Dr Marion Lyons, director of health protection for Public Health Wales (PHW), said they were "concerned" about the outbreak.
However, Public Health Wales (PHW) has warned that despite those immunisations the outbreak has not been brought under control.
In the whole of Wales, PHW said 823 people aged between 10 and 18 received MMR in the same period.
Dr Marion Lyons, director of health protection for PHW, said there were plenty of stocks of the MMR jab available.
But Dr Marion Lyons, director of health protection for PHW, said it appeared the uptake of the MMR was now improving.
Urdd chief executive Efa Gruffudd Jones said organisers were in constant contact with PHW to discuss the best way of advising visitors.
The event's website also has a message from PHW warning people about the outbreak and urging everyone to check they are immunised.
Dr Marion Lyons, director of health protection for PHW, warned there was no sign of the outbreak, which began last November, easing.
Public Health Wales (PHW) said if people are not immunised before going to the festival in Pembrokeshire the infection could spread throughout Wales.
Dr Brendan Mason, a consultant epidemiologist with PHW, said he could not comment on Gareth Williams' death as it was still being investigated.
Public Health Wales (PHW) continues to warn that the outbreak may spread with those not vaccinated "highly likely" to contract the potentially fatal disease.
Another 56 measles cases have been reported to PHW since Tuesday.
Dr Marion Lyons, director of public protection at PHW, has also warned that a death was inevitable if parents did not take action to vaccinate children.
"We know that measles is highly contagious - it spreads from person to person very easily, " Dr Rhianwen Stiff, PHW speciality registrar in public health told BBC Radio Cymru.
Despite this, PHW said large numbers of people aged 10-18 years still have not had a jab which could prevent them from contracting measles, a highly contagious and potentially fatal disease.
BBC: Measles epidemic: 'Slowing but more still need MMR jab'
Dr Marion Lyons, PHW director of health protection, said the increase of cases on Thursday shows the outbreak is not easing up, especially in the 10 to 18 year old age group.
"Whatever the cause of death in this case we should not be surprised if, as the outbreak grows, we start to see deaths in Wales, " said Dr Marion Lyons, PHW's director of health protection.
The appeal by PHW for parents to make sure their children are vaccinated against measles has taken an increasingly urgent tone, following the death of Mr Williams at his home in Port Tennant Road in Swansea.
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.
BBC: Swansea measles epidemic expected to peak in four weeks
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