By Richard Black BBC Science correspondent

It sets down detailed plans for setting up emergency clinics, supplying vaccine and briefing the public.

A year after the anthrax letters began, the US health authorities rightly or wrongly continue to take the prospect of further bioterrorist attacks seriously

Experts believe that smallpox could well be the weapon of choice in any future bioterrorist attack - it is frequently fatal, spreads easily from person to person unlike anthrax, and - as vaccines have not been used for decades in the west - many people have no immunity.

The US Government has gradually been amassing a stockpile of vaccines and equipment which could be rushed to the site of an outbreak.

Current plans allow for the shipment of 75 million doses of vaccine within a day. But mass vaccination programmes need effective staff and facilities as well as the vaccine itself.

Consent forms

The guidelines tell local health authorities how to set up a so-called "model clinic" capable of vaccinating a million people a day.

Israel is already vaccinating emergency workers

They detail the staff numbers necessary and what shift patterns they should work, how the clinic should be designed and plans for security in case of mass panic.

They include forms for obtaining consent from people about to be vaccinated - a crucial issue when much of the vaccine stockpile consists of formulations which have never been properly tested.

There are directions on who to test for HIV, as the vaccines cannot be given to immuno-suppressed people.

The guidelines have been sent to the health commissioners of every US state.

They indicate that, a year after the anthrax letters began, the US health authorities - rightly or wrongly - continue to take the prospect of further bioterrorist attacks seriously.