Zoe McKnight

The same deadly H1N1 flu virus that caused widespread panic in 2009 is back this year, but experts say there's a big difference between that outbreak and this flu season.

Toronto Public Health has confirmed two fatal cases of H1N1, and three other people died elsewhere in Ontario in December. In Alberta, demand for the flu shot spiked after three deaths in Edmonton and two in Calgary from the virus. About 270 others have been hospitalized, and 965 cases have been lab-confirmed.

As of mid-December, there were 52 hospitalizations and 323 reported cases in Ontario.

The same H1N1 strain caused the 2009 "swine flu" outbreak, which was declared a global pandemic by the World Health Organization and killed 128 people in Ontario alone between April 2009 and January 2010, according to a Public Health Ontario report.

The lack of immunity to this strain in children and younger adults made them unusually susceptible to a disease that more typically proves dangerous to elderly people and those with compromised immune systems. Infected pregnant women were hospitalized at a higher-than-average rate, and an infant who died in a London, Ont., hospital was suspected of having the virus.

While health officials and infectious-disease experts say H1N1 is responsible for the majority of flu cases this year on Ontario, those who received a flu shot are probably protected.

Officially, 130 people died of influenza last year in Toronto, but that figure probably captures only one-quarter of the cases; most go unreported. About 2,000 people die in Canada year from influenza or its complications.

Dr. Allison McGeer, infectious disease specialist at Mount Sinai Hospital, said this has been an "average" flu season generally.

"It's mostly H1N1, which means it's mostly in children and younger adults than in older adults and nursing homes," McGeer said. Mount Sinai hospital has also treated several pregnant women, she said.

In the last week of December 2012, Toronto Public Health recorded 199 lab-confirmed flu cases, mostly seasonal strain H3N2. This year, the figure stood at just 83, mostly H1N1.

All new cases diagnosed last week were classified as Influenza A; 15 were confirmed as H1N1, and the others are likely also H1N1, McGeer said. There were zero seasonal H3N2 cases, and one case of Influenza B.

The symptoms of H1N1 are the same as the seasonal flu: fever, aches and pains, chills, sore throat, nausea. Older adults who may have been exposed to it in the 1950s, when H1N1 also appeared in Canada, have some natural immunity because the strain does not mutate as readily as seasonal flu H3N2 or Influenza B. The population generally also has more immunity than it did four years ago, when H1N1 made its reappearance here.

Though this year's flu vaccine protected against H1N1, its efficacy will be reduced as flu season peaks. It takes two weeks after the shot to achieve full protection.

"It's still better than nothing," McGeer said.

There's no need to panic, said Dr. Doug Sider, medical director, Communicable Disease Prevention and Control at Public Health Ontario.

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Reporting deaths from the flu is "totally expected at this point in time in any given influenza season," he said, adding that Ontario data is only updated to mid-December and the full impact won't be known until flu season subsides and full reports are made to public health agencies.

There are probably "a couple hundred" cases of H1N1 in Ontario so far this season, but at this time last year there were 1,400 confirmed cases of influenza, Sider said, virtually all of them seasonal flu.