An H1N1 vaccine tailored for pregnant women won't arrive in Ontario until after the shot for the general population is made available, Ontario's chief medical officer of health, Dr. Arlene King, told a news conference in Toronto on Friday.

Canada has purchased thousands of doses of H1N1 vaccine that lacks a chemical booster called an adjuvant, because there is limited data on the safety of the substance in pregnant women. Adjuvant is used to stretch a vaccine's active ingredient and boost immune response to the serum.

Ontario expects to roll out its H1N1 shots at "the beginning of November," King said. But she confirmed the adjuvant-free vaccine would only arrive the week of Nov. 7, later than the adjuvanted variety.

The federal government had previously said pregnant women would receive adjuvant-free vaccine at the same time as the wider population.

Pregnant women are at greater risk of developing severe complications from the H1N1 flu than the general population, research has shown.

Rollout plans may change

King wouldn't specify why there's a delay in the arrival of the adjuvant-free vaccine.

She hinted that the Ontario government's plans to administer the vaccine may change, saying "the rollout of our programs will have to obviously take into consideration the fact that the unadjuvanted vaccine will not be available until around the 7th of November."

Health Canada is now reviewing data on the adjuvanted vaccine, she said.

"When that information is available, we will be providing it to all of you as quickly as possible — in terms of the conditions for use," she said.