The measles epidemic centred on Swansea is not expected to peak for another four weeks, say health officials.
The number of cases has risen to 693, with health officials warning that too few children aged 10 to 17 are coming forward for MMR jabs.
Drop-in clinics will be held in the Cardiff and Vale, Aneurin Bevan and Abertawe Bro Morgannwg health board areas on Saturday.
Officials have also raised concerns about the number of cases in Powys.
Many schools in the Swansea area reopen again next week after the Easter holidays and Friday is seen as the last opportunity for children to receive the MMR (measles mumps rubella) jab.
A spokesperson for Public Health Wales (PHW) said: "Based on other measles outbreaks, we would expect the current outbreak in the Swansea area to peak in around four weeks' time.
"Getting the MMR vaccination is the only way to stop catching measles, and the vaccine takes time to take effect, so the sooner you can get your child vaccinated the better.
"Contact your GP straight away if your child has not been vaccinated."
Dr Ian Millington, secretary of Morgannwg local medical committee, said: "Slowing down won't be seen for another two to three weeks because of the incubation period of measles."
"I think we would be disappointed if in four weeks we're not seeing some slowdown," Dr Millington told BBC Radio Wales' Good Morning Wales.
He said the immunisation programme would continue when schools reopened after Easter.
He said children would be mixing "from family and friends to a much wider group of children" on Monday.
"That's really why the immunisation is being concentrated on this week and the very beginning of next week," he said.
"There will be an immunisation programme in schools starting next week in four of the biggest comprehensives which are most at risk.
"Just under 2,000 pupils could be immunised in that setting but to deliver an immunisation programme as complex as the whole programme for Wales is not possible in school."
It was reported on Thursday that measles cases had risen by 73 to 693 in the Swansea area.
Meanwhile, Dr Marion Lyons, head of the vaccination programme for PHW, told BBC Wales of worries about the growing number of cases in the old county of Montgomeryshire, where 50 have been reported recently in Welshpool.
"We remain extremely concerned this outbreak is showing no signs of going away," Dr Lyons said on Thursday.
More than 1,700 people received MMR jabs at special drop-in clinics in Swansea, Neath Port Talbot and Bridgend last weekend. Another 900 were vaccinated in the Swansea area last week.
At least 6,000 children have still not had the MMR vaccine in the Swansea area.