The biggest outbreak of measles on Merseyside since 1988 has triggered a record uptake of the MMR vaccine, the Health Protection Agency (HPA) said.
Health officials said the number of children receiving the MMR vaccine was at an all-time high but some teenagers and young adults were still at risk.
There are 301 confirmed cases of measles in Liverpool and surrounding areas, 90 of which are teenagers.
Dr Roberto Vivancos, from the HPA, said it was "not too late" to be vaccinated.
He said: "We are encouraging older children, teenagers and young adults to come forward as well because we know that in previous years the take-up wasn't that good.
"If they haven't been vaccinated they are still at risk of catching measles."
Of the 301 confirmed cases since January, 70 were babies and toddlers under the age of one who were too young to be vaccinated and therefore vulnerable to infection from older contacts.
About 55 were children aged 13-40 months, who were either wholly or partially unvaccinated.
The smallest number of cases was recorded in children aged 40 to 60 months - an age group which is currently well protected by two doses of MMR vaccine.
Fifty four of the confirmed cases and 13 of the suspected cases needed hospital treatment.
A further 148 probable cases are under investigation.