Health officials say they are very disappointed in the uptake of the MMR vaccine in an area at the centre of a measles epidemic.
Public Health Wales said only 100 of the 3,800 susceptible children aged over two in the Swansea area had the vaccine last week.
At that rate, it will take two years to vaccinate all those at risk.
So far there are 432 cases of measles in total and of those 116 have been reported in the last week alone.
Parents have been warned the outbreak could spread to other parts of Wales with fears it could top 1,000 cases.
Public Health Wales (PHW) said the epidemic was spreading at an "alarming rate" and that 51 people have been hospitalised so far.
Although cases have been reported across Wales, the majority are in Abertawe Bro Morgannwg, Powys and Hywel Dda health board areas.
But it is feared the outbreak of the highly infectious and potentially fatal virus has not yet reached its peak.
Dr Marion Lyons, director of health protection for PHW, said she was disappointed the vaccination message was not being acted upon and urged parents to take advantage of the Easter holidays to book an appointment with their GP.
"It could spread more widely," she added. "We know we have tens of thousands of children in Wales who have never had the MMR vaccine".
"However the epicentre is still very much in the Swansea area. Children will mix over the Easter holiday so if they haven't been vaccinated they could be at risk."
Typical symptoms of measles include fever, cough, conjunctivitis and a rash.
Complications are quite common even in healthy people and around 20% of reported measles cases experience one or more complication.
These can include ear infections, vomiting and diarrhoea, pneumonia, meningitis and serious eye disorders.
Dr Lyons said that children who have not been fully immunised face a life-long risk of catching measles but said one dose of MMR would protect 90% of children very quickly and two doses protects 99%.
Last week she warned the current outbreak is likely to continue into the summer.
Dr Gwyn Jones, who works at Gowerton surgery in Swansea, said some cases of measles he was seeing were more worrying than others.
"There have been a number that have gone to hospital but for most people thankfully it isn't a serious condition if it's caught in childhood," he said.
"But for those vulnerable people who maybe have immunal compromise for other reasons or particularly young, it can present as a much more serious problem and it can indeed go on to develop complications in those children affected as well.
"So that's why we're vigilant and cautious about it."
Parent Joanne Jones told BBC Wales Today: "I know someone who didn't immunise. Her son is in the same class as our son and one of the little boys has been hospitalised recently and is quite seriously ill with measles.
"She has taken all her children out of school because they haven't been immunised and is paying privately to have them immunised."
Dr Dai Lloyd, a city GP and former assembly member, said: "Around 80% of children in the Swansea area historically are immune to measles, mumps and rubella. But we need 90% coverage so we don't get epidemics."