The Democratic Republic of Congo (DRC) on Wednesday confirmed 14 new cases of the Ebola virus in its eastern borderlands, the largest one-day increase since the current outbreak was declared in August.
In all, the haemorrhagic fever is believed to have killed 439 people and infected another 274 in the eastern provinces of North Kivu and Ituri.
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The epidemic in a volatile part of the DRC is the second worst ever, according to the World Health Organization (WHO).
The largest outbreak occurred between 2013-2016 in West Africa. More than 28,000 cases were confirmed.
The news comes shortly after drugmaker Merck said it will ship another approximately 120,000 doses of an experimental Ebola vaccine to Congo by the end of next month.
Associate Vice President Lydia Ogden told the World Economic Forum that the company is committed to having a ready stockpile of 300,000 doses and already has shipped 100,000 to the WHO.
Health officials have called the experimental vaccine highly effective against the virus.
Carrying out vaccinations is complicated by rebel attacks, poor infrastructure and in some cases hostility from communities that have never faced an Ebola outbreak before.
Aid groups have been forced to halt Ebola prevention activities in the past due to violence.