The Food and Drug Administration has approved the first vaccine meant to prevent a rare viral infection that affected dozens of people in the Midwest 16 years ago.

The agency said Tuesday that it approved Copenhagen-based Bavarian Nordic’s Jynneos, for the prevention of monkeypox and smallpox. The vaccine is meant for adults who are considered to be at high risk of either infection. In addition to being the first FDA-approved monkeypox vaccine, it is also the first non-replicating smallpox vaccine to win the agency’s approval.

Shares of Bavarian Nordic were up more than 21 percent on the over-the-counter market following the news.

Although smallpox long plagued humankind and was responsible for extremely deadly epidemics around the world – particularly among Native Americans, who lacked natural immunity to it – the World Health Organization certified the eradication of naturally occurring smallpox in 1980, and routine vaccination in the U.S. ended in 1972. However, because of its eradication, many people no longer have immunity.

“Therefore, although naturally occurring smallpox disease is no longer a global threat, the intentional release of this highly contagious virus could have a devastating effect,” said Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, in a statement. “Today’s approval reflects the U.S. government’s commitment to preparedness through support for the development of safe and effective vaccines, therapeutics, and other medical countermeasures.”

Another smallpox vaccine is Sanofi’s ACAM2000, which the FDA approved in 2007.

According to the Centers for Disease Control and Prevention, there is no immediate threat of the use of smallpox in terrorism, and only two labs are certified to have it, namely the CDC itself and the Russian State Center for Research on Virology and Biotechnology. There is, however, lingering concern that older samples may have fallen into the hands of terrorists.

Meanwhile, monkeypox is native to Africa and does not occur naturally in the U.S., being transmitted to humans from animals like primates and rodents. However, the U.S. experienced an outbreak in 2003. According to the CDC, the outbreak – which affected up to 47 people in six states in the Midwest – stemmed from a shipment of animals from Ghana to Texas. The outbreak marked the first time the disease was reported outside of Africa.

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