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The most active early flu season since 2003 is already ‘wreaking havoc' in the U.S.

The flu season is off to an early start.

The 2019-2020 flu season follows two straight unusually bad flu seasons. Unfortunately, two key indicators show this year could be more of the same.

First, "This season is off to an early start, earlier than any season this decade," Dr. Bryan Lewis, professor at the University of Virginia, who works in a research partnership with AccuWeather, wrote in an email. "You have to go back to 2003 before you have as much active transmission in early December as we have now."

Nationwide, 3.5 percent of patient visits reported through the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) were due to influenza-like illness (ILI). This percentage is above the national baseline of 2.4 percent for the fourth straight week, according to the Centers for Disease Control and Prevention (CDC).

Last year, levels of ILI in the U.S. were at or above-baseline normal for 21 straight weeks, the longest above-baseline flu season since the CDC started keeping such comparable records in 2007-2008.

However, this highly active season is already five weeks ahead of last year's first above-baseline week, which didn't occur until week 49. More than half of the states in the U.S. are at the highest level in the last decade for this particular week.

The second unique characteristic of the 2019-2020 flu season is Influenza B is appearing uncharacteristically early and at unusually intense levels, according to the University of Virginia doctors partnered with AccuWeather. Influenza B still causes significant illness in those stricken; however, hospitalizations and death are less frequent than with Influenza A.

"That's the extra-odd thing about this season is at the moment it's being driven by Influenza B," said Dr. Lewis. "These are two sibling viruses that are distinct diseases, though they're very similar. Influenza B is generally less severe and most seasons trails Influenza A, often with a small peak of activity in late winter and early spring."

This early wave of Influenza B may soon peak in many states and start to subside before an anticipated wave of Influenza A picks up steam, according to the University of Virginia doctors partnered with AccuWeather.

Flu season typically begins in October, peaks between December and February and lasts well into March although activity can last as late as May. Every season, flu sickens millions of Americans, hospitalizes hundreds of thousands and kills an estimated tens of thousands, according to the CDC. Last year, the CDC estimated there were between 36,400 and 61,200 flu-related deaths in the U.S. During the 2017-2018 season, the CDC estimated there were 61,000 flu-related U.S. deaths.

An additional oddity to the 2019-2020 flu season, is the role the Australia flu season is possibly playing in the U.S. "Anecdotally, we've seen bad seasons in Australia presage bad seasons in the U.S., but we've also seen the opposite," Dr. Lewis said.

Australia also had an uncharacteristically early and initially severe season, starting two months earlier than usual. However, initial dire projections leveled off as the season continued.

"The Australia angle is interesting," Dr. Lewis noted. "The strain that caused the most trouble over there isn't causing much trouble here, thus their experience is not what is playing out here. However, in reviewing their surveillance data, they closed out the year with an increasing trend of the same strain of Influenza B that is wreaking havoc here.

"So, in some ways, the answer to the question about concerns of their season being like ours is: No, but in a roundabout way, yes," Dr. Lewis said.