A measles outbreak has affected 800 people in Britain, where up to two million schoolchildren are believed to be unprotected due to a scare which wrongly linked the vaccine with autism.
The outbreak is centred on Swansea, in south Wales, but health experts warn there is a risk the virus could spread.
Since 1996 an average of 550 people contracted the disease each year in Britain, according to Health Protection Agency figures. Last year, there were 2,030 reported cases, a near-20-year high, and this latest epidemic has already infected 808 people.
Health officials pointed the finger at disgraced doctor Andrew Wakefield's 1998 report, which appeared in prestigious medical journal The Lancet, that linked the joint measles, mumps and rubella jab (MMR) with autism. Many parents decided not to have their children immunised as a result.
Helen Bedford, from the University College London Institute of Child Health, said: "Children are going to get measles, there is no question, because there is a big group - probably of the order of approaching two million children - who are susceptible who have not been immunised."
She said London was most at risk, as almost half of the children in the capital were not immunised at the peak of the scare.
The government urged parents to vaccinate their children.
Mark Walport, the government's chief scientific adviser, said: "It is an example of how people forget the danger of diseases such as measles. But it's very clear evidence of the huge importance of vaccination."
Measles is a highly contagious virus which can cause serious complications in about one in 15 cases, leading to deafness, brain damage and even death.
However, two doses of the MMR vaccine offer almost guaranteed protection against the disease. Vaccination levels in Britain are now at a high of around 88 per cent.
Wakefield is barred from practising in Britain after a panel ruled he "failed in his duties as a responsible consultant" in his published research.
The Lancet fully retracted the study and noted that some of the report had been falsified.