From May 24 through Sept. 5, 2015, the United States experienced typical low levels of seasonal influenza activity, according to a report in this week's MMWR by Blanton, et al. (2015). Influenza A (H1N1)pdm09 (pH1N1), influenza A (H3N2), and influenza B viruses were detected worldwide and were identified sporadically in the United States. All of the influenza viruses collected from U.S. states and other countries during that time have been characterized antigenically and/or genetically as being similar to the influenza vaccine viruses recommended for inclusion in the 2015-16 Northern Hemisphere vaccine. From May 24 to Sept. 5, 2015, three influenza variant† virus infections were reported; one influenza A (H3N2) variant virus (H3N2v) from Minnesota in July, one influenza A (H1N1) variant (H1N1v) from Iowa in August, and one H3N2v from Michigan in August.
The U.S. influenza surveillance system is a collaboration between CDC and federal, state, local, and territorial partners, and uses eight data sources to collect influenza information, six of which operate year-round. During the period of May 24 through Sept. 5, 2015, World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories in the United States tested 80,345 specimens for influenza; 1,698 (2.1%) were positive for influenza (Figure). Of the 1,698 specimens positive for influenza during the summer months of 2015, a total of 913 (54%) were influenza A viruses, and 785 (46%) were influenza B viruses. Influenza B viruses were reported slightly more frequently than influenza A viruses from late May until mid-June, and influenza A viruses were more commonly reported from late June through early September. Of the 913 influenza A viruses, 551 (60%) were subtyped: 28 (5%) were pH1N1 viruses and 523 (95%) were influenza A (H3N2) viruses. Influenza viruses were reported from the District of Columbia, Puerto Rico, and 47 states in all 10 U.S. Department of Health and Human Services regions.
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Reference: Blanton L, Kniss K, Smith S, et al.Update: Influenza Activity — United States and Worldwide, May 24–September 5, 2015. MMWR. Sept. 18, 2015 / 64(36);1011-1016