(Reuters) -The U.S. Food and Drug Administration granted expanded approval to Emergent BioSolutions’ smallpox vaccine for use by people at high risk of mpox infection, sending the drugmaker’s shares surging 20% before the bell on Friday.

The FDA clearance, announced late Thursday by the company, makes the vaccine, called ACAM2000, the second approved shot against mpox in the United States after Bavarian Nordic’s Jynneos.

The Emergent vaccine, however, cannot be taken by those with weakened immune systems, including people with HIV. Its use was limited earlier during a 2022 outbreak of mpox in the United States, even though it was part of the country’s stockpile.

Both Jynneos and ACAM2000 contain vaccinia virus that is closely related to, but less harmful than, the viruses that cause smallpox and mpox. ACAM2000 uses a live, infectious form of the vaccinia virus.

Still, the approval comes at a time when a new mpox strain known as clade Ib has spread rapidly in Africa. The World Health Organization earlier this month declared mpox a global public health emergency for the second time in two years.

“Mpox has progressed to become an uncontrolled epidemic in Africa … creating an enormous need to use all effective tools to extinguish it as a threat,” said Dr. Amesh Adalja, an infectious diseases expert at Johns Hopkins Center for Health Security.

Last week, Emergent BioSolutions said it would donate 50,000 doses of its smallpox vaccine to the Democratic Republic of the Congo and other impacted countries such as Burundi, Kenya, Rwanda and Uganda to address the mpox outbreak.

Gaithersburg, Maryland-based Emergent BioSolutions’ shares slumped last year amid weakness in its contract manufacturing business, but have tripled so far this year as it announced job cuts and hired former Bausch CEO Joseph Papa to lead a turnaround at the company.

(Reporting by Shivani Tanna and Mariam Sunny in Bengaluru; Editing by Maju Samuel, Subhranshu Sahu and Shounak Dasgupta)