Three new measles cases linked to a school at the centre of a renewed outbreak of the disease are being investigated in a region recovering from a major epidemic.
Concerned health chiefs are urging parents with children showing symptoms of the illness to keep them away from school.
The warning comes as efforts began today to give the measles, mumps and rubella (MMR) vaccine to more than 200 at-risk pupils at a south Wales school.
The measure was ordered when a pupil became the first confirmed new measles case since the epidemic, the biggest ever to hit Wales, ended three months ago.
The urgent task of giving the triple vaccination to pupils at Cwmtawe School, in Port Talbot, south Wales, continues tomorrow.
Of the three new suspected measles cases, two concern pupils attending Ysgol Y Cribarth, in Abercrave, and are connected to the earlier confirmed case at Cwmtawe School.
Public Health Wales (PHW) has sent letters urging parents of unvaccinated children, who attend either school, to arrange for them to have the MMR vaccine.
Of 1,260 pupils at Cwmtawe School, 62 are recorded as having received no doses of the MMR vaccine at all, and 142 have received only one dose. Two doses of MMR are needed to provide the maximum 99% protection against measles.
Due to the small number of unprotected pupils at Ysgol Y Cribarth, Powys Teaching Health Board will arrange for susceptible children to be immunised individually.