U.N. workers and hundreds of volunteers have vaccinated more than 7 million children in Madagascar against measles to prevent a potentially lethal outbreak on the giant Indian Ocean island, a U.N. official said on Monday.

The month-long drive against the world’s biggest vaccine-preventable killer saw trucks and planes delivering 10 million doses of the vaccine throughout the country, which is bigger than France.

The operation, organised by the U.N. children’s agency UNICEF, started in mid-September and aimed to vaccinate 7.7 million children aged nine months to 14 years. The campaign successfully reached 7.3 million.

UNICEF says measles caused nearly half of the world’s 1.7 million childhood deaths preventable by vaccines in 2000.

Prior to the campaign, only two in five children on Madagascar had been vaccinated, leaving the island open to a potentially lethal epidemic.