The country's chief public health officer says few Canadians have suffered serious side effects from the swine-flu shot.

Dr. David Butler-Jones says that of the 6.6 million doses of H1N1 vaccine that have been given so far to Canadians, there have been only 36 serious adverse reactions.

One person is believed to have died from an anaphylactic reaction to the vaccine, but Butler-Jones stressed the death hasn't been conclusively linked to the flu shot.

"It's important to remember that just because a medical event follows vaccination, it may not have been caused by the vaccination; it may have been caused by other factors, as unfortunate events continue to occur with or without vaccine," he told a news conference Tuesday.

Butler-Jones said the serious side effects which have the potential to cause disability or death have included allergic reactions, fevers and convulsions.

With the allergic reactions seen so far, most began within minutes of vaccination and were treated promptly by medical people at the vaccination site. Milder side effects, such as nausea, soreness, headaches and fever, have also been seen.

All the side effects were expected, Butler-Jones noted.

"With any vaccination campaign, we expect to see some cases of serious adverse events. They are very rare but they are part of all mass vaccination campaigns and we expect to see a small number of them," he said.

The Public Health Agency of Canada takes all reports of adverse events seriously and all cases are fully investigated, he said.

Butler-Jones estimated that the frequency of serious reactions from the H1N1 vaccine has been less than one per 100,000 doses distributed so far, "which is what we've seen with other vaccines."

And he stressed that the small risk of side effects should not deter anyone to get the vaccine to protect their health or that of their family.

"The benefit of immunization -- the prevention of serious illness and death -- far outweigh any theoretical risks associated with being immunized," he said.

The country's chief public health officer added that about 20 per cent of Canada's 31 million citizens have received the H1N1 shot in the last three weeks.

"Canada now has the most secure supply and the most number of people immunized as a percentage of population anywhere," Butler-Jones stated.