H1N1 may have hit its peak but the risk of transmission remains high and millions of Canadians could still be infected, Canada’s top doctor warned Wednesday in pleading for Canadians to get vaccinated.

“Even if you are not personally worried about getting H1N1, for the sake of others around you, as well as your loved ones, please get your H1N1 flu shot,” said Dr. David Butler-Jones, Canada’s chief public health officer.

Despite evidence of a pandemic peak in all provinces and territories, flu activity levels are still above the expected range and the number of people with flu-like illnesses seeking medical treatment is three times higher than what is normal for this time of year.

“Passing a peak in activity doesn’t mean we decrease our concern,” Dr. Butler-Jones said. “Coming down the mountain can be just as treacherous as climbing it.”

The latest surveillance data confirms that, while human swine flu can strike anyone, “this is a younger person’s disease,” he said.

• The highest proportion of H1N1 infections are occurring in people under 15; 40% of deaths are occurring in people aged 40 to 65. With seasonal flu, the vast majority of deaths are in the very elderly or those with serious underlying disease.

• Forty per cent of those admitted to intensive care, and one-third of those who have died had no pre-existing medical conditions. Even among those with an underlying illness, “many of them are well-controlled diabetics, well-controlled asthmatics, until they get this disease and it tips them over the edge,” Dr. Butler-Jones said.

• Sixteen per cent of those hospitalized with H1N1 have required intensive care, and up to 80% of those admitted to an ICU have required breathing assistance or ventilation. “This is over three times the frequency for respiratory support for flu and pneumonia seen during the 2007-08 flu season,” Dr. Butler-Jones said.

• Those most seriously affected by H1N1 are 23 to 45 years younger than what is normally seen with seasonal flu. During last winter’s flu season, the median age for hospitalization from flu and pneumonia was 71. For ICU admission, it was 68 and for death, 82.

As of Nov. 21, the median age of H1N1 hospitalization in Canada was 26. For ICU admissions, it was 45, and, for deaths due to H1N1, it was 54.

Dr. Butler-Jones said more than one-third of Canadians are immune to human swine flu, either because of vaccination or infection with the virus.

But it’s not a hard number. The estimate isn’t based on “serology” reports, where a group of randomly sampled people have their blood tested to see how many have been exposed to the virus. Rather, the estimate is largely based on the proportion of flu tests coming back positive, visits to doctor and surveys of how many people have been sick at home with flu-like illness.

“In the big scheme of things, it still means that over half of us, if we’re susceptible, are vulnerable to this virus and that’s millions of people still that could become ill over the next coming months,” Dr. Butler-Jones said.

“I cannot stress enough how important it is to get the vaccination. There is a huge cost associated with not getting immunized and with H1N1.”

His plea comes as some mass vaccination centres said demand for the shot is declining.

In Hamilton, the city’s last flu shot clinic will close its doors on Friday, said Dr. Christopher Mackie.

“Over the past couple of weeks, the rate of illness has plummeted. We’re almost down to normal levels of flu activity,” he said. “We’re pretty confident that our family doctor colleagues and walk-in clinics can pick up the slack.”

At the peak, Hamilton was operating four clinics, which were seeing as many as 3,000 people daily, Mackie said. The one remaining clinic is only taking in about 100 patients each day this week.

In Toronto, two clinics closed their doors Sunday because of a drop in demand. According to the Ontario Ministry of Health, about 4.25 million doses of the H1N1 vaccine have been administered, representing an inoculation rate of 32.5%.

Alberta has also noticed a drop in demand for the shot, said Health Services spokesman Bruce Conway, adding there are currently no plans to close any centres.

“The clinics are sill operating, but the lineups certainly are not significant,” he said “Wait times are minimal, but people are still moving through.”

About 27% of Alberta’s population has got the shot, he said.

In neighbouring Saskatchewan — which estimates that 40 to 50% of its population has rolled up their sleeves — clinics are also moving “full steam ahead,” said spokeswoman Joan Petrie.

“We are very appreciative of the strong response that we’re getting from the general public . . . We’re very encouraged by that and we’d like to see it continue as long as the clinics are open.”

More than 20 million doses of H1N1 vaccine will have been shipped to the provinces by the end of this week, Federal Health Minister Leona Aglukkaq said during a news briefing. Later in the day, she was on the hot seat in the House of Commons health committee where she faced a demand for a public inquiry into the government’s response to the pandemic. The New Democratic Party’s health critic Judy Wasylycia-Leis asked whether the government would agree to an independent investigation after the pandemic is over. Aglukkaq did not give Wasylycia-Leis the “yes or no”answer she asked for, but appeared to dismiss the call.

“We will always evaluate how we have done in responding to this pandemic and we continue to do that on a daily basis,” she said. Aglukkaq said in her discussions with her provincial and territorial counterparts they have agreed to come “back to the table” to review how the pandemic was handled.

As of Tuesday, 329 H1N1-related deaths had been reported to the Public Health Agency of Canada.