Flu season arrives every year, but public health officials are watching this one a little more closely because of a newly dominant strain of influenza A(H3N2) that emerged after this season’s vaccine formula was finalized.
The strain, known as H3N2 subclade K, accumulated enough genetic changes to draw notice from epidemiologists tracking global flu patterns. A December 19 report in JAMA Medical News described its spread in several countries, including early and heavier-than-usual flu activity in parts of Japan, the United Kingdom and Canada.
In the United States, health officials say flu timing remains typical for midwinter. What has changed is which strain is circulating.
Northern Plains News first reported in December on the emergence of the H3N2 subclade as the flu season began to take shape. Since then, the strain has become dominant nationwide as vaccination rates have slipped.
What the data show in South Dakota and the region
In South Dakota, state health officials continue to track influenza through weekly surveillance updates. So far this season, the state has reported six flu-related deaths, more than 5,200 cases and more than 330 hospitalizations, according to the South Dakota Department of Health.
Officials note those figures reflect cumulative, midseason reporting, not a sudden surge. Recent weekly updates continue to describe overall flu activity as low, suggesting the season may be moving past its peak rather than accelerating.
Regionally, influenza activity remains elevated but stable across the Northern Plains. Federal surveillance data show HHS Region 8 — which includes South Dakota, North Dakota, Montana, Wyoming, Colorado and Utah — has reported higher laboratory test positivity than other regions in recent weeks. Public health experts caution that percent positivity often peaks during midwinter and does not, by itself, indicate increasing severity.
Health officials emphasize that “low” activity does not mean zero impact. South Dakota’s older population, long travel distances to hospitals and limited rural health capacity mean even moderate flu seasons can strain care systems, particularly for older adults and young children.
The emphasis, officials say, is preparedness rather than alarm.
What makes H3N2 different
H3N2 viruses are a regular part of seasonal flu, but they are often associated with somewhat more severe seasons, especially for older adults and young children.
Federal surveillance data show that roughly nine out of 10 H3N2 viruses now being characterized in the United States belong to subclade K. Researchers have identified multiple mutations in a surface protein targeted by vaccines, raising the possibility of a less precise match this season.
That does not mean the flu vaccine is ineffective.
What the vaccine still does
Even in seasons when circulating strains drift from the vaccine, flu shots consistently reduce the risk of severe illness, hospitalization and death.
Early real-world data from the United Kingdom, where the new subclade is widespread, show meaningful protection against emergency department visits, particularly among children, according to experts interviewed by JAMA. Antiviral medications also remain effective and are recommended for people at higher risk when started early.
South Dakota health officials advise older adults, caregivers and people with chronic conditions to contact a health care provider promptly if flu symptoms appear, since antiviral treatment works best when started early.
What to watch next
How the rest of the flu season unfolds in South Dakota and the Northern Plains will depend on vaccination uptake, access to timely treatment and how quickly people seek care when symptoms appear.
For now, health officials agree on a steady message: stay informed, take basic precautions and avoid panic.
