Synopsis: During week 51 (December 15-21, 2013), influenza activity continued to increase in the United States.

o Viral Surveillance:Of 6,813 specimens tested and reported by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories during week 51, 1,639 (24.1%) were positive for influenza.

o Pneumonia and Influenza Mortality: The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.

o Influenza–associated Pediatric Deaths: One influenza-associated pediatric death that occurred during the 2012-2013 season was reported.

o Influenza–associated Hospitalizations :A cumulative rate for the season of 4.3 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported.

o Outpatient Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) was 3.0%, above the national baseline of 2.0%. Eight regions reported ILI at or above region-specific baseline levels. Six states experienced high ILI activity; eight states experienced moderate ILI activity; six states experienced low ILI activity; 28 states experienced minimal ILI activity, and the District of Columbia, New York City, and two states had insufficient data.

o Geographic Spread of Influenza : The geographic spread of influenza in 10 states was reported as widespread; Guam and 23 states reported regional influenza activity; 12 states reported local influenza activity; the District of Columbia, Puerto Rico, and four states reported sporadic influenza activity, and the U.S. Virgin Islands and one state did not report.