Flu shot: What we know about this year’s vaccine and ‘virulent’ dominant strain

With flu activity high in Canada, infectious diseases experts are urging Canadians to get their flu shot, as it contains protection against the strain of flu proving most dominant.

With flu activity in Canada reaching well above typical levels, infectious diseases experts are concerned about lower uptake of the flu shot, especially since this year’s dominant strain can cause more severe illness.

The flu shot can be especially beneficial this season, they say, as this year’s vaccine contains protection against the strain of flu proving most dominant, which is not always the case.

As of Friday, the percentage of positive influenza cases rose to 16 per cent from 6.3 per cent the previous week, Canada’s chief medical health officers, Dr. Theresa Tam, said in a briefing last week.

This is more than three times the seasonal threshold of five per cent, which prompted the Public Health Agency of Canada (PHAC) to declare the beginning of a national influenza “epidemic” last week.

The vast majority of flu cases detected and lab tested this year so far have been caused by a strain of the influenza A virus known as H3N2. A very small percentage – just three per cent to date – were the influenza A strain known as H1N1, according to FluWatch data released weekly by PHAC.

Both of these strains are included in this year’s flu vaccine, which also includes two isolates of influenza B virus.

International experts convened by the World Health Organization before every flu season provide advice, based on evidence from global surveillance, on what types of flu viruses are expected to cause the most illness and therefore should be included in the seasonal flu vaccine.

That the most dominant strain circulating this year is included in this year’s flu vaccine is both good news and bad news for Canadians this flu season, says Matthew Miller, director of the Michael G. DeGroote Institute for Infectious Disease Research at McMaster University.