A new and rapidly spreading flu variant is fuelling a significant surge in influenza cases across the United States, with health officials warning that holiday travel is likely to accelerate the outbreak further.
Rapid Spread and Early Impact
Flu activity is currently high or very high in more than half of all US states, according to the latest report from the Centers for Disease Control and Prevention (CDC). The data, which covers the period up to 20 December, estimates there have already been at least 7.5 million illnesses, 81,000 hospitalisations, and 3,100 deaths this season. This grim toll includes at least eight child fatalities.
The driving force behind this early and intense wave is a mutated version of the H3N2 influenza A virus, known as "subclade K." This variant was first linked to early outbreaks in the United Kingdom, Japan, and Canada. In the US, where flu typically begins its winter spread in December, the impact is already severe.
Some regions are experiencing particularly intense transmission. For instance, New York's health department reported that the week ending 20 December saw 71,000 flu cases—the highest weekly total the state has recorded since 2004.
Understanding the Subclade K Variant
Influenza viruses are notorious for their constant mutation. Subclade K is a descendant of the H3N2 strain, a subtype of flu that is historically associated with more severe seasons, especially for older adults. While its mutations are not significant enough to classify it as an entirely new kind of flu, they are sufficient to partially evade the protection offered by this year's annual vaccine.
Virus expert Andrew Pekosz of the Johns Hopkins Bloomberg School of Public Health explained that these changes mean the vaccine is not a perfect match. However, preliminary analysis from the UK indicates the shot still provides valuable partial protection, reducing a person's risk of requiring hospital treatment.
The Critical Importance of Vaccination
Health authorities stress that it is not too late to get a flu vaccination. The CDC and major medical societies recommend the jab for virtually everyone aged six months and older. Vaccination remains the best defence against severe illness, even when infection occurs.
Despite this, uptake has been sluggish. The CDC reports that only around 42% of adults and children in the US have received their flu shot this season. The vaccines available this year are designed to protect against three circulating strains and are considered a good match for H1N1 and Type B flu.
For those who do fall ill, prescription antiviral medicines are available and are usually recommended for people at high risk of complications. These treatments are most effective when started within a day or two of symptoms appearing.
As families reunite and travel over the festive period, the situation is expected to deteriorate. While it is still too early to predict if this season will be as severe as the last, the rapid rise of subclade K serves as a stark reminder of influenza's persistent threat.
