The World Health Organization says 63 confirmed and probable cases of Ebola have been reported in Uganda, including 29 deaths.
WHO chief Tedros Ghebreyesus has warned that the Ebola outbreak, declared two weeks ago, was taking a deadly toll on health workers as well as patients.
There are six variants of the Ebolavirus genus and the one circulating in Uganda is the Sudan Ebolavirus – for which there is no vaccine.
"Ten health workers have been infected and four have died. Four people have recovered and are receiving follow-up care," Tedros told a press conference in Geneva on Wednesday.
Ugandan Health Minister Jane Ruth Aceng Ocero said a 58-year-old anaesthetist had died of Ebola early Wednesday, following the deaths of a Tanzanian doctor, a health assistant and a midwife.
Tedros said the vaccines used successfully to curb recent outbreaks of the Zaire Ebolavirus variant in the neighbouring Democratic Republic of Congo did not provide cross-protection against the Sudan ebolavirus.
"However, several vaccines are in various stages of development against this virus, two of which could begin clinical trials in Uganda in the coming weeks, pending regulatory and ethics approvals from the Ugandan government," he said.
The initial outbreak was discovered in the central district of Mubende.
According to the WHO's Africa regional office, "The mobile nature of the population in Mubende increases the risk of a possible spread of the virus."
Read more on RFI
Threat of double epidemic worries Guineans as Ebola vaccines dispatched
West Africa braces for Ebola outbreak as seven cases confirmed in Guinea
'No evidence' of Ebola in woman who tested positive in Cote d'Ivoire