Parents are being urged to ensure their children have both MMR vaccinations as a measles outbreak has been confirmed.
Fourteen people have caught the highly infectious virus in the Cardiff and Blaenau Gwent areas of south Wales.
School children and young adults are advised to get protected before going away on their Easter holidays or attending half-term play schemes.
"Ahead of the Easter break we are urging all parents with children within the Cardiff and Blaenau Gwent areas that have not had two doses of MMR to contact their GPs and immediately arrange this quick, safe and effective vaccine," said Dr Gwen Lowe of Public Health Wales (PHW).
"This is particularly important before unvaccinated and unprotected children mix with others in play schemes and play settings over the holiday period.
"Similarly we're appealing to young adults from the Cardiff and Blaenau Gwent areas to get up-to-date with their MMR vaccinations before going on holiday to Europe, where a number of measles outbreaks are ongoing."
PHW has written to schools, colleges and universities in the affected areas about the risk.
What is measles?
- Unvaccinated young children are at highest risk of measles and its complications, including death
- Measles is spread by direct contact and through the air by coughs and sneezes
- The virus remains active and contagious on infected surfaces for up to two hours
- The first signs of infection are usually a high fever and cold-like symptoms, such as a runny nose
- You may notice small white spots on the inside of the cheeks as well
- After several days, a rash develops, usually on the face and neck first and then spreading to the body and limbs
- An infected person can pass on the virus to others from four days prior to developing the skin rash to four days after the rash erupts
- There is no treatment, but two doses of vaccine can prevent infection in the first place
It has also asked local health boards to consider undertaking vaccination sessions in schools.
Adults who have never had measles or the MMR vaccine and who work in close contact with children have been urged by PHW to speak to their GP about vaccination.
Public Health Wales said measles is "very contagious" and can cause serious complications and in rare cases can be fatal.
It said anyone showing early signs of the virus - the symptoms of which include fever, cough, runny nose and red eyes - should not go to places of work or school and should avoid social interactions.
The World Health Organisation declared that the UK had "eliminated measles" for first time in 2017.