Hywel Dda University health board has paused booster jab walk-in sessions on safety grounds after 'hundreds' are reported to have turned up.
Up to 200 people are reported to have been queuing for up to six hours to receive the booster vaccine at Dafen Vaccine centre in Llanelli.
Nearly 1.8m people in Wales are due to receive a Covid booster jab in Wales as part of the Welsh Government's rollout to vulnerable groups and the over 50s.
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The coronavirus booster is designed to improve the protection people have received from getting the first two doses of the vaccine, and combat any waning efficiency.
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But to be effective the booster has to be offered at least six months after the second dose. It means anyone who had their second jab in May won't be eligible for a booster until November. Anyone whose second jab was in June won't be offered a booster until December.
The UK government's scientific advisers called the booster the "last piece of the jigsaw" in September when they recommended that everyone over 50 should be offered a third dose of a Covid vaccine, along with frontline medical staff and younger adults with some underlying health conditions.
Hywel Dda invited those who were eligible for their booster jab to just attend their nearest vaccine centre, but many more than expected took them up on the offer.
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A spokesperson for Hywel Dda University Health Board said: "After an overwhelmingly positive response and a significantly greater than expected volume of walk-ins for the COVID-19 booster vaccines, we have made the difficult decision to pause walk-ins in order to maintain the safety around the site and on highways.
"We will, however, continue to receive walk-ins from 12-15-year-olds during the half term week. Your safety is of utmost importance to us, and we are working on the best way forward to deliver the booster vaccines to you when eligible, alongside the appointments. If you are eligible for a booster vaccine, you can still contact us for an appointment. We thank you for your patience and understanding."
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