0846 GMT May 22, 2018
There have since been more than one million suspected cases of cholera in Yemen, and 2,275 recorded deaths, the WHO said, Reuters reported.
Caught early, the disease can be treated with oral rehydration salts.
The oral vaccination campaign, which began in four districts in Aden on Sunday targeting 350,000 people, coincides with the rainy season, which health workers fear could spread the disease further.
“The first four districts are being targeted ... and then the campaign will move towards all the areas at risk in the country, covering at least four million people,” Lorenzo Pizzoli, WHO cholera expert, said in a tweet posted on Sunday.
Yemen’s war has killed more than 10,000 people since 2015.
Saudi Arabia has been incessantly pounding Yemen since March 2015 in an attempt to reinstate former president, Abd Rabbuh Mansur Hadi, a staunch ally of Riyadh, and to undermine the Houthi Ansarullah movement.
The war has also displaced more than two million and destroyed much of the country’s infrastructure, including the health system whose workers have not been paid.
Nevio Zagaria, the WHO representative in Yemen, told Reuters in April that some 1.4 million vaccine doses had been shipped via Nairobi, out of 4.4 million planned.
“The rainy season is starting, so we need to use the window of opportunity to start a vaccination campaign,” he said at the time.
In July 2017, the International Coordinating Group on Vaccine Provision - which manages a global stockpile - earmarked one million cholera vaccines for Yemen.
Some senior Houthi health officials have been known to object to vaccinations, delaying the campaign, aid workers say.