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CDC: Flu season getting off to a high-impact start


Influenza activity continues to increase in the United States, and most of the country is experiencing high levels of influenza-like illness, according to the Centers for Disease Control and Prevention’s latest FluView report.

“Reports of influenza-like illness are nearing what have been peak levels during moderately severe seasons,” Joe Bresee, MD, FAAP, chief of the Epidemiology and Prevention Branch in the CDC’s Influenza Division, said in a news release.

“While we can’t say for certain how severe this season will be, we can say that a lot of people are getting sick with influenza and we are getting reports of severe illness and hospitalizations.”

Bresee said anyone who has not yet been vaccinated should get the vaccination immediately. And “people who have severe influenza illness, or who are at high risk of serious influenza-related complications, should get treated with influenza antiviral medications if they get flu symptoms regardless of whether or not they get vaccinated. Also, you don’t need to wait for a positive laboratory test to start taking antivirals.”

The CDC tracks influenza activity year-round and publishes a report weekly on Fridays ( According to this surveillance, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) in the United States has been elevated for four consecutive weeks, climbing from 2.8% to 5.6% during that time.

Last season, which was relatively mild, ILI peaked at 2.2%. Comparatively, during 1998-1999 and 2003-2004, which were moderately severe seasons, ILI peaked at 7.6%. During 2007-2008, another moderately severe season, ILI peaked at 6%. During the 2009 H1N1 pandemic, ILI peaked at 7.7%.

Although the timing of influenza seasons is impossible to predict, based on past experience flu activity likely will continue for some time. During the past 10 influenza seasons, ILI remained at or above baseline for an average of 12 consecutive weeks, with a range of one week (2011-2012 season) to 16 weeks (2005-2006). During the pandemic, the proportion of visits to doctors for ILI remained above the national baseline for 19 consecutive weeks.

Twenty-nine states and New York City report high levels of ILI and another nine states report moderate levels. Ten states still are reporting low or minimal ILI. (These are California, Connecticut, Hawaii, Kentucky, Maine, Montana, Nevada, New Hampshire, South Dakota and Wisconsin). Washington, D.C., and two states did not have enough information to calculate an activity level.

Information about flu-related hospitalizations is collected from 15 states to calculate a rate of laboratory-confirmed influenza-associated hospitalizations. Cumulative influenza hospitalization rates are 8.1 per 100,000 people, which is high for this time of year, Bresee said.

Influenza-associated pediatric deaths have been reportable to CDC since the 2004-2005 season. To date, CDC has received reports of 18 pediatric deaths this season. More information about reported pediatric deaths is available at

One factor that may indicate increased severity this season is that the predominant circulating type of influenza virus is influenza A (H3N2) viruses, which account for about 76% of the viruses reported. “Typically, H3N2 seasons have been more severe, with higher numbers of hospitalizations and deaths, but we will have to see how the season plays out,” Bresee said.

This season, 91% of the influenza viruses that have been analyzed at CDC are like the viruses included in the 2012-2013 influenza vaccine. The match between the vaccine virus and circulating viruses affects how well the vaccine works. But Bresee cautioned that other factors are involved: “While influenza vaccination offers the best protection we have against influenza, it’s still possible that some people may become ill despite being vaccinated. Healthcare providers and the public should remember that influenza antiviral medications are a second line of defense against influenza.” (See for further explanation.)

CDC has recommendations on the use of antiviral medications (sold commercially as Tamiflu and Relenza) to treat influenza illness. Antiviral treatment, started as early as possible after becoming ill, is recommended for any patients with confirmed or suspected influenza who are hospitalized, seriously ill or ill and at high risk of serious influenza-related complications, a category that includes young children, people 65 and older, people with certain underlying medical conditions and pregnant women. Treatment should begin as soon as influenza is suspected, regardless of vaccination status or rapid test results and should not be delayed for confirmatory testing.


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