The MMR jab protects against measles, mumps and rubella
Health officials in Liverpool are investigating eight reported cases of measles, prompting new fears of an epidemic in the region.
Three of the cases in the outbreak - the fourth in the north west of England in 2008 - have been confirmed by laboratory tests.
The Health Protection Agency (HPA) said an unusual feature was that two of the cases were unrelated adults.
Officials are also still investigating 22 cases in Cheshire.
Earlier in 2008, eight cases were confirmed in Trafford, Greater Manchester and 89 in Blackpool, Lancashire.
'Potential to kill'
After the latest outbreak, the HPA's immunisation lead Dr Rosemary McCann said the region was on "the cusp of an epidimic".
The HPA in the north west is urging parents to ensure children get two doses of the MMR vaccine to stop further outbreaks.
Liverpool Primary Care Trust (PCT) said it was working closely with the agency to ensure people who needed vaccinations could get them promptly.
Dr Roberto Vivancos, of the Merseyside Health Protection Unit, said: "This is the fourth measles outbreak in this region in a matter of months.
"Measles has the potential to cause serious complications, such as meningitis and encephalitis. On rare occasions it can kill.
"The only sure way for a young person to avoid measles is to have two doses of MMR vaccine, preferably by age five, but at any age up to 18 if people have been previously unvaccinated."