Save the Children, a leading charity organization, has said Sierra Leone has the rate of five new Ebola cases per hour. This, explains the charity, means healthcare demands are far outstripping supply.
Save the Children said there were 765 new cases of Ebola reported in Sierra Leone last week, while there are only 327 beds in the country.
Experts and politicians are set to meet in London to debate a global response to the crisis.
It is the world's worst outbreak of the virus, killing 3,338 people so far.
There have been 7,178 confirmed cases, with Sierra Leone, Liberia and Guinea suffering the most.
Save the Children says Ebola is spreading across Sierra Leone at a "terrifying rate", with the number of new cases being recorded doubling every few weeks.
It said that even as health authorities got on top of the outbreak in one area, it spread to another.
The scale of the disease is also "massively unreported" according to the charity, because "untold numbers of children are dying anonymously at home or in the streets".
"We're in a race against time," said Justin Forsyth, the organisation's chief executive.
Speaking on the BBC's Today programme, he said that the figure for Sierra Leone could rise to 10 people every hour before the end of the month if urgent action was not taken.
"Cases are doubling every 20 days," he said. "The disease has now reached every county in Liberia."
Earlier this month, Britain said it would build facilities for 700 new beds in Sierra Leone, but the first of these will not be ready for weeks, and the rest may take months.
Safety trials for two experimental vaccines are under way in the UK and US, the WHO said on Wednesday, and will be expanded to 10 sites in Africa, Europe and North America in the coming weeks.
It said it expected to begin small-scale use of the experimental vaccines in West Africa early next year.
The Ebola Donors Conference in London on Thursday is being hosted by the UK and Sierra Leone governments. Its main agenda is to discuss what the global community can do to provide an effective international response to the epidemic.
It will be chaired by UK Foreign Secretary Philip Hammond, who said he hoped it would "raise even greater awareness of the disease and what is needed to contain it , encourage ambitious pledges and show our solidarity with Sierra Leone and the region."
The President of Sierra Leone, Ernest Bai Koroma, will be unable to attend as the British charter plane sent to the capital Freetown to collect him developed a technical fault. It is hoped he will join the conference by video-link.
Meanwhile the head of the UN's mission to combat Ebola warned that the disease was spreading "very rapidly" and that a "massive international response" was required to deal with the crisis.
Anthony Banbury, who is in Liberia, said more needed to be done to educate remote communities about how to protect themselves from infection.