Kenya on Sunday declared a yellow fever outbreak after three people died and 15 other suspected cases were detected in the northern county of Isiolo.
The health ministry said it had dispatched a rapid response team to the area.
It also said it would prioritise vaccination in at least another half a dozen neighbouring counties that have now been placed on red alert.
Local media report that the first cases of yellow fever were detected in mid-January in Merti and Garba Tula areas, raising questions an why an an alert was only issued on Saturday.
But these are some of the harshest and remotest parts of the country, where families raise livestock moving from one area to another in search of pasture and water.
Health centres are also poorly staffed and equipped.
Yellow fever is caused by a virus that is transmitted to humans by mosquitoes. It’s symptoms include severe fever, headaches, jaundice and in some cases liver failure.
Yellow fever vaccinations levels are low in the country despite being relatively accessible in government hospitals. It is partly due to its high cost of about $18 (£13) to $20 which is out of reach for many Kenyans especially in rural areas.
The last yellow fever outbreak was in 1992/1993.