Merck (NYSE:MRK) announced on Friday that four African countries had approved the use of the company's Ebola vaccine, Ervebo. The Democratic Republic of the Congo (DRC), Ghana, Zambia, and Burundi have all agreed to license the Ebola vaccine, with additional countries expected to approve the vaccine, as well, in the coming months.

Ervebo had already received approval from the U.S. Food and Drug Administration (FDA) back in December, while the European Commission granted the vaccine conditional marketing authorization just one-month prior in November. The use of the vaccine is limited to people who are at least 18 years old, and while Ervebo has demonstrated effectiveness in protecting patients from Ebola, the exact duration of how long this protection lasts is still uncertain.

A medical professional preparing a vaccine.

Image source: Getty Images.

"The approval of the Ebola vaccine by these countries is another milestone in the fight against this unforgiving disease. Africa has rallied to cement hard-fought progress to keep its people safe from Ebola," said World Health Organization (WHO) Director-General Dr. Tedros Adhanom Ghebreyesus.

Further details

While the drug is manufactured by Merck, the WHO has played a crucial role in expediting the approval process for the vaccine. The healthcare organization certified Ervebo back in November in what was the fastest vaccine prequalification process in WHO history.

Ervebo has a 97.5% efficacy rate, according to the preliminary data, while those who are already infected with Ebola see a significant reduction in their chances of death after receiving the vaccine. Merck's stock is up less than 1% following the announcement.