In some schools the vaccination uptake is as low as 14.8%
Health officials say they are "highly alarmed" that the number of measles cases being investigated in mid and west Wales has reached 109.
They have issued an urgent warning that vaccination is the only way to stop the virus spreading but are disappointed with the uptake in schools so far.
There are 11 confirmed cases, five are in Pembrokeshire and six in Llanelli.
There are also suspected cases in Powys, Ceredigion, Swansea, Neath Port Talbot and Bridgend.
Dr Mac Walapu, consultant in communicable disease control for the National Public Health Service said the 109 figure was alarming as there were only 39 cases in Wales last year, 13 in 2007 and none in 2005.
He said anyone who had not received the full two doses of the MMR vaccine was at risk from measles and should come forward for immunisation.
Cases are occurring across all age groups from children as young as five months to adults in their late 40s.
Confirmed measles outbreaks
Llanelli: 43 cases. 6 confirmed, 19 probable, 18 suspected
South Pembs: 14 cases. 3 confirmed, 11 probable
Burry Port: 5 probable cases
There are three separate outbreaks in Llanelli, Burry Port and south Pembrokeshire accounting for 62 of the cases.
Of the 47 other cases, six are in Pembrokeshire, six in Carmarthenshire, four in Ceredigion, one in Powys, 15 in Swansea, five in Neath Port Talbot and 10 in Bridgend.
Two of the six other cases in Pembrokeshire have been confirmed as measles.
There is no clear connection between these and the three outbreaks.
"Last month the NPHS wrote to all parents or guardians of unvaccinated children in south Pembrokeshire and Llanelli urging them to have their children vaccinated but the response was disappointing, particularly in Llanelli," he said.
"In some schools, the vaccination uptake is as low as 14.8 per cent.
"To see so many cases of measles at one time is highly alarming.
"It is of great concern to us that in the Llanelli area alone there are 800 primary school children and 2,100 secondary school children who are vulnerable.
"The virus will continue to circulate until the susceptible population have been infected and the only way this will stop is for people to come forward for the MMR vaccine."
He said measles is highly contagious and could have dangerous consequences.
Many people who catch measles will have a fever, a rash and feel generally unwell.
Typically, people ware infectious from a few days before the onset of their rash until four to five days afterwards.
Dr Walapu added: "If your child is unwell and you suspect it is measles, you should contact your GP.
"Your child should not attend school or nursery for five days after the rash starts.
"Most people make a full recovery but there is a rare possibility of severe complications including serious eye disorders, deafness, brain damage and even death.
"After completing a two-dose course of the MMR vaccine - which also offers protection against mumps and rubella - 99% of children will be protected against measles."