Giving the blood of Ebola survivors to patients didn't seem to make a difference, doctors found in the biggest study so far on the approach, prompting some scientists to say it's time to abandon the strategy.

With no licensed treatment for the devastating disease, doctors have sometimes used blood from survivors to treat the sick, hoping its infection-fighting antibodies might help patients defeat the virus. It seemed to help some patients in the past but there was no clear proof. Amid the world's biggest outbreak of Ebola in West Africa in 2014, scientists decided to put the treatment to the test in Guinea.

At a clinic in the capital Conakry, scientists found no difference in survival between 84 patients who got survivor blood compared to about 400 patients treated some five months earlier, according to the study published in the New England Journal of Medicine on Thursday.

"We would have liked to have seen more dramatic results," said Johan van Griensven of the study, the paper's lead author. "But this doesn't mean (blood) plasma treatment doesn't work by definition."

He said antibody levels are often low in patients who have only recently recovered from Ebola and that doctors might need to use blood from long-term survivors to get a better effect.

Van Griensven and colleagues studied people who got two transfusions of plasma - the clear part of blood that contains antibodies - in Guinea last year. The plasma came from 58 Ebola survivors, most of whom had recovered from Ebola within six months of donating blood and are thought to have immunity to the strain. Scientists didn't know what the antibody levels were in the donated plasma and said it was possible some plasma was more potent than others.

A similar study on the blood of Ebola survivors in Liberia was discontinued because there weren't enough patients and another study in Sierra Leone had treated just three patients by last October. To date, the outbreak first noted in 2014 has killed more than 11,000 people in West Africa.

Guinea was declared free of Ebola last month.