It may not be much of a surprise that public health professionals are reporting a rise in flu cases, but it's the dominant strain that has caught them off guard. It appears that H1N1, or swine flu, which first appeared in Canada in 2009, is back.

Cities in Ontario and Alberta have reported clusters of serious cases of influenza, some of which have been severe enough to put patients in hospital.

Misericordia Hospital in Edmonton has reported that about a dozen patients and staff have contracted serious cases of the flu, and other clusters have been reported in Windsor and London, Ont., and in Calgary.

Dr. Judy MacDonald of Alberta Health Services says some influenza patients are now in intensive care.

"What we are seeing that is different this year is the severity of illness," MacDonald said.

In Alberta, 68 patients have been hospitalized and three flu-related deaths have been recorded since September.

H1N1, or swine flu, has been circulating the globe for about five years and swept across North America in 2009.

Public health officials expected that a majority of people would have developed immunity to the strain after it first circulated, but rising reports of illness suggest otherwise.

"We are seeing a large number of cases of new outbreaks in populations we thought were already protected, and that to me is kind of a surprise," infectious disease specialist Dr. Neil Rau told CTV News.

Testing in British Columbia suggests that only 50 per cent of adults, 35 per cent of young adults and only 20 per cent of children under the age of 5 are immune to H1N1.

Virus evolving

Dr. Danuta Skowronski of the B.C. Centre for Disease Control said tests also indicate that "the virus is starting to evolve."

"I sense we won't see anything like we saw in 2009," Skowronski told CTV News. "But the remaining people who are susceptible, such as the very young and young adults, may be susceptible to this virus."

People who are young and healthy and decided not to get their annual flu shot "should think twice and get the vaccine," Rau said.

He also pointed out that residents in the parts of Canada that have yet to see spike in cases should get the flu shot before flu season there ramps up.

Meanwhile, patients in the U.S. are also ending up in hospital critically ill, and health officials are investigating six deaths from H1N1. Most patients had not been vaccinated, which underscores the importance of this year's vaccine, which offers protection against H1N1.

While flu clinics close over the holidays, the flu shot is still available at pharmacies in most provinces.

With a report from CTV's medical specialist Avis Favaro and producer Elizabeth St. Philip