The MMR jab protects against measles, mumps and rubella
Emergency measures are being implemented to halt a measles outbreak.
Health chiefs in London have ordered NHS trusts to offer MMR jabs in quick succession amid a surge in measles.
There have been over 200 cases in south east London in the first five months of this year. It comes after a record 1,000 were recorded nationally in 2007.
The Health Protection Agency said it hoped that offering the two jabs within months instead of two years apart would help stem the rise.
Similar steps were taken when there was a high concentration of cases in north London last year.
Measles is a highly infectious virus. It starts with a fever and conjunctivitis before a rash develops
The rash often lasts about a week and other complications can include pneumonia and diarrhoea
The MMR jab is used to immunise children against the disease
Before the triple vaccine was introduced in the late 1980s, there were 20 deaths a year on average in the UK
But since the early 1990s there has just been one in total
It is not known exactly what has caused the rise in cases, but take-up of MMR has struggled to recover from being linked to autism in the 1990s.
At least 95% of children need to have the triple-jab to create herd immunity to stop the disease spreading, but London in particular has struggled to reach that.
About three quarters of children across London have had the first jab which is given to babies at 13 months of age.
The second jab, offered to capture the one in 10 who do not get immunity by the first vaccination, is normally given before children start school.
But the Health Protection Agency has told the six trusts in south east London - Lewisham, Lambeth, Southwark, Bexley, Bromley and Greenwich - to give the second jab one to three months after the first.
HPA disease control expert Diana McInnes said: "The increase in measles cases is of concern and we know that large numbers of children are still not fully protected.
"In south east London we are encouraging parents to give the second MMR between one to three months after the first dose to protect their children.
"Children's immune systems have a huge capacity and overloading them with the vaccination is not an issue.
"Our main focus is to remind people that they need two doses of the MMR vaccine to be fully protected."
Health officials in Lewisham, which has had some of the highest rates of infection, is running special clinics to get every child under five immunised.
Parents of children between five and 16 who have not had the second jab are also being urged to go to their GP to get the jab.
A Lewisham PCT spokesman warned: "The disease is still spreading, particularly among school-age children."