Local GP practices are not receiving coronavirus vaccinations for people aged between 50 and 64 years.
It has emerged that due to the "variability" in supplies all vaccinations for that age group will be administered at Newtown's mass vaccination centre.
Powys Teaching Health Board urged people in the age bracket not to call their GP surgery.
Meanwhile, the health board has said the expected dip in vaccine supplies will not affect administration of it.
They anticipate administering the first dose of the vaccination to all 18 to 49-year-olds by the end of July.
"If you are aged 50-64 please do not contact your GP practice for a COVID-19 vaccine as you will be invited by health board to a mass vaccination programme," said Carol Shillabeer, Chief Executive of the health board.
It comes as more than 63,800 people in Powys received their first dose of the vaccine. Another 19,800 have received their second dose.
Carol Shillabeer added: "By mid-April we will have offered vaccination to everyone aged 50+. But, as you may be aware from national media coverage we do expect a dip in UK supplies during April.
"Please be assured that this does not affect second dose appointments, and everyone will normally be invited for their second dose appointment around 11-12 weeks after their first dose appointment.
"We also continue to expect to offer vaccination to everyone aged 18-49 by the end of July."