The MMR vaccine protects against mumps, measles, and rubella
Public health officials say there is no apparent link between two clusters of measles cases in north and south Wales.
The National Public Health Service for Wales (NPHSW) urged parents to give children the MMR jab after 13 cases in Flintshire and Wrexham.
Two people were taken to hospital, but later recovered.
On Tuesday, 15 cases were revealed in Carmarthenshire and Cardiff, and the NPHSW said the the 13 cases in north east Wales were linked to each other.
However, it could find "no evidence that the outbreak of measles in north east Wales is linked to the recent outbreak in south Wales".
On Tuesday, the NPHSW said 13 students and staff at Newcastle Emlyn Comprehensive School in Carmarthenshire had been diagnosed with the illness.
Two family members of one of the pupils who live in Cardiff, were also diagnosed.
Dr Chris Whiteside, consultant in communicable disease control at NPHSW, said: "Measles is a highly contagious infectious disease and children who have not been immunised are at risk.
"Most people who catch measles make a full recovery but there is a rare possibility of severe complications including serious eye disorders, deafness, mental difficulties and even death.
"Parents can easily protect their children by having them immunised with the MMR vaccine.
"After completing a two-dose course of MMR, 99% of children will be protected against measles."
Coughs and sneezes
The NPHSW said further tests were being conducted in relation to the cases in north Wales, and it was continuing to monitor the situation.
Measles is highly contagious and is generally spread from person to person by coughs and sneezes.
Early symptoms include a high temperature, cough and conjunctivitis and a red, blotchy rash that appears about three days later, usually starting on the face and spreading down over the rest of the body.
The NPHSW said any child who develops symptoms of measles should not attend school for five days after the rash begins.