Q: It seems like every year there’s a new strain of flu circulating. How does this happen and is it a cause for concern?

A: It’s a common misconception that a new strain of flu develops every year.

The fact is there are already over 60 known flu viruses that have been identified and the predominance of one or any of them changes from year to year. Every year in January, teams at the World Health Organization (WHO) analyze data submitted from health authorities in various countries to determine which flu strains are the most common or widespread at that point. This information is then used to develop the seasonal influenza vaccine—composed of two strains from the A type and one strain from the B type—that will be widely distributed later that year in November. Despite the 10-month lag, the flu strains which are active in January are usually still predominant in November, a fact which counters the common argument that a vaccine “based on last year’s flu won’t help me this year.”

Occasionally, a new flu strain will appear, such as the H1N1 strain in 2009

This causes a higher-than-normal incidence or severity of flu. The H1N1 strain is still active and it makes up part of the flu vaccine developed for this coming year.

The fact that the flu changes from one year to the next is not a cause for concern, especially if you get vaccinated. Plus, getting your flu shot every year helps you increase your immunity to a number of different flu strains and not just the three most common strains that are active this year. 

Montreal Children's Hospital