One reason for the high flu activity this season may be the specific flu strains that are circulating. The most prevalent strain making the rounds this season is influenza A (H3N2). Typically, flu seasons in which this strain dominates are more severe, and result in a higher number of hospitalizations and deaths, according to the Centers for Disease Control and Prevention (CDC).
The flu season also started early this year — the first week of December instead of the usual January or February. (Flu season is said to officially begin once flu activity reaches a certain threshold.) It was the earliest start to the flu season since the 2003 to 2004 season. [See Earliest Start To Flu Season in Nearly a Decade.]
Years in which the flu season starts early, and in which the dominant strain is H3N2, tend to be worse flu years, the CDC said.
However, the CDC notes that flu season is always unpredictable, so scientists will still have to see how this one plays out.
As of the last week of December, 5.6 percent of doctors' visits were made by people suffering from a flulike illness, meaning they have symptoms such as a cough, a fever and body aches, according to the CDC. Last year, a relatively mild flu season, visits for flulike illnesses peaked at 2.2 percent. During 2007 to 2008, a moderately severe flu season, doctor visits peaked at 6 percent, and during the 2009 pandemic, visits peaked at 7.7 percent.
Twenty-nine states and New York City are reporting high levels of flu activity, and nine other states are reporting moderate levels, the CDC said. Ten states still have minimal to no flu activity. Yesterday, Boston declared a public health emergency after its state reported 18 flu-related deaths. [See What You Can Do About the Raging Flu.]
So far, about 2,257 people have been hospitalized with flulike symptoms, and 18 children have died of flu, the CDC said. In all of last season (from October 2011 to May 2012), 26 children died of flu. During the 2010 to 2011 season, 115 children died of flu, the CDC says. (Last flu season officially began in February 2012, but pediatric deaths were monitored from October of the previous year.)
Pass it on: The dominant strain of flu in circulation, and the early start to the flu season, could be reasons why so many people are sick with the flu.
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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.