Washington, Nov 20 (IANS): Several vials that were labelled "smallpox" and accidentally found at a laboratory in the US state of Pennsylvania raised the alarm for threats of a virus leak to public health, local media has reported.

A laboratory worker found 15 vials while cleaning out a freezer in a facility that conducts vaccine research outside Philadelphia, with five labelled "smallpox" and 10 as "vaccinia", Xinhua news agency reported.

Smallpox is a highly infectious disease caused by the variola virus, which claimed the lives of 300 million people in the 20th century, according to the World Health Organization (WHO).

A possible leak put the Centers for Disease Control and Prevention (CDC) and the Department of Homeland Security on high alert. Under the WHO agreement, there are only two authorised repositories of live smallpox stocks, the CDC headquarters in Atlanta and a research center in Russia.

The CDC said Thursday that it found no evidence the vials contained the variola virus, and there was no indication that lab workers or the public had been exposed to the contents, without explanations of how the vials ended up in the freezer or if the agency investigated a virus leak.

Robert Glatter, an emergency physician at Lenox Hill Hospital in New York, was quoted by The New York Times as saying that smallpox can be lethal "even after it is freeze-dried."