Flu season has arrived and it could be a bad one

Flu season has arrived early to North America this year, and it may end up being a doozy.

Cases of flu tend to ramp up mid- to late-December and peak in January or February, according to Paul Sax, an infectious disease physician at Brigham and Women’s Hospital in Boston. But this year, things are different.

The Center for Disease Control and Prevention releases a weekly flu report where it tracks flu activity across the country. This year, the percentage of patients reporting flu-like symptoms reached the threshold of 2.2% by late November, an indication that flu season has started. 

Not only has flu season arrived early, but the U.S. may be particularly hard-hit. It’s difficult to make predictions about how a flu season will play out, as flu viruses are wildly unpredictable. But the Southern Hemisphere experienced an especially bad season over the summer (their winter), with Australia reporting record-high numbers of influenza cases and a higher-than-average number of hospitalizations and deaths.  

There are a several different strains of flu viruses, and the main culprit in Australia was a strain called H3N2. However, the flu vaccine was only 10% effective against that strain, meaning it reduced a person’s risk of infection by 10%, according to a paper published in the public health journal Eurosurveillance.

That same strain, H3N2, was the most common strain of virus in North America last season, and is likely to be this year as well. Experts worry because the H3N2 portion of the vaccine used here in the U.S. is identical to the one used in Australia.

While 10% seems low, “flu vaccines are usually only about 40% to 60% effective in the best of years,” said Martin Hirsch, an infectious disease physician at Massachusetts General Hospital in Boston and the editor-in-chief of the Journal of Infectious Diseases.

There are two major things that could contribute to this.

“It can be due to a change in what’s circulating in the human population, or a change in the vaccine that leads to a mismatch” between the virus and the vaccine, Hirsch said.

Circulating influenza strains may mutate after the World Health Organization decides what should go into that year’s vaccine. Or the vaccine itself may mutate while it is being grown — currently, vaccines are grown in eggs, but many experts don’t think that’s ideal as the vaccine virus tends to undergo mutations to adapt to growing in an egg.  

But that doesn’t mean you shouldn’t go out and get your flu vaccine, Hirsch cautions.

“Even if the vaccine is only 10% effective against H3N2, the vaccine does protect against other strains that are circulating,” he said. “The most important thing is still to get your flu vaccine.”