MMR uptake is very high on Anglesey, according to the health service
People are being urge to make sure they have had the MMR vaccination after 16 cases of mumps in Anglesey and Gwynedd in the last month.
The National Public Health Service for Wales (NPHS) says 15 of those affected have been given the recommended two doses of the MMR vaccine.
Ten secondary school pupils in Amlwch on Anglesey are understood to have been sent home with the illness.
GPs in the two areas have been alerted to the problem.
Health officials from Anglesey's Local Health Board (LHB) have sent letters to parents through Ysgol Syr Thomas Jones in Amlwch where pupils have become ill.
All the suspected cases have emerged since 27 December.
Anyone in a school or community where mumps cases are reported who have not already been fully immunised with two doses of MMR should be vaccinated immediately
The health service said it is possible, but not common, to get mumps after being vaccinated with MMR which protects against measles, mumps and rubella and is given as part of the routine childhood immunisation programme.
Dr Judy Hart, Consultant in Communicable Disease Control, said: "The MMR vaccine is extremely effective in protecting against measles and rubella, with 99% of those who have received two doses protected for many years.
"However, while it is still very effective compared to other vaccines, MMR does not provide such high levels of protection against the mumps infection compared to measles and rubella."
Dr Hart said one dose of MMR protects around 65% of those who receive it against mumps, with a second dose of MMR improving immunity to around 85% of recipients.
"So it is possible to see mumps in individuals who have been vaccinated with MMR, although this is not common," said Dr Hart
Coughs and sneezes
"MMR uptake is very high on Anglesey so most school children are protected.
"However, young people aged 16 and over were too old to have received a routine second dose of MMR before school entry when it was introduced in 1996.
"They are more likely to be susceptible to mumps and require a second dose now."
Mumps is spread by coughs and sneezes and directly through close personal contact.
Symptoms include swelling on the angle of the jaw on one or both sides of the face.
Beforehand, there may be several days of symptoms such as fever, headache, tiredness, muscle aches, and loss of appetite.
Anyone with these symptoms is being advised to consult their GP and stay away from school or work until five days after the start of the swelling when they are no longer infectious.
Dr Hart added: "Anyone in a school or community where mumps cases are reported who have not already been fully immunised with two doses of MMR should be vaccinated immediately."
There is no upper age limit for vaccination.
The NPHS has issued letters to GPs in Anglesey and Gwynedd to alert them to the increase in suspected mumps.