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TORONTO - The Public Health Agency of Canada is rolling out its first doses of unadjuvanted H1N1 flu vaccine this week, a move that should offer relief to pregnant women unwilling to bare their arms for the adjuvanted shot.

The agency has already begun delivering vaccine without an adjuvant to the provinces and territories and plans to have dispersed almost 225,000 doses by week's end.

An adjuvant is a substance added to a vaccine to improve the immune response. A dose of vaccine with adjuvant uses one-quarter of the amount of antigen (the material that induces an immune response) used in a shot made without such a boosting additive.

The World Health Organization had earlier advised pregnant women - especially those in the first trimester - to get unadjuvanted vaccine if available because of a lack of safety data for that population.

Based on that advice and public concern, Canada ordered two million doses of unadjuvanted vaccine, including 200,000 doses recently purchased from Australia. (Pregnant women who contract H1N1 flu are at an elevated risk for serious complications.)

But on Friday, an expert committee that advises the WHO said H1N1 flu vaccines containing adjuvant and vaccines without the additive appear to be equally safe, so there is no need to recommend that pregnant women get the latter version.

Meanwhile, initial results from a U.S. study of 120 healthy pregnant women showed they mounted a robust immune response following just one dose of H1N1 flu vaccine, an unadjuvanted product made by Sanofi Pasteur in its plant in Swiftwater, Pa.

"For pregnant women, who are among the most vulnerable to serious health problems from 2009 H1N1 infection, these initial results are very reassuring," said Dr. Anthony Fauci, director of the National Institute for Allergy and Infectious Diseases, which sponsored the trial.