TORONTO (CBC) - Ontario's top medical official is having to fight off criticism that the province's goal of getting 2.2 million people inoculated against the H1N1 flu virus by the end of the week may not be achievable.

In order to reach that mark floated by Ontario Health Minister Deb Matthews Monday the province would have to vaccinate 300,000 people a day.

When asked by a reporter at a Tuesday news conference if health officials can reasonably vaccinate people at that rate, Dr. Arlene King, Ontario's chief medical officer of health, said "we're doing our very best to get the vaccine out there."

King couldn't say with certainty if the goal could be met, but told reporters "it's really important to have a target that we're working with and I hope we meet that target."

"I have every confidence that everyone will do their very best to get as much vaccine into people as possible and I look forward to seeing some numbers later on in the week to see how we've done."

The province has focused its vaccination efforts in the past nine days on priority groups people who are at a higher risk of developing complications from the virus than the general public. Healthy adults in some communities including Toronto were due to start receiving shots on Monday.

On Friday, however, the province announced that only those who belonged in priority groups would get the shot this week, due to a shortage of the vaccine.

The opposition at Queen's Park slammed the vaccination rollout, saying there aren't enough clinics.

"Now they're stuck trying to play with the numbers to convince the people of this province that by the end of the week they'll have it under control, and I don't believe they will," said NDP Leader Andrea Horwath.

Progressive Conservative leader Tim Hudak said the "government isn't prepared for this crisis."

Premier Dalton McGuinty said public health officials have the crisis well enough in hand to justify his Wednesday trip to Mexico to support Toronto's bid for the Pan-Am Games.

King, meanwhile, wasn't able to give a hard number on the number of Ontarians who have already received the vaccine, putting that number at the "hundreds of thousands."

She tried to put the impact of the flu in perspective, noting 33 Ontarians had died because of the H1N1 virus since April.

"This is a rate of 2.6 deaths for every million people, King said. "To date, 639 confirmed cases have been hospitalized, representing a rate of 4.9 hospitalizations per 100,000 people."

King said swine flu activity is higher than what is normally seen at the peak of flu season, and also higher than during the first wave of the virus in June.