Levels of flu-like illness in the United States have been at or above baseline for 20 weeks so far this season, tying the previous high of 20 weeks at or above baseline set in 2014-15, according to the Centers for Disease Control and Prevention (CDC).

That would be the longest above-baseline flu season since the CDC started keeping such comparable records in 2007-08. (Prior to that year, the CDC did not calculate a national baseline.)

The percentage of outpatient visits to health care providers for influenza-like illness (ILI) still remains above the national baseline of 2.2%, according to a CDC report that will be issued Friday. The flu season typically begins in October and lasts well into March.

The report will indicate that flu activity continues to decrease; however, activity levels are still relatively high for this time of year, with 20 states continuing to report "widespread" flu activity. The four states experiencing high ILI activity are Louisiana, Missouri, South Carolina and Virginia.

A key reason for the season’s length may be “a second wave of H3N2-predominant activity that is expected to continue for a number of weeks,” CDC public affairs specialist Martha Sharan wrote in an email to AccuWeather. “H3N2 is typically associated with more severe illness in older adults.”

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The CDC estimates there have been between 452,000-549,000 flu hospitalizations and 30,600-50,900 deaths so far this season from October 1, 2018 until March 30, 2019. While that is a large range, this uncertainty could be trimmed moving forward.

"AccuWeather believes the weather and the sunshine intensity is an important factor in the flu season; there is no flu to speak of in the summer because the sun is strong and the weather is warm," the company's founder and CEO Dr. Joel N. Myers said.

There is also no real flu season in the tropics. “There is a lot of evidence that suggests low humidity and low temperatures allow the influenza virus to survive longer, which promotes its transmission,” University of Virginia research associate professor Bryan Lewis wrote to AccuWeather, which has a partnership with the Biocomplexity Institute & Initiative at Virginia.

Flu viruses are more stable in cold air and the low humidity allows the virus particles to remain in the air, according to Peter Palese, who was the lead author on a key flu study in 2007. The viruses float in the air in small respiratory droplets; when the air is humid, those droplets absorb water, grow larger and fall to the ground.

“The colder it is outside, the lower the dew point must be. And when the air inside is heated by the heating system, the lower the humidity will be indoors,” Myers explained. “Because it was colder this season than average, indoor humidities tended to be lower and that’s one reason why there was more flu. It was actually the dryness in the homes driven by cold outside temperatures that helped to propagate flu activity.”

Is there an end in sight to this year’s flu season? The CDC notes that flu activity is expected to remain elevated for a number of weeks.

Similarly, while surveillance data has yet to show it, there are reports of increases in influenza B, which may stretch out this already very long season, according to researchers at Virginia.

"The good news is, warmer weather is coming, the sun is getting stronger with each passing day and both of those are enemies of the flu," said Myers. "So we expect a significant decline in the incidents of flu over the next several weeks."

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