Graph of a remarkably mild flu influenza season.
This from the CDC’s summary.
Last full FluView posted May 25, 2012 Shows U.S. Influenza Activity at Summer Levels
A Late and Mild Flu Season
May 25, 2012 – Today the Centers for Disease Control and Prevention issued the final full influenza surveillance report for the 2011-2012 season. The report – titled “FluView” – shows that influenza activity in the United States is minimal across most of the country, wrapping up a season that began late and was mild compared to most previous seasons for which surveillance data is available. In fact, the season set a new record for the lowest and shortest peak for influenza-like-illness since this type of surveillance began.
Influenza-like-illness (ILI) in the United States typically begins to increase in late December or early January and peaks in February most commonly. This season, ILI remained low through February and did not exceed baseline – and then only slightly -- until mid-March. ILI stayed above baseline for just one week in March and did not exceed baseline again. According CDC’s Dr. Joseph Bresee, “This is the first time since CDC started this kind of influenza-like-surveillance (ILI) that the percentage of patient visits for ILI was elevated for only one week of the season.” Dr. Bresee is Chief of the Epidemiology and Prevention Branch in CDC’s Influenza Division. In past seasons, ILI has remained above baseline for between 8 and 20 weeks, with an average of 13 weeks at or above baseline each season since this type of surveillance began in 1997-1998. Bresee adds, “In terms of ILI, this not only the shortest time we were above baseline, but it’s also the lowest ‘peak’ ever recorded.” The graph below compares ILI from five different seasons, including the current season (2011-2012), the 2009 H1N1 pandemic season, a ‘moderately severe’ flu season (2007-2008), as well as a season classified as ’moderate’ in severity (2002-03).
For a more detailed view of the graphic, please click on the image or visit ILI Weekly National Summary detail.
The CDC has also announced several new interactive data display tools, which show various datasets over time.
Interactive Web Tools
A new interactive web application in “FluView,” displays information – from this season and previous season’s – related to flu-related pediatric deaths. Other interactive tools display ILI over time and ILI and laboratory data on a national and regional level over time.
Interactive View of Pediatric Deaths related to Influenza
As to why this current flu season has been so mild, solid answers are hard to find. The CDC’s Dr. Joseph Bresee is quoted as saying:
“The reason for the mildness and lateness of the season isn’t certain, but it’s likely that there were a number of contributing factors, including a mild winter, the fact that most of the influenza viruses circulating this season were similar to those that have circulated for the past two seasons and the fact that most circulating viruses were similar to the viruses that the 2011-2012 vaccine was designed to protect against.” The low levels of influenza virus “drift” (change) for two consecutive years and “steadily increasing influenza vaccination coverage in the country likely contributed to broad levels of immunity in the population,” says Bresee.
Whatever the cause, it’s probably too much to hope for a repeat next year. Which makes getting the flu shot when it becomes available in the fall, the smart thing to do.