Vaccination centres have seen a rise in "no shows" which has coincided with unseasonably warm weather sweeping across Britain.
Health officials are now urging others to turn up for their jabs after up to 500 people failed to show in the Swansea Bay area in Wales alone on Monday.
Swansea Bay University Health Board said 28 per cent of its 1,750 appointment were missed in that one day, which saw crowds flock to parks and beaches in mid-20C temperatures to soak up the post-winter rays.
One official said: "It is no coincidence that people are not turning up just as the sun comes out.
"We don't know why so many are not here - but they should just get in touch so someone else can take their place."
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Louise Platt, head of operations at the health board, said: "We continue to urge everyone to attend their vaccination appointment if they can do so.
"If not, please telephone or email us to rearrange or cancel at the earliest opportunity."
The missed appointments has prompted Cwm Taf Morgannwg University Health Board in South Wales to launch a "reserve list" of patients.
It is asking those aged between 40 to 49 to sign up online to make sure "no vaccine was wasted."
The health board's director of planning and performance, Clare Williams, said: "We have seen over the past couple of weeks, a small number of people fail to show for their appointments which is something we want to address.
"No vaccine has been wasted because of this, but the reserve list will make sure we continue to use every single dose available."
The Welsh Government said it was looking into why so many people were not attending vaccine appointments.
A Government spokesman said: "We recognise the importance of wasting as little of the vaccines as possible.
"All health boards have arrangements in place to give others, in the priority groups, the vaccine in the event of no-shows or cancellations.
"This has ensured the number of doses unable to be used has been very low, under 1 per cent, significantly below the anticipated levels."
