In Samoa, 70 people have died and there are more than 4000 confirmed cases of the disease.
In Fiji, the number of confirmed cases is rising.
Mass vaccination campaigns have been underway in both countries.
UNICEF's chief of immunisation, Robin Nandy, said he was "deeply concerned and disappointed" by the situation in Samoa.
"We should not be seeing these sorts of outbreaks in 2019 when we've had a very inexpensive, safe and efficacious vaccine for several decades now.