Ivory Coast began a roll-out of vaccinations against Ebola on Monday, after the country recorded its first known case of the disease since 1994, the health ministry said.
"Health workers, close relatives and contacts of the victim" were the first to be vaccinated, getting jabs from 5,000 doses sent from Guinea, spokesman Germain Mahan Sehi said.
Ivorian health workers had previously said that vaccinations of "targeted groups" had already begun on Sunday.
The single identified case of Ebola in the country was identified over the weekend in Abidjan, Ivory Coast's economic hub, in an 18-year-old Guinean woman who had arrived in the country on Wednesday after travelling by road from Labe in Guinea, the authorities said on Saturday.
The World Health Organization (WHO) said genetic sequencing of a virus sample would determine whether the case was linked to a recent flare-up in neighbouring Guinea.
The fact that it had occurred in a conurbation of more than four million people was of "immense concern," it said.
Ebola causes severe fever and, in the worst cases, unstoppable bleeding.
It is transmitted through close contact with bodily fluids, and people who live with or care for patients are most at risk.
The death rate varies from 25 to 90 percent, according to past outbreaks, although the chance of survival rises significantly if the disease is spotted at an early stage.
Combatting Ebola outbreaks mainly involves the time-honoured technique of tracing and isolating people who have been in contact with patients.
This has recently been joined by a vaccine that was extensively deployed against an epidemic that ran in eastern DR Congo from August 2018 to June 2020, claiming more than 2,200 lives.
The discovery in Ivory Coast comes nearly two months after the UN's health agency declared an end to Guinea's second outbreak of Ebola, which started last year and claimed 12 lives.
Five WHO experts have been sent from Guinea to help train several dozen Ivorian health workers in using the vaccines.