushion, I'm absolutely delighted."
Prof Jonathan Van-Tam, England's deputy chief medical officer, said when volunteers started taking part in vaccine trials, the UK had no idea which vaccines would succeed or that vaccines would be needed for travel abroad.
He said the UK wanted to support them, given "the sacrifice they made to help others".
He said they must be able to travel "for deeply personal and important reasons".
"We hope it will be a solution for triallists - as they so richly deserve," he said.
Vaccine trial volunteers, starting with those who had Novavax, will be offered one-to-one counselling before they decide whether to have further doses of an approved Covid vaccine. Novavax volunteers will be offered two doses of Pfizer, eight weeks apart.
But they can decide to wait for the vaccine they've already had to be approved in the coming months, if they don't have a travel deadline.
Like others in the UK, people in the trials will be offered a booster jab, if eligible.
Trials on the safety of administering four doses of different Covid vaccines have not been carried out, but vaccine experts say they do "not expect any major safety concerns".
The UK has ordered 60 million doses of the vaccine which were set to be made in north-east England.
The government said the Medicines and Healthcare products Regulatory Agency (MHRA) and the vaccine taskforce were working with vaccine companies to ensure their vaccines were authorised as soon as possible if they met the required safety and quality standards.
Around 52,000 people are taking part in clinical trials across the UK, with 21,000 given a vaccine not yet approved for widespread use. Novavax and Valneva vaccine trial volunteers make up the majority of people in this group.
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