With Ebola nearly stamped out in West Africa, vaccine trials will probably fail to provide enough useful data on how well they protect people against the deadly virus, the World Health Organisation (WHO) said on Tuesday.
Liberia was declared free from Ebola by the government and the WHO on Saturday after 42 days without a new case of the virus, which killed more than 4,700 people there during a year-long epidemic.
Guinea reported seven cases in the week of May 4-10, while Sierra Leone had two, Dr. Marie-Paule Kieny, WHO Assistant Director-General for Health Systems and Innovation, told a news briefing in Geneva.
"The best news is we are going to zero cases, there is absolutely no doubt about that," she said.
But two experimental Ebola vaccines - developed by GlaxoSmithKline and jointly by Merck and NewLink Genetics - being tested on volunteers may not yield sufficient data on efficacy as case numbers fall, Kieny said.
"It is not clear whether it will be possible to have even a hint of efficacy from these two vaccines," she said, noting that they already had been proven safe.
"To have efficacy we must see if people are actually protected, as the number of cases is going down it is not clear whether there will be a strong robust answer to this question at the end of epidemic," she said.
Two drugs - Zmapp made by Zmapp Pharmaceuticals and sIRNA by Tekmira Pharmaceuticals - are also being tested and it is hoped that they will produce some limited results on efficacy, Kieny said.
The UN agency this week hosted a two-day experts' meeting on Ebola research and development after the world's largest epidemic that has killed more than 11,000 since December 2013. The aim is to draw up a plan to speedily develop vaccines and drugs for use in clinical trials during any future outbreak of any infectious disease.
Consultations will be held in coming months on issues including developing protocols, data sharing and storing biological material including the virus and blood serum of patients, Kieny said.
Asked when a framework deal on research and development could be reached, she said: "Tentatively we try to go for the end of the year."