Health authorities are also debating the proposed use of an adjuvant in the pandemic shots, which boosts the effectiveness of the vaccine so a lower dose of the active ingredient can be used.

Canada's seasonal and pandemic flu vaccine contract is with GlaxoSmithKline, which makes the vaccines at its facility in Quebec City.

GSK plans to sell H1N1 vaccine that includes 3.75 microgram doses with adjuvant — 25 per cent of the size of the doses the United States is buying from pandemic vaccine suppliers that are not using adjuvants.

The federal government has ordered 50.4 million doses of vaccine, most of which will contain an adjuvant. About 1.2 million doses have been set aside that won't include the adjvuant and are intended for pregnant women. Little research has been done on the effect the adjuvant could have on them.

Other unresolved questions include whether an adjuvant is needed at all...