Flu season has arrived in force in London with strains A and B making the spread unpredictable and deadly. 

The Middlesex London Health Unit said four people have died and 90 have been hospitalized in the past three weeks.

There have also been 22 new outbreaks between December 17 and January 6. 

What's most unusual, the health unit's statistics show, is that both strains of influenza have arrived at the same time. Typically, stain A arrives first, with strain B following in March. 

"Influenza is so unpredictable and we never know from year-to-year, the patterns change and while we have seen this pattern in the past, it doesn't come up very often," said Tracy Gordon, manager of the health unit's Vaccine Preventable disease team.

It's not too late to get the flu shot, Gordon said, recommending frequent hand-washing, coughing into your sleeve, staying home if sick and not preparing meals for others if you're ill. 

It's possible this year's flu vaccine may have limited effectiveness against the dominant strain of influenza A. 

H3N2, a subtype of influenza A, has tended to cause more severe illness among vulnerable people, including the elderly and children, Canada's chief public health officer has said.