A worrying rise in a highly infectious disease has been detected in Calderdale.
An MMR catch-up programme is being launched after 12 cases of measles were reported in the district.
Of the 12 cases, all reported since last month, nine were in Hebden Bridge or Todmorden.
The Health Protection Agency says measles is a highly infectious and dangerous illness. It spreads very easily through direct contact with an infected person or through the air when an infected person coughs or sneezes.
It says MMR immunisation rates in Hebden Bridge and Todmorden are known to be lower than other parts of the Calderdale district.
A spokeswoman said: “This, together with the recent upsurge in cases locally, is of great concern to immunisation experts at West Yorkshire Health Protection Unit and public health leads at NHS Calderdale and Calderdale Council.
“This week local parents, GPs and schools are being urged to ensure all children and young adults have received two doses of the MMR vaccine as soon as possible.”
Those who have not had an MMR jab or who have only had one dose can be vulnerable to the virus.
The Health Protection Agency says being vaccinated protects individuals and stops the virus from spreading in the community.
Dr Ebere Okereke, consultant in communicable disease control and immunisation expert at West Yorkshire Health Protection Unit, said: “All families should check their children and young adults are fully up to date with MMR immunisation. If they are not, they should make arrangements with their GP to begin vaccination as soon as possible.
“We realise that this is a busy time of year for families, but it is particularly important that parents act now due to the recent upsurge in cases locally in Hebden Bridge and Todmorden and recent outbreaks that have affected other parts of our region and the nearby North West in 2012.
“It’s never too late to get immunised with this safe and effective vaccine and we must not forget that it also protects against mumps and rubella, which also have the potential to be very serious illnesses.”
Symptoms of measles usually develop about 10 days after being in contact with an infectious person and last for up to 14 days from the first signs.
The early warning signs, which can last two to four days, can include fever, conjunctivitis, spots inside the mouth and a cough and runny nose.
The characteristic measles rash of flat red or brown blotchesusually appears around the fourth day of the illness, spreads and lasts for about a week.
If measles is suspected, health advice should be sought from a GP or other healthcare professional by telephone to prevent further spread of the illness.
Dr Graham Wardman, Director of Public Health for Calderdale, said: “Where measles occurs, complications can be quite common and can include severe coughs and breathing difficulties, ear and eye infections and pneumonia.
“In rare cases, measles can cause serious complications affecting the brain and nervous system, and even deaths on rare occasions.
“We must remember that measles is preventable and two doses of MMR are needed for optimum protection.
“If you or your child has not been vaccinated, or you are unsure, contact your GP or health visitor to arrange this. We cannot stress enough that measles is serious and in some cases it can be fatal. Delaying immunisation could put children at risk.”