US regulators have approved the first treatment designed to mitigate the impact of a potential outbreak of smallpox caused by bioterrorism.

Smallpox is a contagious, disfiguring and often deadly disease that has affected humans for thousands of years.

Naturally-occurring smallpox was eradicated worldwide by 1980, the result of a huge global immunisation campaign, but samples of the virus were kept for research purposes, and there are fears that it could be used as a biological warfare agent.

No cure or treatment for smallpox exists, and while a vaccine can prevent smallpox, the risk of the vaccine's side effects is too high to justify routine vaccination for people at low risk of exposure, SIGA noted.

TPOXX, also known as tecovirimat and ST-246, is an orally administered and IV formulation antiviral drug for the treatment of human smallpox disease caused by variola virus.

FDA approval is based on data from 12 clinical trials of oral TPOXX in over 700 healthy human volunteers, which showed no drug-related serious adverse events.

Also, the drug significantly cut both mortality and viral load in non-human primates infected with monkeypox virus (MPXV) and in rabbits infected with rabbitpox virus.

“The FDA approval of TPOXX achieves an important objective for both SIGA and our lead partner in the US Government, the Biomedical Advanced Research and Development Authority (BARDA),” said Dr Phil Gomez, Siga’s chief executive.

“The approval validates this novel smallpox therapy as an important medical countermeasure in response to a potential smallpox outbreak.”