The study comes amid an outbreak of the Sudan Ebola virus, which has already killed a nurse and infected others.
Uganda has kicked off a clinical trial of a vaccine against Ebola that killed one person in the outbreak declared last week.
Health workers and other people exposed to the strain are being targeted in the trial which began four days after Uganda announced the death of a nurse in the capital, Kampala.
Two more cases were confirmed on Monday in relatives of the first victim.
It is the sixth time Uganda has been hit by an outbreak of the Sudan strain of the virus, for which there is no approved vaccine. Of the five other Ebola species, just one has licensed vaccines.
The deadliest Ebola epidemic killed more than 11,300 people in West Africa between 2013 and 2016.
“This vaccination trial was initiated with record speed,” the World Health Organization (WHO) chief Tedros Adhanom Ghebreyesus said on X.
A system for candidate vaccines was put in place during a previous outbreak of the Sudan Ebola virus in Uganda in 2022, paving the way for a trial during the next outbreak, the WHO said.
The doses are being used in a “ring” vaccination scheme where first jabs are given to all contacts of confirmed Ebola patients, and contacts of contacts.
The WHO said the first ring defined Monday involved 40 direct contacts, and contacts of contacts of the health worker who died.
Human-to-human transmission of Ebola happens through body fluids, with the main symptoms being fever, vomiting, bleeding and diarrhoea.
Of eight previous outbreaks of the Sudan Ebola virus, five were in Uganda and three in Sudan.
The Democratic Republic of the Congo has had more than a dozen Ebola epidemics, the deadliest killing 2,280 people in 2020.
