The irony is that although keeping them allows scientists to develop better defences, it also makes it possible for a would-be bioterrorist to get hold of the virus

There are officially only two centres in the world which keep stocks of smallpox virus - the Russian Vector laboratory, and the United States Centres for Disease Control.

Following the successful completion in 1979 of the global programme to eradicate smallpox, it has been presumed that at some stage these two batches of virus would be destroyed.

In 1999 the World Health Organisation set a target date of the end of this year for destruction.

Now its expert committee says the stocks should be kept for longer to allow work on drugs and new vaccines to be completed.

Dangers

Without the events of 11 September and the incidents involving anthrax which followed, it is doubtful whether the committee would have reached the same conclusion, although US President George W Bush said a couple of months ago that he favoured retaining the American samples.

The irony is that although keeping them allows scientists to develop better defences, it also makes it possible for a would-be bioterrorist to get hold of the virus.

There are unanswered questions too about who can authorise destruction.

In the early 1980s, many countries sent their stocks to the Vector and CDC labs for safe keeping.

There has been no agreement on whether those countries retain rights to their smallpox or whether the present keepers can decide when to destroy it.