Federal health officials warned Friday that this year's flu outbreak is more severe than any other since the 2009 swine flu pandemic, and that its intensity is still increasing.

Officials at the Centers for Disease Control and Prevention (CDC) said in a phone call with reporters that the number of people experiencing flu-like symptoms is increasing. Hospitalizations for influenza are also rising.

What's more, flu activity has stayed at the same level nationwide for about three weeks, said Dr. Dan Jernigan, the director of the CDC’s Influenza Division. That sets it apart from many flu seasons, in which activity wanes in certain parts of the U.S.

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"We often see different parts of the country 'light up' at different times, but for the past 3 weeks, the entire country has been experiencing lots of flu, all at the same time," he said.

The deaths of seven children from the flu were reported to the CDC this week, bringing the total number of child deaths this flu season to 37, CDC Director Dr. Brenda Fitzgerald said.

Perhaps driving the latest increase in those reporting flu-like symptoms is the influx of kids returning to school after winter vacations, Jernigan said, pointing to a higher rate of flu cases in children.

The number of people who have gone to see a doctor for influenza-like illness also rose this week to 6.6 percent. That means that 6.6 percent of all people who went to clinics or emergency rooms had a flu-like illness.

The number of people going to see doctors for such illnesses is at its highest rate since the 2009 swine flu outbreak, when it rose to 7.7 percent, Jernigan said.

Most people with the flu this season are infected with the H3N2 virus, one of the most severe — and deadly — strains of the seasonal flu. What's more, Jernigan said, the 2017 to 2018 flu season likely still has weeks to go.

"In seasons where H3N2 is the main cause of influenza, we see more cases, more visits to the doctor, more hospitalizations and more deaths, especially among older people," he said.