Frozen vials apparently containing the deadly virus smallpox have been discovered in a freezer at a US facility that conducts vaccine research.

Federal health authorities confirmed the vials “were incidentally discovered by a laboratory worker” who was cleaning out the freezer in Pennsylvania.

“CDC, its administration partners, and law enforcement are investigating the matter and the vials’ contents appear intact,” CDC spokesperson Belsie González said.

“The laboratory worker who discovered the vials was wearing gloves and a face mask.”

She added: “There is no indication that anyone has been exposed to the small number of frozen vials.”

The CDC would not confirm where in Pennsylvania the vials were found.

Smallpox is a deadly, infectious disease caused by the variola virus.

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The last naturally occurring case of smallpox was diagnosed in October 1977 in Somalia, and the World Health Organisation (WHO) certified the global eradication of the disease in 1980.

It plagued the world for centuries and killed nearly a third of the people it infected – with higher death rates among babies.

The initial symptoms included fever and vomiting, along with body aches, before victims suffered spots and blisters that would leave pitted scars.

While the origin of smallpox is unknown, the earliest evidence of the disease dates back to the third century BC in Egyptian mummies.

The US ended routine childhood vaccination against smallpox by the early 1970s, and the last natural outbreak in the country occurred in 1949.

Two sites designated by the WHO store stocks of variola virus for research – the CDC facility in Atlanta and a centre in Russia.

US research of smallpox focuses on the development of vaccines, drugs and diagnostic tests to protect people against the disease in the event that it is used as an agent of bioterrorism, according to the CDC.

In 2014, a government scientist cleaning out an old storage room at a Bethesda, Maryland, research centre found six decades-old glass vials containing freeze-dried smallpox.

The samples had been packed away and forgotten in a cardboard box, officials said, calling it the first discovery of unaccounted-for smallpox in the country.

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