Parents are being urged to check their children's immunisation status as they return to school and group activities.
Health authorities are continuing to deal with an outbreak of whooping cough in the Nelson-Tasman and Marlborough regions.
Between October 28, 2017, and January 26, 2018, there have been 266 cases notified in the regions.
While the rate has slowed down over the holidays, they're worried about it picking up again now school is back.
Dr Stephen Bridgman from the Nelson Marlborough Public Health Service said the number of notified cases has decreased from its peak of about 50 a week before Christmas to 10 to 20 cases a week currently.
"It's quite possible that cases might start to increase again with the start of the school year, as large numbers of children mingle again because the disease is very contagious."
He said it can be a very severe illness, particularly for younger children, in whom it can be fatal.
"Rarely, children can die from it, so our concern is to try and make sure people are aware of this, and ensure their children are vaccinated."
Dr Bridgman is appealing to parents who haven't immunised their children to urgently reconsider their choice.
He said pregnant women should also be vaccinated to protect their babies - but the uptake of the free vaccine for pregnant women is low.