Situation Update: Summary of Weekly FluView

Key Flu Indicators

Weekly reporting of the key flu indicators has ended for the 2011-2012 season and will resume on October 12, 2012, for the 2012-2013 season.

According to this week's FluView, flu activity in the United States is approaching summertime levels in most of the country. The week 20 FluView marks the final full influenza surveillance report for this late and mild flu season in the United States (see "2011-2012 Season Draws to a Close"). While one state continues to report widespread geographic flu activity, flu intensity is low nationwide. The late start of this season and the fact that flu viruses circulate year-round in the United States means that some flu activity will likely continue to occur in the coming weeks. Below is a summary of the most recent key indicators:

• The proportion of visits to doctors for influenza-like illness (ILI) remains below the national baseline. All 10 U.S. regions reported ILI activity below region-specific baseline levels as well, which is the same as last week.
• One state reported widespread influenza activity (the same as last week). Regional influenza activity was reported by two states (a decrease from 5 states last week). Eight states reported local influenza activity (three less than last week). Thirty states (the same as last week), the District of Columbia, Puerto Rico and Guam reported sporadic influenza activity. U.S. Virgin Islands and eight states reported no influenza activity (an increase from three last week) and one state did not report.
• Based on data reported to the FluSurv-NET participating sites since October 2011, the cumulative number of patients hospitalized with laboratory-confirmed influenza is 2,374. This remains lower than the numbers reported at this time during previous influenza seasons.
• The proportion of deaths attributed to pneumonia and influenza (P&I) based on the 122 Cities Mortality Reporting System is slightly higher than the previous week but remains below the epidemic threshold at 6.4%.
• Two influenza-related pediatric deaths were reported this week. One of these deaths was associated with a seasonal influenza A (H3) virus and the other was associated with an influenza B virus. Twenty-six influenza-associated pediatric deaths occurring during the 2011-2012 season have been reported to CDC to date.
• Nationally, the percentage of respiratory specimens testing positive for influenza viruses in the United States was 13.9%. The regional percentage of respiratory specimens testing positive for influenza viruses ranged from 3.6% to 27.4%.
• All three types and subtypes of seasonal influenza viruses have been identified this season: 2009 H1N1 viruses, influenza A (H3) viruses and influenza B viruses. This week, 150 of the 286 influenza positive tests reported to CDC were influenza A and 136 were influenza B viruses. Among the 150 influenza A viruses identified this week, approximately 58.7% were influenza A (H3) viruses and 14% were 2009 H1N1 viruses; 27.3% were not subtyped.
• Although influenza A (H3) viruses have predominated nationally this season, circulating viruses differ from region to region, and even from state to state. In recent weeks, the proportion of influenza B viruses has been increasing.
• Tested viruses this season remain susceptible to the antiviral drugs oseltamivir and zanamivir with the exception of 16 2009 H1N1 viruses that were oseltamivir resistant (the same number of resistant viruses reported in last week's report). Eleven of the 16 oseltamivir resistant viruses detected are from Texas.

FluView is available – and past issues are archived – on the CDC website.