A mumps outbreak in Mid Sussex has led to jabs being offered to all 16 to 18-year-olds in the area.
Pupils aged between 16 and 18 are being offered the MMR jabs
The Primary Care Trust (PCT) said a "nationwide mumps epidemic" had reached Mid Sussex with cases already up and more expected now school had resumed.
Health workers said the age group is at greater risk because they would only have had one MMR jab as children, whereas toddlers now have two.
Vaccinations will be offered at all Mid Sussex secondary schools by Christmas.
Claire Flowerdew, PCT spokesman, said: "We had only eight reported cases of mumps last year.
"This year, up until the end of August - so that is only part way through the year - we have had 61 reported cases.
"There is an increase, and now that students are back at school we are expecting it to go up considerably."
And Yvonne Woodcocks, a school nurse in Haywards Heath, said: "I think a lot of parents think that if they have had one dose then they are fine and they are covered, but actually they are not.
"They only have about 75% cover from the first dose.
"If they have the second dose it goes up to about 97% cover - so it's much better to have the second dose."
'Worse with age'
Parental consent is not required for the vaccination of young people who are over 16 years old, but information letters are being sent to families explaining the vaccination programme and any possible side effects of the jabs.
The students are being warned whereas mumps is often a minor illness in childhood, it becomes more unpleasant with age.
Before the introduction of the MMR vaccine in 1988, mumps was the commonest cause of meningitis in Britain, the PCT said.