(making clear the risk involves cowpox)

CHICAGO (Reuters) - There is a small but real chance that cowpox, a smallpox vaccine side effect, could be spread by contact if officials decide to carry out widespread inoculations to counter the threat of a potential biological attack, experts said on Tuesday. "The available data from the 1950s and 1960s show that there is a risk of ... transfer from a primary vaccinee to an unimmunized individual in contact with the vaccinee, but the risk is not large," they said in a commentary published in this week's Journal of the American Medical Association.

The overall contact transmission rate was in the range of two to six for every 100,000 primary vaccinations and in all of the studies this required close contact, rarely happening outside the home, the report said.

Smallpox vaccine is made from live vaccinia virus, which causes cowpox. The report said the cowpox virus "can be transmitted inadvertently from vacinees to others, sometimes causing serious and even fatal adverse reactions."

Health officials are weighing a possible mass inoculation program in the wake of last year's airliner attacks on New York and Washington.

The head of the Centers for Disease Control and Prevention has said the United States has enough vaccine for every American now, but officials are still weighing whether to vaccinate everyone just in case of a biological attack.

A contingency plan under consideration by the CDC calls for the vaccination of some 10 million U.S. health care workers, police and emergency technicians. Britain is planning a similar scheme to vaccinate key health care workers as a first line of defense.

Vaccinations ceased after smallpox was eradicated from the world's population in 1979 but bioterrorism experts have said for years the United States is vulnerable to a biological attack using the disease.

The JAMA essay primarily addressed the contact spread issue and not the risks to those being vaccinated but it said serious adverse reactions are more common with smallpox vaccinations than with any other kind.

It also said there are more people with eczema and other skin conditions and compromised immune systems than there were in the 1960s who may be more susceptible to adverse reactions from contact. Skin problems can cause a smallpox rash to spread.

"An orderly, systematic approach along with careful screening to identify potential ... susceptible individuals and household contacts and close monitoring for adverse effects are essential to reduce the risk of transmission ... following smallpox vaccination," the report concluded.

The commentary was written by John Neff of the University of Washington School of Medicine, Seattle; J. Michael Lane, formerly of the Smallpox Eradication Program at the Centers for Disease Control and Prevention, Atlanta; Vincent Fulginiti of the University of Arizona School of Medicine, Tucson; and Donald Henderson of Johns Hopkins University Center for Civilian Biodefense Strategies, Baltimore.