Doctors in South Yorkshire are urging secondary school pupils not vaccinated against measles to visit their GP over the summer holidays to get the jab.
The NHS says a ‘significant number’ of children in the county remain unprotected against the illness.
Letters have been sent to youngsters who have not had the MMR – measles, mumps and rubella – injection.
According to the most recent figures, there have been 10 cases of measles so far in Sheffield this year, compared with five in the whole of 2012.
No cases have been seen in Rotherham and Barnsley, with two in Doncaster.
Fiona Jorden, NHS ENgland consultant in public health, said: “I would urge everyone who has received a letter to go to their GP to get vaccinated. Cases of measles, mumps and rubella are rare.
“However, these conditions can have serious, potentially fatal, complications.”
A catch-up programme is being run to give the MMR jab to as many unvaccinated and partially vaccinated young people aged 10 to 16 as possible ahead of the next school year.