Parents in Cardiff and Vale of Glamorgan are being urged to ensure their children have received two doses of the MMR vaccine, following a measles outbreak in nearby Newport.
Public Health Wales is investigating the small outbreak among people with links to Lliswerry High School.
Dr Tom Porter at Cardiff and Vale health board said uptake of the jab was “lower than we would like it to be”.
He said this meant further outbreaks were “likely”.
Vaccine sessions have already taken place at Lliswerry High School and the primary schools in Newport linked to it.
Dr Porter, a consultant in public health medicine, said: “We know that around one in seven children and young people in Cardiff and the Vale of Glamorgan are currently not fully protected against measles.
“We would urge parents to check that their children have had two doses of MMR and, if not, to contact your GP surgery to make an appointment for vaccination.”
According to latest Public Health Wales figures, the MMR uptake among 16-year-olds for Wales as a whole is 87.2%, meaning roughly one in seven are unvaccinated across the country.
Dr Porter said: “There is no immediate cause for concern in the area at the moment, but because there are pockets of low uptake there’s likely to be further outbreaks.
“This campaign is aiming to try and prevent further outbreaks from happening.”
The first MMR dose is usually given at 12 months and the second at 40 months.
Symptoms of measles include a fever, cough, runny nose, conjunctivitis and a distinctive red rash.