Health officials are investigating four suspected cases of measles and one confirmed case linked to a school in Neath Port Talbot.
Two pupils and a member of staff have been affected by the outbreak at Cwmtawe School.
There are two other suspected cases, believed to be relatives who are pupils at another school.
Over 200 students are being offered the MMR jab after it was confirmed a pupil had the virus last week.
Public health officials said two probable cases have been identified in children at Ysgol Y Cribarth, Abercrave, Powys, who are connected to a case at Cwmtawe.
Health officials said a disappointing number of children at Cwmtawe had been vaccinated.
Parents of children at the school not protected against measles who show symptoms have been told to stay home.
Of 1,260 pupils, 62 are recorded as having received no doses of the MMR vaccine and 142 have received only one dose.
As the number of unvaccinated children at Ysgol Y Cribarth is so small, Powys teaching Health Board is making arrangements for children to be immunised individually.
It comes three months after Wales' biggest measles outbreak ended.
Dr Jorg Hoffmann, of Public Health Wales, told BBC Radio Wales: "Disappointingly, after six months of onslaught of media coverage, after children being seriously ill, admitted to hospital and after a death in the area, we still estimate, or it appears to be, we have 60 cases in the school completely unvaccinated and about 140 who have only had one MMR.
"And so we would expect about 10% of those to be susceptible as well, we're speaking of about 75 children at very credible risk here."
Dr Hoffmann urged parents whose children had not been vaccinated and developed flu-like symptoms - and attended schools where measles was circulating - to keep them at home for a couple of days.
He said health officials were concerned to see a confirmed case at a secondary school once again.
"We are disappointed that there are still sufficient numbers out there that outbreaks are still possible," he added.
Abertawe Bro Morgannwg University Health Board has arranged for a vaccination programme to be carried out at the school on Tuesday.
The Swansea area epidemic saw suspected cases first reported last November before it spread to mid Wales.
By the time the outbreak was declared over in July, it had resulted in 1,219 notifications of measles cases in the Abertawe Bro Morgannwg, Hywel Dda and Powys health board areas.
Some 88 people visited a hospital due to measles during the outbreak and more than 75,000 were vaccinated throughout Wales.
Gareth Colfer-Williams, 25, from Swansea, died from pneumonia after contracting the virus at the height of the epidemic.
The symptoms of measles include a fever, fatigue, runny nose, conjunctivitis and a distinctive red rash.
Two MMR doses are needed to provide the maximum 99% protection against measles, say experts.