Fearing that a terrorist attack could create an outbreak of smallpox, Health Canada plans to vaccinate about 500 Canadians against the disease, CBC News has learned.

The decision to inoculate doctors, nurses and lab technicians against a disease that was eradicated more than 20 years ago comes after consultations with the provinces over the past year.

• INDEPTH: Smallpox

"We're thinking of a number around 500 who would need vaccination," said Dr. Paul Gully, senior director general at Health Canada.

Extreme reaction

The World Health Assembly announced in 1980 that smallpox had been wiped out, and told countries around the world to stop vaccinations. The last known case of naturally occurring smallpox was diagnosed in 1977 in Somalia.

But the virus still exists in laboratories, and Russian scientists have talked to CBC's the fifth estate about how they turned it into a weapon – a weapon that could make it into the arsenals of rogue states, or the hands of terrorist groups.

Expert says it's a 'dangerous' vaccine

Grant McFadden

With about 350,000 doses of smallpox vaccine still in storage from the 1970s, Health Canada wants to have the 500 vaccinations done by the end of the year.

Some 500,000 front-line medical and lab workers in the United States will be vaccinated against smallpox.

The World Health Organization continues to recommend against mass vaccinations, because the vaccine comes with some nasty side effects. Grant McFadden, a microbiology and immunology professor at the University of Western Ontario in London, calls it a "dangerous" vaccine.

"The percentage of complications can be one in 1,000," he said.

Those complications can include sever eczema and encephalitis. A product called vaccinia immune globulin (VIG) can counter the reactions. But Health Canada doesn't have any, and has none ordered.

"The requirement for vaccinating 500 people would be relatively small," said John Langstaff, president of Cangene, a Winnipeg company that makes VIG.

"The best way to mitigate your risk of potential serious adverse reactions would be to have some VIG on hand."

The U.S. government, which plans to start vaccinating frontline hospital workers, has ordered 100,000 doses of VIG from Cangene.

Health Canada's vaccination program will be voluntary.