Officials within the World Health Organisation in Geneva, which in 1990 recommended the destruction of the stocks kept in Russia and the US, said yesterday that the 31 December deadline will not be met despite an agreement by the two countries to destroy their virus samples by then.
An intense debate among scientists has recently cast doubt on whether it is wise to destroy the remaining samples kept in secure laboratories at the US Centers for Disease Control and Prevention in Atlanta, Georgia, and the Research Institute for Viral Preparations in Moscow.
The latest evidence against destroying the stocks comes from scientists from the US National Center for Infectious Diseases in Atlanta, who say in today's issue of the journal Nature that much remains to be learned about the reasons for the virulence of smallpox compared to other, harmless, pox viruses.
In 1967 there were 10 million cases of smallpox in 40 countries, mostly in the developing world, but a successful vaccination campaign organised through the WHO quickly brought the epidemics under control. The last natural outbreak of the disease occurred in Somalia in 1977.
A year later, however, a photographer working in a laboratatory at Birmingham University died after accidently becoming infected with smallpox kept in a supposedly secure laboratory some distance from the room where she worked.
Her death gave weight to a call to put remaining laboratory stocks of smallpox under international control. At the end of 1979, the Global Commission for the Certification of Smallpox Eradication recommended all remaining stocks should be destroyed or transferred to designated centres.
By 1983 the only known stocks were transferred to either the Moscow institute, which has kept about 150 smallpox strains, or the Atlanta centre, which has kept about 450 strains from around the world.
Because advances in genetic engineering during the 1980s made it possible to determine the complete genetic structure of the virus, the US government in 1990 called on the then Soviet government to consider destroying the stocks, a proposal endorsed by smallpox experts at the WHO. However, in the past year a rift has developed within the scientific community between those in favour of destroying the stocks, and those against.
David Bishop, head of the Institute of Virology at Oxford, has argued that destruction of the smallpox stocks does not remove the threat. Frozen bodies exhumed from Siberian permafrost, for example, have been found to contain evidence of infection with smallpox.
There is also the risk of monkeypox virus - which causes a disease in humans that resembles smallpox - mutating into a more virulent form that could spread easily from one person to another.
More importantly, the opposers of smallpox destruction argue, the virus will be needed to shed light on how it causes disease: scientists are only just beginning to understand how viruses work, they say.