TORONTO - The number of Canadian swine flu cases moved up to 19 with the confirmation of six new cases, three each in Ontario and British Columbia.

And while the majority of the cases across the country involved people who recently travelled to Mexico, both Ontario and B.C. had one case each where the travel history wasn't immediately clear.

All the cases were mild, but Canada's chief public health officer David Butler-Jones said that as surveillance continues he expects more cases and perhaps deaths as well.

In Geneva, the WHO raised its pandemic alert to Phase 5, one step short of a full pandemic.

The increase in the alert level may make Canadians anxious but donning face masks in public and giving healthy people antiviral drugs would only make matters worse, Butler-Jones said in Ottawa.

"I'm sure many people will be anxious upon hearing this, however, for Canada it does mean a change in what we are facing and how we approach it," he said.

"We already have cases in Canada and as we continue our surveillance we will find more and more cases. As for any form of influenza, some will be more severe and unfortunately we may see some deaths as well."

Butler-Jones advised people not to start wearing masks in public, with the exception of patients in hospitals and clinics who are displaying flu symptoms and health-care workers.

"Our research shows that wearing masks actually does very little to reduce the spread of infection in the general population, and that improper use may actually increase your risk of getting ill," he said.

Antiviral drugs should be reserved for the seriously ill or those at high risk of influenza complications.

"Inappropriate use could increase the risk of resistance and the drug would not be as effective if we needed it in the event of more severe illnesses," said Butler-Jones.

Officials at the B.C. Centre for Disease Control said the three new cases involved people who had "direct contact with Mexico."

"Two out of the three have travelled back from Mexico recently. The other has had extensive contact with travellers," said Dr. David Patrick, the centre's director of epidemiology services.

"What this means for B.C. is five out of the six confirmed cases we've had to date have had that direct contact with Mexico."

The sixth case is "newly reported and public health is still interviewing and trying to find out a bit more," he added.

"It's possible that could represent a case of secondary transmission in British Columbia."

In Ontario, six of the seven Ontario residents diagnosed with swine flu had recently visited Mexico, but officials are still not sure about the seventh person.

Butler-Jones said there has been an increase in the presence of quarantine officers at Canadian airports that have direct flights from Mexico arriving.

"As the situation evolves we will continue to evolve our response."

The WHO moving to Phase 5 is less important to Canadians than what local officials report about potential person-to-person transmission in the community, said Dr. Vivek Goel, chief executive of the Ontario Agency for Health Protection and Promotion.

"We're already at the level the disease is present in our country, the next issue is to going to go to community-level transmission from person-to-person and that's probably what the public should be looking at more," Goel said at a briefing in Toronto.

"Right now we do not have evidence in Ontario of community transmission.

"In Canada, the only cluster is the one in Nova Scotia which is within a school dormitory -- very close quarters."

The 19 confirmed cases across Canada included four in Nova Scotia and two in Alberta. One person in B.C. was hospitalized overnight.

Public health officials said the three Ontario women added to the list Wednesday are all in their 20s -- the same age group as the first four Ontario cases -- and all are mild cases of the flu.

The three cases are from York, Durham and Peel regions, which all border Toronto.

Also Wednesday, the Public Health Agency's national microbiology lab agreed to assist Mexico with the testing of some 200 samples.

"We know by now that infectious diseases just do not respect any borders," Butler-Jones said.

"So anything that we can do to help our neighbours helps us to protect our own citizens as well."