Some parents are skipping the second vaccination.
A catch-up campaign to immunise children against measles, mumps and rubella (MMR) has started in Bristol.
GPs will write to the parents or guardians of those under 18 who have not received both MMR jabs.
A spokeswoman for NHS Bristol said the vaccine is a safe and effective way to protect children against three potentially serious childhood diseases.
In Bristol, in 2007-2008, 81% of children had the first dose by age two but only 71% had both doses by five.
Lisa Patel, public health specialty registrar, said parents in the city were very good at taking their children for the first inoculation but some were putting off having the second.
"I hope that parents take this catch-up opportunity to fully protect their children from these unpleasant diseases - and in some cases prevent them from getting serious complications," Ms Patel added.
Measles is a highly contagious viral disease which spreads easily between unvaccinated children.
About one in every 15 children who catches measles will develop more serious complications. These can include chest infections, fits, swelling of the brain and brain damage. In extreme cases measles can be life threatening.
The most effective way to protect against the disease is to have two doses of MMR vaccine.
The first is normally given at around 13 months and the second at 40 months.
However the vaccination can be given to anyone of any age who is not protected.