What to know about the ongoing flu season.

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A flu vaccine remains the best way to protect yourself against infection. Getty Images

Flu season continues across the country, and a more severe strain of the condition is among what’s spreading.

According to the Centers for Disease Control and Prevention (CDC), 56 children have died of flu-related complications in the United States so far this season.

“Each season different flu strains dominate, either H1N1 or H3N2,” said Julia Sammons, MD, a hospital epidemiologist and the medical director of the Department of Infection Prevention and Control at Children’s Hospital of Philadelphia.

The H1N1 strain was the dominant form of the flu at the start of the season in the fall, but now more than half of the reported recent cases are of a variant of H3N2, which is a non-human version of the flu, the CDC reported last week. H1N1 and H3N2 are the strains that circulate year to year and cause seasonal epidemics in humans.

While this H3N2 variant is also a type of swine flu, it’s not the same strain that caused the 2009 swine flu pandemic.

“The 2009 pandemic was caused by a new subtype of H1N1 that’s become a regularly circulating flu virus since 2009,” a CDC spokesperson told Healthline.

The strain named (H1N1)pdm09 virus was the strain implicated in the swine flu of 2009. It included genes not formerly found in people or animals, Sammons said.

“This year the 2009 H1N1 strain was the most common strain circulating,” she said. “In more recent weeks, we’ve seen a change to an increased proportion of H3N2 viruses, also of swine origin. The shift to H3N2 viruses is important because they tend to cause more severe disease year to year.”

The CDC says that H3N2 is a concern because it seems to spread easily from pigs to humans compared to other swine flu viruses. The CDC also notes that children born after 2001 have little to no immunity against it. H3N2 strains are typically considered more severe.

Last year’s flu season was one of the most severe in recent history and was mostly an H3N2-dominant season.

H3N2 has been more dominant this year only in the Southeast states of Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee.

As of February 23, the CDC estimates that up to 23 million people have been sickened, up to 302,00 have gone to the hospital, and up to 26,700 people have died.

The CDC estimates that the flu is widespread in 49 states and Puerto Rico.

The CDC still urges people to get vaccinated if they haven’t already. This year’s flu shot is 44 percent effective against H3N2, the CDC reported.

“How well the current influenza vaccine performs against the H3N2 strain is yet to be fully determined,” Sammons said.

If you have symptoms such as fever, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, and fatigue, seek medical attention promptly to avoid potentially serious complications.