A Northland baby has been diagnosed with the new W strain of meningococcal disease.
The seven-month-old child is the first case in the region since an outbreak of the virulent new strain was declared last spring.
The Northland District Health Board said 14,000 children in Northland had been vaccinated against the disease in the past three months in a campaign that included schools.
But Northland DHB said that left 8000 children eligible for the free shots who had not been immunised.
In some parts of the region, such as Hokianga, only 50 percent of those eligible have been vaccinated and that was not enough to protect the community, according to Medical Officer of Health, Dr Jose Ortega Benito.
"We now know that meningococcal W is still circulating ... and if we don't increase the number of eligible children vaccinated, we are at risk of another outbreak," he said.
The DHB and Ministry of Health had targeted children under five and teenagers with the free vaccine because they were generally the groups most affected by meningococcal disease, he said.
But at seven months, the baby that had now fallen ill was too young to be vaccinated.
"Vaccinating 13 to under 20-year-olds is also really important because this is the age group that generally carries the disease ... and even if they have no symptoms they can infect those around them," Dr Benito said.
A seven-year-old Whangārei girl, Alexis Albert, was one of six people who died of meningococcal W last year, after falling ill with a headache, vomiting and diarrhoea.
Northland DHB said it was now working closely with Hokianga Health to offer community clinics in the area.
Most GPs in Northland are still offering free vaccinations to eligible children, and the shots are also available in Whangārei, Kerikeri and Kaitaia, at designated pharmacies.