New variant of the flu virus drives surge of cases across US, Canada

MASSACHUSETTS: After a sharp uptick in flu cases in mid-December 2025, flu activity across the US and Canada remains high.

Although cases are trending downward in Canada as of Jan. 9, 2026, the season has yet to peak in the US, according to data from the Centres for Disease Control and Prevention.

As an emergency room pediatrician in central Massachusetts, I’m seeing a tremendous amount of flu over the past few weeks. I’m hearing from colleagues in emergency rooms across the country that they are experiencing a similar explosion of flu cases.

In early January, New York state recorded the highest number of flu cases in a single week on record. Several other states, such as Colorado, are also experiencing record flu levels, and 44 out of 55 states and other jurisdictions are reporting high or very high flu activity, according to the CDC.

The spike in flu cases – widely referred to in media reports as a “superflu” – is largely driven by a newly identified subtype of the virus called subclade K.

Flu seasons vary every year. In 2024-2025, influenza cases rose fast beginning in October 2024. In contrast, the 2025-2026 season started out slow, but then it’s as if a switch was flipped in early to mid-December.

In the last week of December, throughout the United States, over 8% of doctors visits were for flu symptoms – the highest weekly rate since 2005. Emergency department visit rates for flu are spiking throughout the country, most notably in the South and Northeast, where rates exceeded the national average.

Over this same time period, more than 26% of flu tests taken throughout Canada were positive for influenza. As of Jan.

15, the CDC estimates that flu has caused 15 million illnesses, 180,000 hospitalizations and 7,400 deaths this season. One small saving grace is that the flu this season hasn’t come on concurrently with either a respiratory syncytial virus.or

COVID-19 surge.