The World Health Organisation on Tuesday began what it said was the “second-largest oral cholera vaccination campaign ever”, near Cox’s Bazar in Bangladesh, where most Rohingya refugees from Myanmar arrive. Doses of the vaccine will be given to nine lakh people over the next few weeks and are being distributed by more than 200 mobile vaccination teams, the United Nations agency said.

A cholera outbreak is possible across the refugee settlements, WHO said, adding that over 10,000 cases of diarrhoea have been reported and treated in the last week.

“This is the second-largest oral vaccination campaign in the world after [Hurricane Matthew in] Haiti in 2016,” said Edouard Beigbeder of Unicef, which is supporting the vaccination drive. “Cholera is a dangerous disease, especially among children living in cramped, unhygienic conditions. Prevention is essential.”

WHO is also monitoring water quality and working with local authorities to increase local laboratory capacity.

More than half a million Rohingyas have fled from Myanmar’s Rakhine state to Bangladesh since August 25, fearing violence and persecution by security forces. The community has been denied citizenship in Myanmar and are classified as illegal immigrants.