Thirty-one of the 48 confirmed and probable cases of Ebola have died since the outbreak began in central Democratic Republic of Congo early this month, the World Health Organization said Thursday.

Sept. 18 (UPI) -- Thirty-one of the 48 confirmed and probable cases of Ebola have died since the outbreak began in central Democratic Republic of Congo early this month, the World Health Organization said Thursday.

The outbreak in the Bulape Health Zone, Kasai Province, was declared Sept. 4.

Thursday's death toll marks a sharp increase over the 16 reported on Sunday.

Local and international health officials have been rushing to stem and contain the outbreak, launching a vaccination campaign for frontline workers and those infected with the virus starting earlier this week.

WHO officials said Thursday that 523 frontline health workers and contacts in Bulape have been vaccinated as of Tuesday.

The Ebola outbreak response was "accelerating," the WHO officials said, with more than 90% of contacts of the 38 confirmed cases being monitored, up from 19% two weeks ago. The number of contacts being monitored, the officials said, was 943.

Sixteen patients were receiving treatment at a Bulape center where a 34-bed unit has been established for the outbreak, with two patients having recovered from the disease and discharged on Monday, the officials said.

"While we're witnessing important improvements in the response, we're still in the early days of the outbreak," Mohamed Janabi, WHO regional director for Africa, said in a statement.

"A determined action is vital to consolidate these positive steps, gain ground against the virus, end its spread and protect the population."

On X, the WHO African Region said it requires $21 million to scale up its Ebola response and stop the spread of the disease.

"We are already on the ground with government & partners making a difference," it said. "With a fully funded budget, we won't just end the outbreak. We save lives!"

Officials have identified the index patient, meaning the first known case in an outbreak, as a 34-year-old woman who was 34 weeks pregnant.

The woman first reported symptoms of the deadly and contagious disease on Aug. 10, died 16 days later and was buried within the community without following what WHO officials called "safe and dignified burial practices."

"No sample was collected prior to burial," it said in a situation report from earlier this week. "The source of infection of the index case is still under investigation."

At least four health workers who contracted the disease are among those who have died.

One was a nurse who provided care to the index patient and developed symptoms on Aug. 22. They died on Sept. 1.

Two other healthcare workers -- a laboratory technician and a second nurse -- experienced symptoms of the disease on Aug. 24 and Aug. 28, respectively. Both have since died, though the date of death was not made public.

This is the 16th recorded outbreak of Ebola in the DRC since it was first discovered in the African nation in 1976.

The last outbreak of the disease in the DRC was reported on Aug. 15, 2022 in Beni City, North Kivu province. One case was reported, and the patient later died. The outbreak was declared over on Sept. 27 of that year.

The last outbreak in Bulape was in 2007.

Ebola is a severe, often fatal disease that is transmitted to people from wild animals and spread through the human population through direct contact with blood and other bodily fluids of infected individuals, according to the WHO.

It has an average fatality rate of 50%, though case fatality rates vary from 25-90%, it said.