Guinea started an Ebola vaccination campaign on Tuesday, the World Health Organization (WHO) said, as authorities race to contain the first resurgence of the virus there since the world’s worst outbreak in 2013-2016.

The vaccination was launched in Gouecke, a rural community in N’Zerekore prefecture, where the first cases were detected on Feb. 14, the WHO said, adding that the launch started with vaccination of health workers.

“The vaccination uses the ‘ring strategy’ where all people who have come into contact with a confirmed Ebola patient are given the vaccine, as well as frontline and health workers,” the WHO said in a statement.

The resurgence of the virus, which causes severe bleeding and organ failure and is spread through contact with body fluids, has alarmed governments in the region and international health organisations, concerned that a major outbreak could overwhelm health infrastructures already battling a pandemic.

“The last time Guinea faced an Ebola outbreak, vaccines were still being developed,” WHO Director-General Tedros Adhanom Ghebreyesus said in the statement. “With the experience and expertise it has built up, combined with safe and effective vaccines, Guinea has the tools and the know-how to respond to this outbreak.”

Guinea has recorded four confirmed and four probable cases, including five deaths, in the first resurgence of Ebola since the 2013-2016 outbreak that killed 11,300 people in Guinea, Liberia and Sierra Leone.

The WHO said in the statement that genome sequencing is underway in Senegal’s Institute Pasteur to identify the strain of the Ebola virus.