The oral vaccination campaign has begun in four districts in Aden on Sunday, targeting 350,000 people, just a portion of 4.6 million vaccine doses allocated for the campaign.
The U.N. health organization however said that they were still negotiating with health authorities in Yemeni capital Sanaa to extend the vaccination campaign to all at-risk areas.
There have been more than one million suspected cases of cholera in Yemen, and 2,275 recorded deaths since Nov 2016, the WHO says.
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 WHO said that it could not rule out the fact that the cholera outbreak could be even bigger this year, with conditions for its spread still in place.
In July 2017, the International Coordinating Group on Vaccine Provision (ICG), which manages a global stockpile – earmarked one million cholera vaccines for Yemen.
But the WHO and local authorities decided to scrap a vaccination plan on logistical and technical grounds and the doses were diverted to South Sudan.
Some senior Houthi health officials have been known to object to vaccinations, delaying the campaign, aid workers said.