Ruth MacNeur, a consultant with Mount Sinai Hospital, receives her H1N1 flu vaccine on Monday in Toronto. (Nathan Denette/Canadian Press)

Toronto's Mount Sinai Hospital has been hit with an outbreak of the H1N1 flu.

Dr. Barbara Yaffe, the city's associate medical health officer, confirmed the outbreak at a news conference Wednesday afternoon.

In an interview with CBC News, Allison McGeer, director of infection control at the hospital, said the outbreak was possibly linked to the adult intensive care unit and affected both staff and patients. Three people have contracted the flu, she said, without specifying further due to confidentiality rules.

"We're not sure yet that they are, in fact, linked in any way. We're not even sure that they were all acquired in the hospital — obviously, there's a lot of influenza around in the community," she said.

At a news conference later Wednesday evening, McGeer said the three became ill last Friday.

She played down the outbreak declaration, calling it a precautionary measure. The outbreak classification is also a way for hospitals to communicate how they are dealing with the situation.

"When you hear outbreak, you may think [it's] the title of a movie," she said.

"Outbreaks do not mean that anything serious is going on. Outbreaks mean that somewhere in the hospital, we have a level of concern that transmission is happening."

Resources at the intensive care unit are tight, McGeer said, and “we know we are going into a time where we're going to see more patients admitted with H1N1."

"And so we were not interested in taking any risks that we might have influenza transmission in our ICU,” she told reporters.

Most Mount Sinai staff have been vaccinated, McGeer said.

It generally takes about a week for the vaccine to start working. Both staff and patients in the intensive care units have been given antivirals in the meantime, she said.

H1N1 cases up sharply

Meanwhile, the city's medical health officer said the number of H1N1 cases this fall has doubled in the past week.

There have been 115 confirmed cases of the H1N1 type of influenza since September, 68 of which were reported last week, said Dr. David McKeown, Toronto's medical health officer.

"These are confirmed cases that have been tested in the laboratory and found to be due to the H1N1 strain," he said at the news conference.

"Clearly, there are many more mild cases which are occurring in the community that don't get reported."

But McKeown said the increase is not surprising.

"I think it is unfolding the way influenza outbreaks unfold," McKeown said.

"We expect to see at this time of year an increase in influenza activity. H1N1 is the dominant influenza strain which is circulating this season."

'Please be patient'

The city has already vaccinated "several thousand" health-care workers, and those in priority groups are next in line, said Dr. Barbara Yaffe, the city's associate medical officer of health.

"We are asking all members of the general public who are not in one of the priority groups but are anxious to get the vaccine to please wait until Monday," said Yaffe.

"When our public clinics open, we'll manage the lineups to accommodate those individuals from priority groups who will be given priority access.

"For the general public, we encourage everyone to please be patient with the process and be prepared for lineups."

Clinics for people in priority groups will be open Thursday through Saturday. They were originally set to open next Monday, but public health officials moved up the schedule in light of the increase in H1N1 cases.

The city began opening vaccination clinics for health-care workers two days ago, but members of the public have already turned out en masse to the clinics.

Toronto's Sick Children's Hospital said Tuesday morning it treated "double" the normal number of cases on Monday.

The long lineups at clinics and emergency rooms came shortly after the news that the death of a 13-year-old Toronto boy was the result of H1N1.

Evan Frustaglio died Monday at St. Joseph's hospital in the west end, three days after he began experiencing minor cold symptoms.

H1N1 vaccination clinics in the Toronto area.