Authorities routinely monitor vaccines for any signals of problems, such as the allergic reactions that do occur, rarely, every year.
Company spokeswoman Gwenan White said that GlaxoSmithKline advised medical staff in Canada last week to refrain from using one batch of the vaccine while they look into reports that that it might have caused more allergic reactions than normal.
"One batch has seen a slightly increased rate of anaphylaxis," she said. Anaphylaxis is life-threatening type of allergic shock.
Six people in Canada had suffered an allergic reaction, said Tim Vail, the spokesman for Canada's health minister. The batch contained about 170,000 doses. It was not immediately clear how many had been administered, although Vail said the majority had been.
GlaxoSmithKline said it has distributed a total of 7.5 million doses of the vaccine across Canada.
"We're not seeing any thing wild or spooky or crazy about our vaccine at all," Vail said, arguing it may have been a statistical anomaly that the reactions occurred.
The vaccine chief of the World Health Organization, Marie Paule Kieny, said Monday that the number of people who went into anaphylactic shock after getting the vaccine in Canada was "a bit higher than normal" but that the numbers were still small. She said Canadian authorities are investigating the components of the vaccine to see if there might be a problem.
She said the problem has so far only been detected with one lot of the vaccine and that they haven't seen evidence of a higher rate of side effects with other lots.
GlaxoSmithKline, the world's second largest drug maker by revenue, is only investigating the one batch of its swine flu vaccine in Canada. White said no other doses of its swine flu vaccine around the world are affected.
White said London-based GlaxoSmithKline wrote to Canadian health care professionals advising them to stop using the batch on Nov. 18.
Canada is honoring the company's request, but urging people not to be alarmed. Dr. Joel Kettner, Manitoba's chief medical officer of health, said that allergic reactions sometimes occur shortly after inoculations, but haven't lasted long and have not led to long-term health problems.
The provincial Alberta government was also holding back the vaccine, although it had not seen a jump in reactions.