It's an unexpected phenomenon that public health experts are still trying to decode.
While federal officials were hoping that the flu season had already reached its peak, Anne Schuchat, acting director of the Centers for Disease Control and Prevention (CDC), had some unfortunate news that she shared with the Washington Post. It also reported the highest rate of flu-like illnesses since the flu pandemic of 2009.
Swine flu, which swept the globe in 2009 and 2010, sickened 60.8 million Americans, hospitalised 274,304 and killed 12,469, according to CDC data. That was a slight increase from 10 outbreaks a week earlier, and brought the state total to 137 so far this season.
"I don't know if we've hit peak yet", Schuchat said.
Schuchat said that while this year's virus "isn't new in terms of antigenic drift", virologists are studying it to see if there are "other explanations for the more severe disease we're seeing". Vaccination of children against the flu is a relatively new phenomenon that may help limit the pediatric toll, she said.
United States researchers have not yet projected vaccine effectiveness for this season, but Canadian researchers estimate the vaccine is about 55% effective against influenza B, a vast improvement over the 17% effectiveness estimated for influenza A, a finding that wasn't even statistically significant.
"This season is a wake-up call about how severe influenza can be and how we can never let our guard down", she said.
The organization does not keep a correct tally of grown-up deaths, but rather almost tracks youngster deaths as one measure of a flu season's seriousness.
There were 40,414 deaths in the U.S. during the third week of 2018, the most recent data available, and 4064 were from pneumonia or influenza, according to the CDC data.
Schuchat said the country is now in the eleventh week of the 2017-2018 flu season. Among them were for six flu-related child deaths in New York City. The CDC said this is the highest rate in recent years and a level not seen since the swine flu pandemic nine years ago. These numbers do not include all the people who have had the flu, as many do not see a doctor when sick. "People can also become infected by touching surfaces or objects contaminated with influenza viruses and then touching their eyes, mouth or nose", according to the ISDH. Half had no other medical conditions that would place them in the high-risk category, and only about 20 per cent were vaccinated.
The current hospitalization rate is approaching the final rate seen at the end of the severe 2014-15 season. This week's data showed outpatient visits for influenza-like illness comprised 7.7% of all visits - up from 7.1% in last week's data.