West Vernon Children Centre employee Sharyn Stokes cleans plastic toys in the preschool classrooms in Vernon, B.C. The centre runs the after-school children's programs for Beairsto Elementary School, which was recently closed because of a case of swine flu. (Jeff Bassett/Canadian Press)

As the number of swine flu cases continued to climb across the country on Friday, Canada's chief public health officer expressed some optimism about the outbreak.

The national total of confirmed cases, so far, stands at 55. The cases are all described as mild, and there have been no deaths as a result of the virus, which has been identified as a new strain of the H1N1 subtype of type A influenza. This particular virus strain is believed to have originated in Mexico and has since appeared in Canada, the U.S., Europe and elsewhere.

Public Health Officer David Butler-Jones said the more public health authorities look, the more cases they are likely to find. He warned that the severity will vary but that some cases might be fatal as is the case with every seasonal flu.

He also pointed out that influenza viruses constantly change and mutate, pointing out that the first round of the 1918 Spanish flu was much milder than the second.

But he said there were reasons to be optimistic.

"What gives me short term optimism is that we are at the end of what would be normally flu season. So, the chances are that this will die down over the summer," Butler-Jones said.

Assuming it follows the normal pattern for influenza, researchers will be in a position to develop a vaccine over the summer, he said.

He also stressed that the cases so far in Canada and the U.S. have been more typical of mild seasonal flu.

"This is the first time in human history where we're actually watching the development of a potential pandemic," Butler-Jones said. "That's never happened before; nor have we ever had the kind of science capacity, hospital and treatment capacity etc. that we do now."

B.C. has highest number of cases in the country

Ontario confirmed four additional cases Friday, bringing its total to 12.

British Columbia confirmed three more cases Friday, raising its total to 19, the most of any province. The virus has killed at least 10 people, nine in Mexico and one in the United States, according to the World Health Organization.

Two B.C. schools have been closed because of swine flu concerns.

Decker Lake Elementary School, northwest of Prince George, was closed early for the weekend on Friday after more than 20 students called in sick, though it's not yet clear whether they have contracted swine flu.

Beairsto Elementary School in Vernon was closed on Thursday when a student tested positive for swine flu after returning from a vacation in Mexico and will remain closed throughout next week.

B.C. health officials decided to keep a third school, Vancouver's Killarney Secondary School, open despite having one student who was diagnosed with swine flu.

The student was recovering well at home and the risk to others at the school was low, officials said.

Nova Scotia recorded six additional cases, bringing its total to 14. Alberta also confirmed two more mild cases for a total of eight and New Brunswick recorded its first case, as did Quebec.

So far, Saskatchewan, Manitoba, Newfoundland and Labrador, P.E.I. and the territories have not confirmed any cases of this particular flu strain.

B.C. Premier Gordon Campbell cautioned that health authorities do expect more cases will be confirmed. He also warned some deaths from the virus are possible, just like normal influenza, and people do have a legitimate reason to be concerned.

The WHO raised its global pandemic alert to Level 5, its second-highest level, on Wednesday. A Level 5 alert means there has been human-to-human spread of a virus in at least two countries in one region, according to the organization's pandemic response guidelines.

The classification means a pandemic is imminent, and countries must finalize preparations to deal with the outbreak, officials said.

The WHO's emergency committee, which can recommend a change to the pandemic alert level, is not scheduled to hold any meetings on Friday, though the committee will convene if the flu outbreak widens, officials said.

To declare a full-blown pandemic, the WHO would have to be convinced the new virus is spreading in a sustained way among communities in another region besides North America.

With files from The Canadian Press