US on smallpox alert after vials labelled with virus 'incidentally' found in vaccine lab

“We will provide further details as they are available.”

The contents of the vials were also described as appearing “intact”.

The discovery is worrying because there are only two places in the world where smallpox samples are allowed to be stored – in Russia in Atlanta, US.

Two years ago, the Russian facility near Novosibirsk in Siberia, known as the State Research Center of Virology and Biotechnology, suffered an explosion.

The site houses many terrifying pathogens, such as Ebola, anthrax and the Marburg virus.