American health officials have devised a huge inoculation campaign to ward off threats from a bioterror attack.

The New York Times reported that 500,000 health care and emergency workers would be vaccinated against smallpox, with plans in preparation to safeguard the public too.

Smallpox

Has existed for 3,000 years Spreads through the air Fatal in up to 30% of cases No known cure Vaccination before exposure or 2-3 days after offers almost complete protection Vaccination 4-5 days after exposure may prevent death US stopped routine vaccination in 1972 Adults vaccinated as children may no longer be protected Vaccine can have serious side effects Sources: CDC and WHO

The aggressive tactics are now possible because the vaccine has been produced rapidly and stockpiled since the 11 September attacks on the US, the newspaper quoted government officials as saying.

A health department spokesman told the Reuters news agency there would be vaccinations, but the numbers of people who needed to be protected had not yet been decided.

The plan to increase the number of workers vaccinated comes amid talk of an attack on Iraq, which some experts suspect of hiding smallpox stocks.

While the disease was eradicated for natural transmission in 1979, the US and Russia kept stocks of the disease for research and it is feared other countries also have access, which could lead to terrorists getting hold of it.

The US Government had planned to vaccinate only a few thousand health workers against the highly contagious disease.

Options now available

But Dr Donald Henderson, senior science adviser to Health and Human Service Secretary Tommy Thompson, told the New York Times the plans were changing.

"Now we can act differently because we have more vaccine," he said.

"We can make vaccine available on request throughout the community," added Dr Henderson, who played an important role in the global smallpox eradication programme led by the World Health Organization (WHO).

About 100 million doses of the smallpox vaccine are now reported to be available and by late this year there will be enough for every American - more than 280 million people.

Recommendations pending

Bill Pierce, spokesman for the health department, said that final decisions still had to be made.

The 15-member Advisory Committee on Immunization Practices, which sets vaccine policy in the United States, recommended on 20 June that the number of people who get smallpox vaccinations should be expanded, but they stopped short of advising vaccinations for the general public.

Mr Pierce said the recommendations were very general and required more work by his department and the Centers for Disease Control (CDC) to identify which groups should be vaccinated.

It was not yet known when Mr Thompson would take the ultimate decision.