The virus may be more stable than a study in August found.
The WHO says 79 confirmed cases were reported, down from 150 the week before.
Guinea's trial involves ring vaccination, a strategy used to used to stamp out smallpox in the 1970s.
MSF says health groups were slow to respond, but affected nations were, too.
The WHO may have delayed an emergency declaration for 2 months partly for political reasons.
Sierra Leone announces a quarantine for its western regions, as more US health workers are evacuated.
Though patterns look promising in Sierra Leone and Liberia, cases spiked in Guinea.
Other Ebola developments include the departure of Cuba's team from Liberia, new information on evacuee contact monitoring, and outbreak case totals.
An American health worker is in critical condition, and several US health worker contacts, and one from the UK, have been flown home for observation.
As an infected American health worker arrives at the NIH, potentially exposed US and UK health workers are flown home.
