Mozambique health officials are in a rush to stop the spread of the cholera, which has claimed the lives of at least two people in the port city of Beira and made more than 1,000 others ill.
The vaccination target represents around 80 percent of the people affected by the cyclone, said Marie Benigna, a deputy director in the health ministry.
"Hundreds of thousands of people are living in awful conditions in temporary settlements without safe drinking water and sanitation, putting them at serious risk of cholera and other diseases", said Dr Tedros Adhanom Ghebreyesus, Director-General of WHO. The vaccine is given in one dose orally, and takes effect within a week.
Along with ensuring that a second dose of vaccine is made available later this year, preventing people from drinking dirty water is a key priority, the World Health Organization insists. "The important thing is to get sick people into treatment as soon as possible". The burden of cholera is disproportionately carried by children and vulnerable groups.
"It is noted with sadness that the figure of human fatalities has now risen to 268 and is still expected to rise as the search efforts continue". "Access to essential health services and the situation most likely will worsen therefore".
Numerous missing may have found shelter in one of 155 displaced persons sites across Sofala, Manica, Zambezia, and Tete provinces.
Some 843 people were killed by the storm and subsequent flooding in Mozambique, Zimbabwe and Malawi. Malaria, measles, and starvation may follow unless help reaches enough people quickly.
The United Nations has appealed for $392 million to fund the humanitarian response to the disaster in southern Africa for the next three months.
Speaking in solidarity with survivors of the disaster, UN Deputy Secretary-General Amina Mohammed on Tuesday paid tribute to the "local, national and global responders" who offered their help "from the earliest moments of this crisis". In the fifteen years between 1997 and 2012, just 1.5 million doses of oral cholera vaccine were used worldwide.
The bodies were washed downstream from Chimanimani's Rusitu Valley, one of the worst-hit areas during Cyclone Idai.