One vaccine dose, taken by mouth, takes about a week to work and then protects for up to six months, the W.H.O. said.
But cholera is just the first and most imminent threat.
Although the cyclone and floods killed most mosquitoes and their larvae, standing water, where mosquitoes lay their eggs, is everywhere.
Millions of them are hatching, and many Beira residents already carry malaria parasites in their blood, which can be picked up and spread by mosquitoes. Experts fear a major outbreak in the coming weeks.
Measles is another looming danger.
More than 130,000 displaced Mozambicans are living in more than 150 makeshift camps on high ground. Under such conditions, in a chronically undernourished population, measles outbreaks can kill 10 percent of all children infected, according to the W.H.O.
“We’ll do an emergency measles vaccination campaign soon,” Mr. Holden said.
Dengue fever, which is spread by mosquitoes, and leptospirosis, a lethal bacteria found in the urine of many animals and commonly spread by floodwater, are also both endemic in the area.
There are still other threats to public health, agencies warn. The storm also tore the roofs off dozens of health centers, soaking the medicines, supplies and equipment inside.
Pregnant women need places to give birth, and large numbers of people taking H.I.V. and tuberculosis drugs need to get back on treatment.