Health professionals concerned that children are not immunized against measles, mumps and rubella have carried out a survey to measure the problem.
Numbers of children having the triple jab are lower than NHS targets
The survey found that in north Essex almost one in four children has not been fully immunized.
Two doses of the MMR vaccine are needed to give children sufficient protection.
In Colchester 75.1% and in Tendring 77.7% of children are immunized - the NHS sets a target of 95% cover to avoid an epidemic if the diseases return.
Brain damage fears
A spokesman said that mumps was the biggest cause of viral meningitis in children.
"It can also cause deafness and babies can be born with terrible defects if their mother gets German measles (Rubella) during the first three months of pregnancy.
Infection control and immunisation nurse Grant Crawshaw said: "Vaccination will protect children from these very serious diseases.
"Worldwide, every hour an estimated 60 children with measles die, and there are many more who suffer from brain damage and blindness.
"Mumps has the potential to cause meningitis and sterility and its numbers are increasing rapidly in this country."