Why this year's flu season is worse than last year so far

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This year's flu season started earlier and is more intense than last year.

Last year's flu season was historically bad, and the 2019-20 season is off to an even worse start.

The percentage of visits for influenza-like illnesses (ILI) rose for the second straight week, according to the Centers for Disease Control and Prevention (CDC). Nationwide, 5.1 percent of patients reported through the U.S. Outpatient ILI Surveillance Network (ILINet) were due to ILI.

This percentage is above the national baseline of 2.4 percent for the seventh straight week. Last year, levels of ILI in the United States 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 2007-08.

Also, CDC notes that already 170.7 million doses of flu vaccine have been distributed. Last year, the total for the whole season was 169.1 million, which was the highest since at least 1980, according to the CDC. As recently as the 2004-05 season, the season total was just 57 million doses.

While the increased vaccination may be a reflection of the severity of the last two flu seasons, the high total speaks to the overall heightened concerns.

As the CDC chart below shows, the 2019-20 flu season started earlier and is more intense than last year.

Flu experts predict most states in the U.S. will remain at moderate to high levels of activity for the next several weeks, with high activities in Washington, New Mexico, Minnesota and Tennessee, according to researchers at the Biocomplexity Institute at the University of Virginia who work in a research partnership with AccuWeather.

The researchers note that the Influenza B wave that started the season early has begun to decline but remains quite active in most regions of the U.S. Influenza A has started to grow more strongly in the north and is moving southward. As the B wave wanes and Influenza A replaces it, the researchers expect a second surge in activity.

Influenza B still causes significant illness in those stricken; however, hospitalization and death are less frequent than with Influenza A.

The CDC estimates that so far this season there have been at least 4.6 million flu illnesses, 39,000 hospitalizations and 2,100 deaths from the flu.

The 2019-20 flu season follows two straight unusually bad flu seasons. Last year, the CDC estimated there were between 36,400 and 61,200 flu-related deaths in the U.S. During the 2017-18 season, the CDC estimated there were 61,000 flu-related U.S. deaths.

Flu season typically begins in October, peaks between December and February and lasts well into March although activity can last as late as May. 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. For example, there is no real flu season in the tropics.

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

Download the free AccuWeather app to see the forecast for your location. Keep checking back for updates on AccuWeather.com and stay tuned to the AccuWeather Network on DIRECTV, Frontier and Verizon Fios.