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).
ILI nationally was low despite the fact that high percentages of respiratory specimens tested positive for influenza in parts of the country from early February until late May, and even then, the percent of specimens testing positive for flu remained above 10% for the Week 20 report. High percentages of respiratory specimens testing positive for flu usually mean high flu activity or flu outbreaks and historically, the 10% mark has been used as an informal marker for the beginning and end of the flu season. This season, improved screening and testing procedures at labs may have contributed to higher levels of percent positive tests than have been seen previously. It’s also possible that the lateness of the season contributed to higher levels of specimens testing positive for flu for longer. According to Bresee, “With little else circulating that would result in influenza-like-illness, it would make sense that we would find a higher proportion of flu viruses since that is pretty much all that is out there right now.” CDC epidemiologists will be looking at this indicator closely over the next couple of years to see whether a “new normal” for percent of respiratory specimens testing positive for flu may have emerged as a result of laboratory and testing improvements. Nevertheless, according to Bresee “the high numbers of percent positive specimens that we saw this season were indicative of flu activity. While these localized influenza outbreaks occurred from February through late May, they were never prolonged or extensive enough to raise the national level of ILI substantially.”
Other key flu indicators were low this season as well. As of May 25, 2012, 26 pediatric deaths occurring during the 2011-2012 season had been reported to CDC; this is the lowest number of pediatric deaths reported during a season since such record-keeping began. These are deaths in children younger than 18 who test positive for influenza. These deaths have been legally reportable by states to CDC since 2004. Since that time, the number of pediatric deaths has ranged from a low of 46 deaths during the 2005-2006 season, to a high of 282 deaths reported during the 2009-2010 season, which included pediatric deaths occurring during the 2009 H1N1 pandemic. It’s possible that additional deaths will be reported for the 2011-2012 season as delays in reporting sometimes occur and surveillance for pediatric mortality occurs year-round.