The Food and Drug Administration on Friday approved the first drug intended to treat smallpox — a move that could halt a lethal pandemic if the virus were to be released as a terrorist bioweapon or through a laboratory accident.
The antiviral pill, tecovirimat, also known as Tpoxx, has never been tested in humans with smallpox because the disease was declared eradicated in 1980, three years after the last known case.
But it was very effective at protecting animals deliberately infected with monkeypox and rabbitpox, two related diseases that can be lethal. It also caused no severe side effects when safety-tested in 359 healthy human volunteers, the F.D.A. said.
“This new treatment affords us an additional option should smallpox ever be used as a bioweapon,” said Dr. Scott Gottlieb, the F.D.A.’s commissioner.
Having a drug that usually cures smallpox is an important medical breakthrough, according to several medical experts not associated with the F.D.A. or the company making the drug.
F.D.A. approval is “definitely a good thing,” said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases.
Research on tecovirimat — originally designated ST-246 — began at the institute after the 9/11 terrorist attack on the World Trade Center, Dr. Fauci said. The research accompanied efforts to stretch the national stockpile of smallpox vaccine by safely diluting it.
“It all started back then, but developing a licensed product took until today,” he added.
The F.D.A. approval of the drug went to SIGA Technologies of Corvallis, Ore., a private company that developed the medicine under a federal biomedical defense contract.
Although circulating smallpox has been eradicated, two known stores of the virus exist in laboratory freezers — one in Russia and one at the Centers for Disease Control and Prevention in Atlanta.
Bioterrorism experts fear that other stocks may exist; for example, in 2014 several forgotten vials containing smallpox were found at the National Institutes of Health.
More worrisome, experts say, is the possibility that a terrorist lab or even a sophisticated amateur could use modern gene-editing techniques to rebuild the virus and then unleash it, deliberately or accidentally, on an unprepared world.
Because routine smallpox vaccination stopped after 1980, almost everyone under the age of 40 is unprotected. The disease kills almost a third of people who get it, and is even more lethal to babies.
Finding a medicine was vital because — unlike, for example, measles or whooping cough vaccine — smallpox vaccine is too dangerous to give everyone, said Dr. Peter J. Hotez, president of the Sabin Vaccine Institute and dean of the National School of Tropical Medicine at Baylor College of Medicine.
The vaccine is now routinely given only to some members of the military, lab workers and others likely to come in contact with the virus in a bioterrorism event. It cannot be given to pregnant women, or to anyone with H.I.V., under cancer treatment or with any other immunosuppressive condition; nor can the vaccine be given to anyone with eczema or several other skin diseases, Dr. Hotez said.
So a medicine like tecovirimat would be useful for treating anyone infected in the first wave of any release of the virus, as well as the millions of Americans who cannot be vaccinated.
Dr. William Schaffner, a professor of preventive medicine at Vanderbilt University Medical School, noted tecovirimat also could be useful for treating monkeypox, which infects humans and has been increasing rapidly in Africa since smallpox vaccination ended.
Monkeypox sometimes travels internationally; in 2003, there was an outbreak of 47 confirmed and suspected cases in the United States. According to the C.D.C., the virus arrived in a shipment of 800 small mammals from Ghana, including African giant pouched rats and rope squirrels intended for the pet trade. They infected prairie dogs at an Illinois pet warehouse; the prairie dogs in turn infected children who bought them as pets.
Despite its fearsome reputation, smallpox actually spreads slowly compared to more common diseases like measles or chickenpox, Dr. Schaffner said.
Symptoms like fever, exhaustion and headache typically begin 10 to 14 days after infection. These are followed by a rash of small bumps that become pus-filled sores, which can cause permanent scarring.
In severe cases, the infection causes loss of large areas of skin and bleeding. The virus can also reach the brain, leading to encephalitis, and can cause blindness by blistering the eyeballs.
When tecovirimat was tested in humans, the most common side effects it caused were headache, nausea and abdominal pain, the F.D.A. said.
Results of testing by Siga Technologies were published in the New England Journal of Medicine on July 5.
The F.D.A. gave Siga several valuable incentives toward its application for approval, including fast-track and priority review designations.