(HealthDay News) – In the fourth week of 2013, influenza activity remained elevated in the United States, with the proportion of pneumonia and influenza-linked deaths above the epidemic threshold, according to FluView, a weekly influenza surveillance report prepared by the Influenza Division of the U.S. Centers for Disease Control and Prevention.

Researchers from the CDC collected data from World Health Organization and National Respiratory and Enteric Surveillance System collaborating laboratories, located in all 50 states and Puerto Rico, to examine the number of respiratory specimens tested for influenza during the fourth week of 2013.

The researchers found that 25.5% of the 10,581 specimens tested and reported were positive for influenza. Of these, 79.3% were influenza A (2.5% 2009 H1N1, 49.2% H3, and 48.3% subtyping not performed) and 20.7% were influenza B. During week four of 2013, 9.4% of all deaths were due to pneumonia and influenza, above the epidemic threshold of 7.4%. There were eight influenza-associated pediatric deaths. The cumulative rate of laboratory confirmed influenza-linked hospitalizations was 25.9 per 100,000 population; >50% of these were among adults aged ≥65 years. Nationwide, 4.2% of outpatient visits were due to influenza-like illness, which was above the national baseline of 2.2%. Widespread geographic influenza activity was reported in 42 states.

“Since the start of the season, influenza A (H3N2) viruses have predominated nationally,” the authors write. “Over the course of the season the predominant circulating virus has varied by state and by region.”