Almost 300 fewer people are known to have died from Ebola in Liberia than previously thought, while more than 200 have been added to Sierra Leone’s death toll, the World Health Organization said in a regular Ebola update on Wednesday.

The WHO said it had tried to clean up data after laboratory tests weeded out many false positives — “probable” and “suspected” deaths that turned out not to be Ebola — with the end result that the total death toll stood at about 4,922, unchanged from the previous count last Friday.

Experts say that monitoring the spread of Ebola is key to defeating it and more accurate data will help to gauge the success of efforts to contain the virus.

The WHO has always given strong warnings that although the Ebola figures it presents are the best available, numbers are subject to revisions.

“Many people who were ‘probable’ turned out not to be Ebola after all,” said WHO spokesman Tarik Jasarevic. “The numbers are changing all the time. It’s not that from now on it’s always going to be the definitive number.”

Even the latest, revised death toll contains unconfirmed Ebola cases that could later be removed, and the data will be revised again in future, he said.

Apart from the sheer number of Ebola cases, experts say that there are many other feverish illnesses that can be mistaken for Ebola. Jasarevic cited the case of a village where many people had died and the deaths were ascribed to Ebola. But on checking, “it turned out many of them were not Ebola cases”.

The WHO document gave figures totalling 4,920 deaths and 13,703 cases in the eight countries affected by the outbreak, as of the end of Oct. 27. That would imply two fewer deaths than in the previous update.

WHO Assistant Director General Bruce Aylward had already disclosed the number of cases earlier on Wednesday, and said it had surged by more than 3,000 since last week largely because a backlog of old cases had been added to the database