Shares for mpox vaccine maker Bavarian Nordic continued to surge on Friday after the World Health Organization declared a public health emergency over an escalating outbreak of the disease in Africa, part of a wider rally for linked health stocks as nations mobilize to combat the virus and the company seeks approval to use the shot in adolescents.

Emergent's ACAM2000 is designed for use against smallpox. Smallpox, or variola, is one of the deadliest pathogens to have struck humanity—in the best of cases, around a third of patients with smallpox would die and some versions were almost always fatal—and one of history's biggest killers. It is the only human infectious disease to have been eradicated and it has not been routinely vaccinated against for decades since its eradication in 1980. Shots like ACAM2000 are believed to be incredibly effective against smallpox and, as the variola virus is related to the monkeypox virus, it is believed they would induce strong immunity against that as well. There are also immense supplies of the shot readily available as many countries—including the U.S.—maintain large stockpiles of ACAM2000 in case smallpox is deployed in a biological or bioterrorist attack. Officials are reluctant to use it in a widespread vaccination campaign, however, and it has drawbacks that make it a less favorable alternative to shots like Bavarian Nordic's. In particular, the shots have a risk of potentially dangerous side effects that were considered acceptable against a threat like smallpox but are generally considered unacceptable against a less significant threat like mpox. The shot is also inappropriate for use in pregnant people, infants and immunocompromised people, many of whom are considered to be most at risk of mpox, as it uses a live virus to induce immunity (another relative of smallpox and mpox called vaccinia). The vaccine is delivered using a more cumbersome technique than used to deliver most other vaccines today, meaning many vaccinators would require additional training and other equipment to deliver the shots at scale.