Around half of people who were due to get a Covid jab at a mass vaccination centre in Glasgow failed to turn up over the weekend.
The BBC has learned a “considerable” number of no shows were recorded on both Saturday and Sunday at the Hydro.
The venue can administer a minimum of 4,000 vaccinations each day, with a capacity to scale up to 10,000.
It comes amid efforts to accelerate vaccination following a rise in cases in the city.
The BBC understands that the number of missed appointments at the Hydro was higher on Sunday than on Saturday.
Both NHS Greater Glasgow and Clyde and the Scottish government said they were looking into the issue to establish if it was a localised issue or more widespread.
The Scottish Conservatives suggested there may have been an issue with people receiving their vaccination appointment letter on time while the Lib Dems pointed to younger age groups who they said “move frequently” and have less contact with their GP.
In Scotland, people are sent their appointments directly by letter, rather than booking slots themselves.
Health Secretary Humza Yousaf, who received his first jab on Monday, said there had been a “slight increase” in those not attending appointments.
He said: “My plea would be to please attend and if you can’t attend the appointment you’re given of course you do have details where you can switch appointments, rearrange and reschedule – which is really important to do.”
Glasgow City currently has the second highest coronaviruS rate in Scotland at 136.8 cases per 100,000 people, having recorded 866 new positive tests in the seven days to 21 May.
Clackmannanshire has overtaken it for the highest case rate, at 139.7 per 100,000 although the number of positive tests is much smaller, at 72.
Glasgow is the only council area in Scotland under level three Covid restrictions amid concerns over the prevalence of the variant first identified in India – referred to as the April 02 variant by the Scottish government.
People aged 18 to 39 who live in postcodes G41, G42, G5, G51 or G52 in the southside of Glasgow are being offered jabs early to tackle surging cases.
The national clinical director, Prof Jason Leitch, confirmed there were bigger numbers of non-attendees than expected and that it was being looked into.
He told Radio Scotland’s Drivetime with John Beattie: “It’s not simple, it will be a mixture of reasons. Some of it will be our fault, because we didn’t send letters in time or the appointments didn’t go where they were supposed to go.
“Some of it will be vaccine hesitancy and some will be complacency. But it doesn’t matter if you didn’t attend, we just want you to come. Make a new appointment and come back.”
He also said he was optimistic that case numbers in Glasgow would stabilise and that the city might be able to move down to level two next week.
18-29 year-olds encouraged to self-register
The Scottish government is now inviting 18-29 years to self-register after a successful trial for unpaid carers.
A spokeswoman said: “This age group is particularly mobile and having the ability to text these groups means they will receive details of their vaccination appointment regardless of where they are.
“This is particularly relevant for students who may have registered with a GP near their term time address but be heading home for the holidays. Therefore, once they receive their appointment details vie text or email they simply contact the helpline should they need to change the location.”
The service is open between 24 May and 4 June and those who register for their vaccination during this period will be allocated the first available slots from mid-June until the end of July.
But anyone in this age group who does not register for whatever reason will still receive a blue envelope appointment.
To sign up, visit nhsinform.scot/under30register or call 0800 030 8013.
‘A matter of practicality’
University of St Andrews psychologist Prof Stephen Reicher, who advises the UK and Scottish governments, said the message should be that it is important to get both jabs. But he said it was not as simple as assuming people were to blame for not going for their vaccinations.
He said: “Sometimes it’s a matter of practicality. Getting to the SECC if you have caring responsibilities over the weekend, or if you are a single parent and you have got to take a couple of buses to get there, it is quite tricky.
“It makes sense that rather than ask people to come to you, to go into communities, take mobile labs there to get people vaccinated.
“We also need to talk to people and understand what their concerns are.”
Hospital admissions in Glasgow have increased recently, and the health secretary said the Scottish government would be monitoring whether this figure translates into more severe cases which require ICU treatment
Mr Yousaf also said this new variant meant it was “vitally important” that people received both doses of the vaccination when it was offered.
“The second dose offers greater and longer lasting protection, and should not be missed,” he said.
“We want everyone to come forward for a vaccine and we continue to work with community organisations to address any barriers people may experience to ensure that everyone is able to get an appointment.
“The vaccines we have are extremely safe and highly effective.”
The Scottish Conservative health spokeswoman Annie Wells said the number of missed vaccine appointments was “extremely concerning” and called for further transparency from the government on the issue.
She said: “If there are any issues with people receiving their appointment letters on time, then SNP ministers must give health boards the resources to ensure that people’s details are fully up-to-date.
“That is even more important now as we encourage younger people to take up the vaccine and ensure we can safely ease restrictions in the coming weeks.”
Scottish Liberal Democrat health spokesperson Alex Cole-Hamilton MSP added: “Younger age groups are more likely to move frequently and have less contact with their GPs so the government should be exploring a range of options as to how they can get people registered and get their jab.
“If you did not receive your letter, please check with your GP.”