More than 200 schools are being visited by nurses
A mass vaccination of more than 10,000 children is beginning in Cheshire to head off a measles epidemic.
The Health Protection Agency (HPA) said there had been 75 reported cases of the illness in central and eastern parts of the county in 2008.
Nurses are visiting more than 200 schools to "nip it in the bud", said HPA spokesman Hugh Lamont.
The agency has written to thousands of parents asking for their consent for unprotected children to be vaccinated.
Health officials have identified 10,534 children - 17% of Cheshire's school population - from the Child Health Register as not having the MMR or the pre-school booster jab.
Cheshire has the UK's highest rate of reported measles outside of London, which officials say is a legacy of the fears over the safety of the MMR jab which emerged in the 1990s.
Health officials said some parents avoided the combined measles, mumps and rubella jab after the now discredited claims of a link to autism.
Mr Lamont said: "The real worrying thing is that we have quite a large cohort of older children and teenagers who didn't get the MMR vaccine when they should have had it - when they were babies.
"They're vulnerable to measles. We've got an outbreak now and measles is highly infectious. When it gets in among them it can spread like wildfire.
"In the next three weeks nurses will be going into 177 primary schools and 33 secondary schools to nip it in the bud."
Central and Eastern Cheshire Primary Care Trust (PCT) said 20 of the cases had now been confirmed by laboratory tests.
Dr Guy Hayhurst, Consultant in Public Health for the PCT, said officials were dealing with a "large and escalating outbreak".
"The only way to halt it in its tracks is by vaccinating children and teenagers who missed out on MMR protection when they were younger.
"MMR is a safe and proven vaccine that is used effectively in 90 countries throughout the world.
"Measles is a nasty illness with a potential for serious side effects. In rare cases it can kill."