The U.S. Centers for Disease Control and Prevention and the Food and Drug Administration took part in a World Health Organization meeting to discuss flu vaccine composition, an official at the UN agency said on Friday.

The World Health Organization is investigating "another cluster of illness" in northern Congo, as a deadly mystery disease spreads in the region.

In previous cases, the cases are linked to a known disease, but a lack of available testing leads to lack of certainty. For example, there was a separate report of an "unknown disease" in December of last year in Congo that was later attributed to illnesses from malaria and respiratory illnesses.