Sierra Leone's western area is seeing intense Ebola activity, and in Guinea a surge of cases in one area propelled the country into its highest weekly case incidence so far.
The authors say the findings have proved useful in creating a more potent version.
The CDC director said Ebola outbreak conditions have improved since his first visit, but tough challenges remain.
In the first stop of a 3-day tour, the UN director warned against easing up on response efforts.
New information now indicates phase 2 trials will most likely start in Africa in February.
Case counts vary, but progress is being made toward the UN goal of isolating & treating, and safely burying where necessary, all Ebola patients.
A report in MMWR details challenges investigation teams in West Africa have faced.
The nation announced a ban on Christmas and New Years' festivities and a month-long response surge.
Survivors face stigma and other complex issues because of the unique nature of the disease.
Officials discuss health system recovery in West Africa, while Swiss vaccine trials hit a snag.
