After several relatively mild — and nonexistent — seasons, the flu is back with a vengeance this year.

“This is one of the most severe influenza seasons that we’ve had in many years,” said Dr. Timothy Jenkins, an infectious disease physician at Denver Health.

As of February, the United States had been experiencing its worst flu season in more than a decade with the Centers for Disease Control and Prevention (CDC) estimating 24 million illnesses, more than 300,000 hospitalizations and 13,000 deaths nationally.

The number of flu deaths this season was not immediately known. Historically, Colorado has about 469 flu-related deaths annually, according to the CDC.

The state health department tracks three respiratory illness: COVID-19, flu and RSV, a common disease that can be serious for the very young and old.

Flu season typically occurs between the fall and winter, peaking in February. But the flu can extend into May.

As of the week ending February 15, half of adults in the state were vaccinated against the flu, ranking the state 12th nationwide, according to the Colorado Department of Public Health and Environment.

But vaccinations this year are waning.

At this point in last year’s season, nearly 45,000 more Coloradans were vaccinated, Hope Shuler, a spokesperson for the state health department, said in an email to The Denver Gazette.

“The low patient vaccinations could be contributing to this,” Jenkins said.

Those at most risk for serious complications and death are people older than 65 and children younger than 2.

Tamiflu is an antiviral medication that can effectively treat influenza, provided its administered within 48 hours of the onset of symptoms, which include body aches, chills and fatigue.

“Having a flu vaccination lowers the risk of hospitalization,” said Dr. Sarah Goodpastor is an internal medicine physician at Mercy Internal Medicine.

For those who hold out on getting inoculated believing doing so caused the flu, Goodpastor has this word of caution: “The actual flu is a living, replicating virus in your body that will feel 10 times worse.”