More than 20 children who shared a hospital ward with a youngster who was later found to have measles, are being offered the MMR vaccine.
Parents of 22 children are being offered the vaccine
Durham Primary Care Trust says the move is a precautionary measure after the three-year-old boy was treated at the University Hospital of North Durham.
Hospital staff who were also in contact with the child are being offered MMR if they are not already immune to measles.
The boy was on the on Treetops ward on 4 and 5 September.
Trust director of nursing Laura Robson said: "The child was nursed in a single cubicle while he was on the ward so the risk to other children is small.
"However, as a precautionary measure we are contacting all of the families of 22 children who may have been in contact with the child when he was on the ward to offer them the MMR vaccine if they haven't already had the recommended two doses.
"This is also an opportunity to emphasise the importance of all children having two doses of MMR vaccine to make sure they are fully protected against measles mumps and rubella.
"Cases of measles are increasing in this country and it's important that parents make sure their children are fully protected against this very serious illness."
The trust said if given to non-immune people within three days of being exposed, the vaccine can significantly reduce the chance of them developing the illness.