Update: influenza activity - United States, September 29-December 7, 2013

Abstract

CDC collects, compiles, and analyzes data on influenza activity year-round in the United States (http://www.cdc.gov/flu/weekly/fluactivitysurv.htm). The influenza season generally begins in the fall and continues through the winter and spring months; however, the timing and severity of circulating influenza viruses can vary by geographic location and season. Influenza activity in the United States continued to increase from mid-November through the beginning of December. This report summarizes U.S. influenza activity* during September 29-December 7, 2013.

FIGURE 1

Number^* and percentage of respiratory specimens testing positive for influenza reported by World Health Organization and National Respiratory and Enteric Virus Surveillance System collaborating laboratories in the United States, by type, subtype, and week — United States, September 29–December 7, 2013^{† *} N = 4,183. ^† Data reported as of December 13, 2013.

FIGURE 2

Percentage of visits for influenza-like illness (ILI)^* reported to CDC, by surveillance week — Outpatient Influenza-Like Illness Surveillance Network, United States, September 29–December 7, 2013, and selected previous influenza seasons ^* Defined as a temperature of ≥100°F (≥37.8°C), oral or equivalent, and cough and/or sore throat, without a known cause other than influenza. ^† The national baseline is the mean percentage of visits for ILI during noninfluenza weeks for the previous three seasons plus two standard deviations. Noninfluenza weeks are defined as periods of ≥2 consecutive weeks in which each week accounted for <2% of the season’s total number of specimens that tested positive for influenza. National and regional percentages of patient visits for ILI are weighted on the basis of state population. Use of the national baseline for regional data is not appropriate.