Hampshire's rate of vaccinations against MMR in children is behind the target needed for herd immunity, figures show.
NHS England is urging parents to ensure their children are booked to get the jab, after the uptake rate across the country fell to its lowest level in more than a decade.
The MMR jab protects against measles, mumps and rubella, highly infectious conditions which can easily spread between unvaccinated people and have life-changing consequences.
For a child to be fully protected, they should receive two vaccinations, the first around their first birthday and the second when they are three.
NHS Digital figures show 91.3% of youngsters in Hampshire were fully vaccinated by their fifth birthday in 2021-22.
This was behind the 95% target set by the World Health Organisation, and a decrease from 92.5% in 2020-21.
Across England, the uptake rate has fallen to the lowest level seen in 11 years – 85.7% of five-year-olds across the country were fully vaccinated against MMR in 2021-22, compared to 86.6% the previous year.
Dr Doug Brown, chief executive of the British Society for Immunology, said the latest NHS figures, which also show a decrease in national uptake for other childhood jabs such as the six-in-one vaccine, are "extremely worrying".
"Immediate action to reverse this alarming multi-year downward trend and protect our communities from preventable diseases is urgently needed," he said.
"Vaccines save lives and are the safest and most effective method to protect our children against disease.
"Low levels of vaccination coverage matter as it means these diseases have the potential to spread within our communities, infecting unvaccinated people, including vulnerable individuals unable to have vaccinations such as young babies or people with cancer. We can and must do better."
MMR uptake rates vary significantly between England's regions – just 74.2% of five-year-olds in London were fully-vaccinated in 2021-22, compared to 91.7% in the North East.
In the South East, 88.2% of youngsters have had both doses of the MMR jab.
The NHS is now contacting the parents of around 740,000 children aged between one and six who have not yet had one or both doses of the MMR vaccine, encouraging them to make an appointment with their child’s GP practice.
Steve Russell, NHS director of vaccinations and screening, said: “It is important that parents make sure their children are protected against MMR as they return to school, and are up to date with their flu vaccination if eligible as we head into the winter months.
“The MMR vaccine is one of the most studied vaccines in the world, with millions of doses given every year – it is safe for your child, and will protect them, their friends and the wider community from these unpleasant but preventable diseases."