WINNIPEG (CP) - Health Canada now plans to stockpile enough smallpox vaccine to vaccinate everyone in the country in the event of a terrorist attack, a senior official said Tuesday.

Even though the risk of such a smallpox attack is considered low, the department is already close to concluding a deal announced late last year for an additional 10 million doses of vaccine. Dr. Ron St. John said it was also decided that enough for the entire population will be stockpiled under the containment strategy he and provincial officials discussed this week at a two-day meeting.

"If you use the vaccine in roughly the first four days after the person is exposed, you can prevent them from getting the disease, so you have a window of response," said St. John, director general of Health Canada's Centre for Emergency Preparedness and Response.

"By vaccinating those people who are potentially exposed, you set up a ring of immunity around the case," he said.

"That ring could expand to include the neighbourhood, it could expand to include a community, it could expand to include a province or it could expand to include the whole country.

"What we call the surge capacity is to have enough in case, just in the unlikely event we have to vaccinate the entire country."

The 10 million doses should be in the hands of health officials by year end.

St. John couldn't say how much longer it might take to add enough for the rest of the population, about another 21 million people. He said the price is still being negotiated but smallpox is not an expensive vaccine.

There are no plans to vaccinate the general population as a precaution, only to keep vaccine in storage in case of an attack. Canada currently has a stockpile capable of protecting between two million to three million people.

Smallpox isn't an easy biological agent to deliver. It spreads through person-to-person contact, but it has been speculated it might be delivered in some form of aerosol cloud.

It's also not the most deadly biological agent, with an upper mortality rate of perhaps 30 per cent compared with up to 90 per cent for plague.

But smallpox is also the only biological agent on the A-list of potential terrorist weapons for which there is a reliable vaccine. The vaccine was used to eradicate the disease from the general population in the last century.

Smallpox symptoms begin with fever, nausea, vomiting, headache and backache. Then severe abdominal pain and disorientation can set in as small, round sores erupt all over the skin.

Dr. Joel Kettner, Manitoba's chief medical officer of health, attended the two-day meeting and said there is a big difference between deciding to stockpile a vaccine and deciding to use it.

"The capacity of having vaccine available is a completely different question from the decision about using the vaccine," he said.

He also said it's up to Canadians to weigh the cost of the vaccine against potential risks and decide whether it's worth the expense.

The smallpox vaccine comes with its own risks, including a mortality rate of perhaps one in a million. Perhaps one in 300,000 may develop encephalitis, a swelling of the brain that can lead to permanent disability.

As a result of increased tensions in the Middle East, Health Canada announced late last year that it would offer the vaccine to selected emergency workers across the country as a precautionary measure.

Iraq is one of the countries which experts say may have the capability of using the virus as a weapon.