More than a quarter of those scheduled to receive their Covid vaccine in Swansea on Monday did not turn up, according to the local health board.
Swansea Bay University Health Board said that 28% of its patients missed their arranged appointments.
This means that 495 people out of 1,175 did not attend their vaccination appointment on Monday.
The numbers were slightly lower on Tuesday with 16.5% of the health board's patients (over 200 people) failing to turn up to their appointment. This continues to be much higher than their daily average of 6%.
Louise Platt, Head of Operations for the Covid-19 Vaccination Programme in Swansea Bay University Health Board, told ITV News when people fail to notify them, it is "logistically challenging for the whole team".
"It’s incredibly disappointing. We've got people desperate for the vaccine. We have a reserve list and if people did phone to reschedule or cancel, then we could access the patients on the reserve list.
"But, no vaccine goes to waste. What we will do is pull people off the reserve list which means we put on clinics at the end of the day which is a challenge for our immunisation workforce. We would schedule additional clinics during the week so it just adds additional pressure.
Discussing the consequences of people not attending their appointments, she said, "It is the overarching risk to the community. We are all working together throughout the health board. Everyone wants to return to normal and this vaccination is our opportunity to return to normal. But we do understand why people don’t, but just let us know.“
Public Health Wales figures for Tuesday showed a total of 1,413,710 first doses have now been administered in Wales - an increase of 12,960 from the previous day.
It added that 424,016 second doses have also been given, an increase of 7,154.
There are fears vaccine rollout efforts across the UK have been hampered by so-called anti-vaxxers spreading myths and misinformation.
A number of countries, including France and Germany, paused their rollout of the Oxford vaccine earlier this month over fears it could cause blood clots.
But there has been no confirmation that any reported blood clots were caused by the vaccine.
Figures from the Office for National Statistics show 44% of black or black British adults have reported vaccine hesitancy - the highest of all ethnic groups.
It is believed low take-up of the jab within ethnic minority communities could be partly down to misinformation.
Film stars Chiwetel Ejiofor and Thandie Newton, author Malorie Blackman and radio DJ Trevor Nelson are among the signatories of an open letter by Sir Lenny Henry urging black Britons to get the Covid-19 vaccine.
In the letter Sir Lenny acknowledges the "legitimate worries and concerns" that people feel, adding: "We know change needs to happen and that it’s hard to trust some institutions and authorities."