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The drug, ZMapp, arrived in two boxes on a commercial flight from the United States carried by Liberia's Minister of Foreign Affairs, Augustine Ngafuan, and was unloaded at the VIP terminal, a Reuters witness said.
It will be taken to a hospital in the capital and administered to Liberian doctors Zukunis Ireland and Abraham Borbor, who officials said contracted the disease while attending to patients, including a late colleague.
The world's worst outbreak of Ebola has claimed the lives of 1,069 people and there are 1,975 probable and suspected cases, the vast majority in Guinea, Liberia and Sierra Leone, according to new figures from the World Health Organisation.
Three people have died in Nigeria.
The U.N. health agency said only around 10 to 12 doses of the drug have been made and this raises difficult ethical questions about who should get priority access.
The doctors will be the first Africans to receive it, though it has been given to a Spanish priest who later died and two U.S. aid workers who are reported to have shown signs of recovery.
Authorities are also concerned that ZMapp's unproven status could leave them open to the charge that humans are being used as guinea pigs.
"This is not the panacea to the problem. It is at the risk of the patient," Liberia's Assistant Health Minister Tolbert Nyenswah told journalists at Monrovia's main airport.
Information Minister Lewis Brown told Reuters the drug merely offered a "glimmer of hope" and its use was little more than a gamble.
Even so, the clamour for it is strong given that the contagious haemorrhagic disease is killing more than half of its victims and there is no known cure or vaccine.
"I welcome it. It is very good. Our nurses are dying. If you bring them the medication it will make them stronger to fight Ebola," said stationery seller James Liburd, in Monrovia.
ANOTHER DOCTOR DIES
In evidence of the ethical dilemma, Melvin Korkor, the first Liberian doctor to survive Ebola, said he would not have used ZMapp when he was fighting for his life because U.S. authorities said they were not responsible for any adverse effects.
"Any drug that has not been approved by FDA should not be administered," he told Reuters.
One of the epidemic's most tragic consequences is the toll on health care workers who rushed in as first responders only to become infected themselves due to inadequate protection measures or diagnoses of patients that came too late or were inaccurate.
The World Health Organization said this week that 170 health care workers had been infected and at least 81 had died.
Sierra Leonean doctor Modupeh Cole became the latest medical practitioner to die of Ebola, a health ministry spokesman said on Tuesday.
He contracted the disease after treating a patient who later proved to have the virus and died. The country's leading Ebola doctor, Shek Umar Khan, also died last month.
Eight Chinese health workers are in quarantine in Sierra Leone because they may have contracted Ebola, according to the spokesman for the Chinese embassy in Freetown, Xu Zhou.
The seven doctors and one nurse treated patients at two Chinese-run hospitals in Freetown who later died from Ebola. One of the doctors has emerged from quarantine after a 21-day observation period, Zhou told Reuters.
GUINEA CLOSES BORDER
Despite the stir caused by ZMapp, preventive public health measures will be crucial to containing the outbreak, according to the U.N. health agency.
As a result, West African and other governments, including some which have seen no cases of the virus, have taken measures intended to prevent the spread of the disease.
Guinea-Bissau has decided to close its frontier with eastern neighbour Guinea, Prime Minister Domingos Simoes Pereira told a news conference. Germany on Wednesday urged its nationals to leave Liberia, Guinea and Sierra Leone, though the request did not apply to medical workers or German diplomatic staff, a foreign ministry spokesman said.
The outbreak has brought fresh attention to efforts to find a cure. Scientists in the United States studying Ebola say they have found how it blocks and disables the body's ability to battle infections in a discovery that should help the search for potential cures and vaccines.
The scientists found that Ebola carries a protein called VP24 that interferes with a molecule called interferon, which is vital to the immune response.
"One of the key reasons that Ebola virus is so deadly is because it disrupts the body's immune response to the infection," said Chris Basler of the Icahn School of Medicine at Mount Sinai, New York, who worked on the study.