Unicef aid workers in Vanuatu are aiming to immunise a thousand children a day over the next ten days.
Relief agencies said post Cyclone Pam conditions are among the most challenging they have faced, with concerns mounting that disease and a lack of clean water could lead to more deaths.
Unicef's spokesperson in Port Vila, Alice Clements, spoke to Radio New Zealand before she flew to Tanna Island this morning.
"So we've already vaccinated 1,000 children under the age of five in the last 24 hours and we're aiming for 10,000 within 10 days."
Alice Clements said the next focus will be getting children back into schools as soon possible, to try reduce their levels of stress with the regular routine of classes.