Flu: The Last Flu Story, We Hope (cont.)

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Update: Influenza Activity --- United States, January 11--17, 2004

The number of states reporting widespread influenza activity* continued to decrease during the reporting week of January 11--17, 2004. Health departments in five states reported widespread influenza activity. A total of 31 states and New York City reported regional activity, eight states reported local activity, and sporadic activity was reported by six states, the District of Columbia, Guam, and Puerto Rico (Figure 1). The percentage of outpatient visits for influenza-like illness (ILI) continued to decrease in all surveillance regions during the week ending January 17. For the first time since the reporting week ending November 8, 2003, the national percentage for ILI (2.0%) declined below the national baseline of 2.5%. The percentage of specimens testing positive for influenza also decreased, but the percentage of deaths attributed to pneumonia and influenza (P&I) was unchanged.

Laboratory Surveillance

During the week ending January 17, World Health Organization (WHO) laboratories reported testing 1,544 specimens for influenza viruses, of which 123 (8.0%) were positive. Of these, 26 were influenza A (H3N2) viruses, 95 were influenza A viruses that were not subtyped, and two were influenza B viruses.

Since September 28, WHO and NREVSS laboratories have tested 76,311 specimens for influenza viruses, of which 19,968 (26.2%) were positive. Of these, 19,853 (99.4%) were influenza A viruses, and 115 (0.6%) were influenza B viruses. Of the 19,853 influenza A viruses, 4,652 (23.4%) have been subtyped; 4,651 (99.9%) were influenza A (H3N2) viruses, and one (0.1%) was an influenza A (H1) virus.

Antigenic Characterization

Of the 573 influenza viruses collected by U.S. laboratories since October 1, 2003, and characterized antigenically by CDC, 565 were influenza A (H3N2) viruses, two were influenza A (H1) viruses, and six were influenza B viruses. The hemagglutinin proteins of the influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99. Of the 565 influenza A (H3N2) isolates that have been characterized, 106 (18.8%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 459 (81.2%) were similar to a drift variant, A/Fujian/411/2002 (H3N2)**. Five influenza B viruses characterized were similar antigenically to B/Sichuan/379/99 and one was similar antigenically to B/Hong Kong/330/2001.