The World Health Organization has warned that it could take up to nine months before a vaccine becomes available for the current Bundibugyo strain of Ebola spreading in parts of Africa.
WHO adviser Vasee Moorthy said two possible candidate vaccines are currently under development, but neither has yet completed clinical trials.
WHO Director General Tedros Adhanom Ghebreyesus said the outbreak has already resulted in around 600 suspected Ebola cases and 139 suspected deaths, though the numbers are expected to rise further because of delays in detecting infections.
According to WHO figures, 51 cases have so far been officially confirmed in the Democratic Republic of the Congo, where the outbreak began, while two confirmed cases have also been reported in neighbouring Uganda.
The outbreak is mainly concentrated in Ituri province and parts of North Kivu in eastern Congo.
WHO declared the situation a Public Health Emergency of International Concern on Sunday, although officials stressed that it has not reached pandemic level.
Tedros said the organisation currently assesses the outbreak risk as high at national and regional levels but low globally.
Health officials also expressed concern over the number of healthcare workers infected or killed during the outbreak.
Medical workers in affected regions reported that hospitals and treatment centres are becoming overwhelmed by the growing number of suspected cases.
Médecins Sans Frontières emergency programme manager Trish Newport said several facilities have run out of space for patients and continue operating with limited protective equipment.
Meanwhile, the government of the United Kingdom announced financial support of up to £20 million to help contain the outbreak through improved disease surveillance, infection control and frontline medical support.
Investigations are still underway to determine how long the virus may have been spreading before detection.
The first known patient was reportedly a nurse in Bunia who developed symptoms and died on April 24 before the virus was identified.
Health authorities say the outbreak has created fear among local communities, with many residents changing daily habits such as handshakes to reduce infection risks.
Ebola spreads through direct contact with infected bodily fluids and can cause severe bleeding, organ failure and death.
