One in five Bay of Plenty children are not fully immunised when they start school, with the district health board recording the second-lowest vaccination coverage rate in the country.
Ministry of Health immunisation figures showed 81.6 per cent of Bay children who turned 5 last year had completed their age-appropriate immunisations - a shade over four out of every five.
The DHB has increased this rate every year since at least 2013, when it recorded 67 per cent coverage.
However, last year's figure was the second-lowest rate of the 20 district health boards in the country, in front of only Northland at 80.1 per cent.
It was also below the national average of 87 per cent, and well below its target of 95 per cent.
Bay of Plenty DHB spokesman Brian Pointon said the health board was concerned with the number of new entrants who were not appropriately immunised.
"The BOP is vulnerable to measles outbreaks with less than 90 per cent of school children vaccinated," Mr Pointon said.
"It appears that we may be starting the next cycle of whooping cough disease, so it is important that every child is vaccinated against whooping cough."
He said more parents in the Bay turned childhood immunisations down than the rest of the country.
He also said those living in rural or high deprivation areas had difficulties accessing health services.
"The BOPDHB has undertaken a review of all childhood immunisation milestones and is looking to strengthen the current childhood service support model."
Western Bay Primary Health Organisation immunisation co-ordinator Diane Newland said the Bay's rate was a worry, but it had not caused any major issues.
They're concerned that something might happen to their child if they have the vaccine.
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"The rates across all of the age groups are a large concern for our community," Ms Newland said.
"There is greater risk in our community that these diseases could come in and cause an outbreak."
The rate of vaccinated 2-year-olds in the Bay was 88.7 per cent last year - seven percentage points higher than the rate of 5-year-olds.
She said the main reason the Bay did not hit its targets was because the Bay had a higher rate of parents declining or delaying immunisations than the rest of the country.
"They're concerned that something might happen to their child if they have the vaccine.
"In recent years, there has been nothing that has linked an event to an actual vaccine - usually there's something else going on for the child at the same time."
The vaccinations usually scheduled at 4-years-old are "boosters" - additions to ones already administered earlier in childhood. Ms Newland said the jabs at age 4 were "no less important".
She said the Bay was lucky the low vaccination rate had not caused any widespread issues.
"It all depends whether an outbreak happens in the first place - we've been very lucky. We've had outbreaks in Auckland and Hamilton, and we hold our breath in the holidays and hope none of our kids . .. come back with measles.
There were a number of groups in New Zealand and around the world who either opposed vaccination or encouraged parents to further investigate the benefits and risks.
New Zealand group WAVES - Warnings About Vaccine Expectations - is an organisation representing parents, caregivers and health professionals "who have concerns about the safety and efficacy of vaccines [and] wish to make an informed decision about whether or not to vaccinate themselves or their children".
Its website says it believes the body's natural immunity and a healthy diet and lifestyle more effectively prevented disease than artificial immunity.
"Most childhood illnesses serve to strengthen and mature the child's immune system and provide lifelong immunity," it says.