Why Is Flu Season So Bad This Year?

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Flu season is off to an early — and severe — start, with rates of hospitalizations and deaths from flu higher than what's typical for this time of year.

But why is the flu so bad this year?

During the week that ended Dec. 30 (the most recent period for which data is available), 46 states reported widespread flu activity, up from 36 states the week before, according to the U.S. Centers for Disease Control and Prevention (CDC). At this time last year, just 12 states reported widespread flu activity.

What's more, the rate of flu hospitalizations from the beginning of October to the end of December 2017 was about 14 hospitalizations per 100,000 people, according to CDC data. The rate was highest among adults ages 65 and older, at about 57 hospitalizations per 100,000 people. At this time last year, the rate of flu hospitalizations was just 5 hospitalizations per 100,000 people.

The flu is notoriously unpredictable, with the timing, severity and length of flu season varying from year to year, according to the CDC. [6 Flu Vaccine Myths]

But the relatively harsh season the U.S. is currently having is likely related to the particular flu strains that are circulating, said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security.

"This year is particularly bad because it's dominated by the H3N2 [flu strain], which tends to be more severe and causes more severe symptoms than other strains of flu," Adalja told Live Science. Indeed, flu seasons in which the H3N2 strain predominates tend to have higher overall flu hospitalization and death rates, according to the CDC.

Officials can't predict what the final outcome for flu season will be this year in terms of illnesses, hospitalizations and deaths, but Adalja suspects it will be a notable one.

"I don't know what the final tally will be in this flu season, [but] I suspect it will be one of the worst ones we've had in recent years," Adalja said.

Another factor that may affect the severity of a flu season is 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 a recent CDC report. 

However, 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.

"You don't end up with the same vaccine viruses that you started with" because of these genetic changes, Adalja said. This appears to have happened with the H3N2 component of this year's flu vaccine, according to the CDC, and the changes may lower the effectiveness of the vaccine.

Researchers are finding that yearly flu shots are typically less effective against H3N2, compared with other strains of flu. A study published in 2016 found that, from 2004 to 2015, the flu shot was only 33 percent effective against H3N2 viruses, compared with 61 percent effective against H1N1 and 54 percent effective against influenza B viruses, which are another strain. This may be because, compared with other flu viruses, H3N2 viruses are more likely to acquire genetic changes that impact the effectiveness of the vaccine, the CDC said.

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. And studies have found that, even if a person does catch the flu, their illness is milder if they've been vaccinated. "Even lower levels of protection" are better than none, Adalja said.

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.