In the absence of proven drugs to treat Ebola, doctors have turned to a variety of experimental methods to try and stem an outbreak that has ravaged West Africa, killing more than 3,300 people. And the World Health Organization (WHO) has thrown its weight behind one of the more unusual of those methods: treating patients with blood from people who beat the often-deadly virus.

While Ebola-stricken American missionaries Kent Brantly and Nancy Writebol received the experimental drug ZMapp on the way to making full recoveries last month, American aid worker Rick Sacra — in addition to receiving a drug called TKM-Ebola — was given a transfusion with blood from someone who had recovered from Ebola.

The WHO believes the method holds promise for helping fight the disease. Last month the organization issued guidance (PDF) on treating those infected with Ebola with the blood or plasma of people who have survived it, “as a matter of priority.”

The idea is that using survivors’ blood for transfusion treatments, called “convalescent plasma serums,” could work by giving infected patients the antibodies that survivors developed to eventually overcome the virus.

That was why Sacra, while being treated at the Nebraska Medical Center in Omaha, received two transfusions from a recently recovered Brantly.

Some scientists have suggested that blood transfusions, which where used early in the 20th century to treat Spanish influenza, could be promising for other infectious diseases like H1N1.

After convening a panel of more than 150 experts in September, the WHO said the severity of the current Ebola epidemic meant that pursuing experimental treatments that showed signs of promise satisfied many concerns regarding medical ethics. The world body cited an example from the 1995 Ebola outbreak in the Democratic Republic of Congo, during which eight patients were given transfusions of blood from people who recovered — and seven of the recipients recovered.

But establishing causation between convalescent plasma serum treatments and full recovery from Ebola remains an open question.

The eight people who recovered in the Congo epidemic may have received better care than those who were infected at the beginning of the outbreak. Or they might have survived anyway, virologists say.

The blood transfusion treatment “still remains very controversial,” said Thomas Geisbert, a virologist at the University of Texas Medical Branch in Galveston who specializes in hemorrhagic fevers like Ebola. “There’s no controlled data in a laboratory that shows it works.”

Geisbert said he and his team had previously conducted an experiment where they gave blood from so-called convalescent monkeys — those who had survived Ebola — to monkeys they had infected with the virus, to see whether the convalescent blood theory would hold up.

“They all died,” he said.

Moreover, since Ebola has such a high death rate in the current crisis, some infected patients are being given multiple kinds of treatments, making it hard to know what actually works.

“[Blood] transfers probably don’t hurt,” Geisbert said, adding that because Ebola patients are so dehydrated and exhausted, receiving a unit of blood itself might give a much-needed boost. But, he said, “I just don’t know that they do good, either. My only concern all along is that you make sure you’re transferring blood that’s not contaminated with something else like hepatitis or HIV.”

Still, other virology experts point to recent research showing that antibodies are critical to beating Ebola, meaning the serum technique could be a good method — and one that is readily available.

A recent study of an experimental Ebola vaccine from the National Institutes of Health (NIH) and Oregon Health and Science University showed that antibodies induced by the vaccine “appeared to be critical” in protecting monkeys against Ebola, according to NIH.

“The serum is already over there [in Africa] in the survivors,” said Ben Neuman, a virologist at the UK’s University of Reading. “It’s already a potential homegrown remedy, and one that could potentially work on a large scale.”

Because it will be a few months before scientists are able to carry out more testing and production of other promising experimental Ebola drugs such as ZMapp, this is an opportune moment to try the plasma technique, he said. “To find out if it actually works, we need a big test, and now I think is the perfect time to do one of those tests.”

It will be important, Neuman said, to examine a broad collection of antibodies from Ebola survivors across the population. Because every individual’s immune system reacts to a virus differently, and because every infected person has a different viral load, the best odds for combatting the virus will involve finding a very particular dose of antibodies.

“Candidly, I don’t think it will work,” Neuman said. But in the absence of proven treatments, he said now is a good time to try it. “So far, I don’t think there are any obvious indications that it’s made people sicker,” he said.

Jonathan Epstein, an epidemiologist with EcoHealth Alliance, a New York-based conservation group, agrees that it can’t hurt to try any possible solution, as long as it’s known to be safe.

“In light of the current crisis, it’s definitely a viable approach to look into,” he said. “I think we have to look at every option in the arsenal, and this is certainly one of them.”

“This is the big challenge here, that we’re kind of throwing the kitchen sink at these patients,” he added. “Which is appropriate when you have a life-threatening illness."