Ontario will start administering the H1N1 vaccine to school-age children and seniors with underlying health conditions starting next week.

Children under the age of 13 will be immunized because "they are highly susceptible to the H1N1 virus," Ontario's chief medical officer of health, Dr. Arlene King, said Friday.

"Immunizing children will help to reduce transmission of the disease," she told a Toronto news conference.

"Adults aged 65 and older seem to have more protection against getting H1N1 flu than the general public, but those with underlying conditions are more vulnerable to serious complications and death if they do contract the disease," she said. Some of those chronic conditions could include problems such as heart disease, diabetes, or cancer, King said.

Children and seniors will be able to get the shots at public health clinics and at family doctor's offices, she added.

Until this week, Ontario had only been vaccinating people in certain priority groups, including pregnant women, children between the ages of six months and five years and those under 65 with underlying health conditions.

On Tuesday, the province expanded the vaccination program to include members of the following groups:

• First responders, including firefighters and police officers.
• Frontline workers at correctional facilities and youth facility workers.
• Adults aged 65 years or older who live in long-term care homes.
• Between 100 and 120 members of the Ontario Ministry of Health and Long Term Care.

"The plan to expand our immunization program to more groups is based on the expectation that we would receive about 700,000 doses of vaccine per week," King said.

Rollout depends on supply

Next week, the province will receive 470,000 doses of vaccine with adjuvant, a chemical substance that boosts the immune system's response to the serum, King said. The federal government will also send an additional 375,000 doses without the adjuvant that will be administered to pregnant women.

"If the federal government ships less vaccine, the program will take longer to administer, and access may be more limited at times," King said.

Officials estimate there are about 2.25 million elementary school children and seniors with underlying chronic conditions, and it will take several weeks to immunize everyone in these two target groups.

The expansion "assumes that we will be receiving a more firm supply of vaccine than we have been receiving lately," said King. "However, we made the decision to expand now in order to protect as many people as possible with whatever vaccine we have."

King couldn't say when the province would start administering the vaccine to the general public. She said the rollout of the vaccine was being determined on an ongoing basis as the province learns how much vaccine she will be receiving.

"So at this point, I can tell you that we have been advised that [public health units] will have enough vaccine to immunize everyone by Christmas. Again, we are working towards the target of matching our delivery with that supply," she said.

King also said the province has now confirmed 61 deaths from the H1N1 virus, up from the 52 announced Tuesday.