A hospital in Oakville, Ont., is dealing with an influenza outbreak in one of its largest inpatient units.

Halton Healthcare Services say nine patients at Oakville Trafalgar Memorial Hospital have a respiratory illness. Three of those patients have tested positive for Influenza A (H3N2), a strain that tends to cause more severe illness than others. It's also the same strain which this year's flu vaccine may not be as effective against, the U.S. Centers for Disease Control recently warned.

The outbreak in Oakville was confirmed by Halton Healthcare on Wednesday. The hospital has since implemented its Infection Prevention and Control Action Plan to contain the spread of the influenza.

Dr. Neil Rau, an infectious disease specialist, says the outbreak is especially worrisome given the hospital's population.

"The problem with this kind of population is that most people are over 65 or with underlying medical diseases, and these are the people who can have very serious consequences from getting the flu," Rau told CTV Toronto.

He adds that because not all of the flu virus strains currently circulating in Canada are well-matched to this year's vaccine, hospitals cannot rely on the preventative antiviral medication as their main defence strategy.

"Usually we rely on staff vaccination to protect our patients from getting infections from people in the community (but) when you have a bad vaccine match you really can't rely on that," he said.

According to the regional health group, the patients contracted the influenza during their stay in the hospital's Medicine 4C unit. All 40 patients in the ward have been given preventative antiviral medication. Hospital visits are also being restricted during the outbreak.

Across the province, there has been a wave of influenza cases. Public Health Ontario says there have been 342 so far -- nearly double the 182 this time last year. Of those cases, 120 were hospitalized and three have died from influenza.

With a report from CTV Toronto's Heather Wright