Flu Season Is Already Off to a Bad Start

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Flu season is underway in the United States, and a new report shows that flu activity is already higher than typical for this time of year.

During the week that ended Nov. 25 (the most recent period for which data is available), three Southern states — Louisiana, Mississippi and South Carolina — reported high levels of flu activity; one state (Georgia) reported moderate flu activity, and the rest reported either low or minimal flu activity, according to the report from the Centers for Disease Control and Prevention (CDC). At this same time last year, no states were reporting high levels of flu activity.

The new CDC report, published Dec. 7, also said that during the week of Thanksgiving, the percentage of people visiting the doctor for flu-like illness was 2.3 percent, which is slightly above the "national baseline" for flu visits — the threshold for what's typically seen in the off-season — which is 2.2 percent. At this time last year, the percentage of people visiting the doctor for flu-like illness was only 1.9 percent.

"Influenza activity in the United States … has been increasing since early November," the researchers wrote in their report. "It is difficult to predict when influenza activity will peak for the current season; however, influenza activity will increase in the coming weeks," the researchers said.  [6 Flu Vaccine Myths]

Some health officials have already warned that this flu season could be a bad one, based on reports from the Southern Hemisphere, where the flu season recently ended for the year.

The flu viruses in circulation change each season, and the predominant strain this year is H3N2, according to the new CDC report. Flu seasons in which H3N2 predominates tend to have higher overall flu hospitalization and death rates, according to the CDC.

Indeed, so far this flu season, there have been five reported flu deaths among U.S. children, compared with no deaths among children at this time last year. (The CDC specifically tracks flu deaths in children, but not adults.)

What's more, the rate of flu hospitalizations from Oct. 1 to Nov. 25 this year was 2 hospitalizations per 100,000 people. The rate was highest among adults ages 65 and older, in which there were about 7 hospitalizations per 100,000 people. At this time last year, the rate of flu hospitalizations was just 1.3 hospitalizations per 100,000 people.

There's another factor that may affect the severity of flu season: whether the flu strains included in the yearly flu shot match the ones circulating in the public. So far this year, the circulating flu strains do appear to match the flu strains that were selected for the vaccine, according to the CDC report.  

However, in order to make the flu vaccine, manufacturers typically use chicken eggs to "grow" the flu virus strains. During this process, the flu strains may acquire genetic changes that make the strains slightly different from those in circulation. This appears to have happened with the H3N2 component of this year's flu vaccine, the report said, and the changes may lower the effectiveness of the vaccine. (However, there is a "cell-based" vaccine available this year in which the H3N2 component was grown in cell culture.)

Health officials still recommend a yearly flu vaccine for everyone ages 6 months and older, because it's still the best way to prevent flu.

"Even with influenza vaccine effectiveness in the range of 30 percent to 60 percent, influenza vaccination prevents millions of infections and medical visits and tens of thousands of influenza-associated hospitalizations." Last year's flu shot was 39 percent effective at preventing flu; estimates for the effectiveness of the 2017-2018 flu vaccine will be available later this season.

Original article on Live Science.

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.