Influenza activity continues to increase in the U.S. and most of the country is now experiencing high levels of influenza-like-illness (ILI), according to CDC's latest FluView report.
"Reports of influenza-like-illness (ILI) are nearing what have been peak levels during moderately severe seasons," according to Joe Bresee, MD. CDC continues to recommend influenza vaccination and antiviral treatment when appropriate at this time. "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," says Bresee, who is Chief of the Epidemiology and Prevention Branch in CDC's Influenza Division.
"Anyone who has not already been vaccinated should do so now," Bresee continues. "And it's important to remember that 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 got vaccinated. Also, you don't need to wait for a positive laboratory test to start taking antivirals."
CDC tracks influenza activity year-round and publishes a report weekly on Fridays. According to this surveillance, the proportion of people seeing their health care provider for ILI in the U.S. has been elevated for four consecutive weeks, climbing sharply from 2.8% to 5.6% during that time. Last season, which was relatively mild, ILI peaked at 2.2 percent.
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 6.0%. During the 2009 H1N1 pandemic, ILI peaked at 7.7%.
While the timing of influenza seasons also is impossible to predict, based on past experience it's likely that flu activity 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 1 week (2011-2012 season) to 16 weeks (2005-2006 season). 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 are now reporting high levels of influenza-like-illness and another 9 states are reporting moderate levels of ILI. Ten states are still reporting low or minimal ILI. (These are California, Connecticut, Hawaii, Kentucky, Maine, Montana, Nevada, New Hampshire, South Dakota and Wisconsin). The District of Columbia and 2 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. Right now, cumulative influenza hospitalization rates are 8.1 per 100,000 people. According to Bresee, "This is high for this time of year."
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 the Influenza-Associated Pediatric Mortality web application.
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 percent of the viruses reported. Bresee explains "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."
So far this season, most (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 is one factor that impacts how well the vaccine works. But Bresee cautions 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," says Bresee. "Health care providers and the public should remember that influenza antiviral medications are a second line of defense against influenza." (For more information about why people may become sick with influenza after vaccination, see 2012-2013 season Questions and Answers.)
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, including 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.
To estimate how well influenza vaccines work each year, CDC has been working with researchers at universities and hospitals since the 2003-2004 influenza season conducting studies using laboratory-confirmed influenza as the outcome. Interim VE estimates will be published as soon as they are available. Bresee concludes, "These estimates will provide more information about how well this season's vaccine is working."