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People vaccinated against Covid-19 outside the UK are reporting problems when they try to register for a digital health pass in Britain.
A lack of easily available appointments means they are being told to travel hundreds of miles to get their medical records updated.
It adds a further potential burden for people travelling home to Britain for the Christmas holidays, with rules on international travel already tightened.
The Omicron variant is driving cases to near-record levels in the UK, with ministers told the true number of infections is likely to be far higher - perhaps as many as 200,000 per day.
Having the NHS Covid Pass became more important in England from Wednesday, after MPs voted to make it mandatory at certain venues - despite a Conservative backbench rebellion that dealt a blow to Prime Minister Boris Johnson's authority.
Mr Johnson's office said football grounds including London's Wembley Stadium would soon open as mass vaccination sites as the country races to hand out booster vaccines.
Registering previous overseas doses in Britain opens the door to booking a booster. But the app and booking service rely on UK medical records, which do not automatically update when people are vaccinated abroad.
An in-person appointment in Britain is needed to rectify this.
Booking such an appointment involves waiting in the same online queue as people trying to arrange a vaccination.
Thousands of people were waiting for the booking service on Wednesday after booster doses were opened to people aged 18 to 30.
Once people reach the front of the queue, they are sometimes told that their nearest venue for an overseas vaccine registration is nowhere near their home in the UK.
In one case, somebody living a short distance outside London was directed to a centre more than 300 kilometres away in north-east England.
Another patient living in Cornwall, in south-west England, had no closer options than Peterborough in the east of the country.
Others were told that they could not book an appointment because of an unspecified technical problem.
Health staff are “not able to register your overseas vaccinations unless you book in advance,” Department of Health guidance says.
Wera Hobhouse, an opposition Liberal Democrat MP, raised the concern in a parliamentary debate on Tuesday.
She said some of her constituents, who were initially vaccinated abroad but were now in the UK, were unable to book booster doses in Britain.
The lack of easy access to the NHS app was “a problem that has been noted since the summer,” she said.
The approved vaccines are Pfizer-BioNTech, Moderna, Johnson & Johnson and Oxford-AstraZeneca.
If the vaccine procedure is too complicated, people can alternatively get an NHS Covid Pass by taking a negative test.
As for a booster dose, it is possible to go to a walk-in centre in the UK and get the shot without an appointment.
Two doses are needed for the health pass, which is now required for access to nightclubs, crowded indoor events and some outdoor events.
Health Secretary Sajid Javid suggested on Tuesday that this would be raised to three doses when everyone has had a “reasonable chance” to get a booster.