Health agencies have declared an outbreak of measles after four children in south Belfast contracted the infectious disease.
According to the Public Health Agency, the four young people who caught the disease had not been vaccinated.
The agency said it had also made contact with the parents of those who may have been exposed to infected children.
It is advising parents and guardians of children who have been in contact to protect them with the MMR vaccine if they have not already done so.
GPs and medical staff have also received letters advising them to be vigilant for measles patients in the coming days and weeks.
Dr Richard Smithson is a consultant in health protection with the Public Health Agency.
He said those across Northern Ireland have had a “high uptake levels for MMR immunisation” which is why the region has seen “very few cases of measles here compared with the rest of the UK and the Republic of Ireland”.
“However, cases which have |occurred here in unvaccinated people are of serious concern and remind us that there is no room for complacency,” he said.
Dr Smithson said those who had been exposed to measles and feel unwell should stay at home.
“This is an infectious condition even before the associated rash develops.
“If medical advice is needed, then they should phone the GP or out-of-hours service so that arrangements can be made to see the patient without putting others at risk,” he said.
The PHA said it was reminding parents about the importance of “protecting their children and themselves” against the disease — which can be life-threatening.
Although most common in children, it can occur at any age and is highly infectious for those who have not been protected by vaccination.
Measles can cause a temperature of over 40C and a red-brown spotty rash.
The initial symptoms appear around 10 days after you get the infection and can last up to 14 days. It can include cold-like symptoms — such as a runny nose, watery eyes, and sneezing — along with red eyes and sensitivity to light, as well as a mild to severe temperature.