Flu activity in the U.S. reaches highest level in 25 years

(CNN, KYMA) - Flu activity in the United States has reached the highest levels seen in 25 years. All but four U.S. states are at high or very high levels of flu activity.

A new flu strain called subclade K is responsible for the spread. Subclade K began to spread after strains were chosen for this year's flu shots, which means flu vaccines likely only offer partial protection against it.

In terms of severity, the number of people hospitalized due to the flu is not a standout, but doctor visits for flu symptoms haven't been this high since 1997.

Montana, Vermont, South Dakota, and West Virginia are the only states reporting low or moderate levels of flu activity.