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Update: Influenza Activity -- United States

Outbreaks of type A(H1N1) influenza affecting primarily children and young adults are continuing. For the week ending January 17, seven states* and Puerto Rico reported widespread outbreaks of influenza-like activity, and 21 states** and the District of Columbia reported regional outbreaks of influenza-like illness. This is the fourth week with more than 20 states reporting outbreak activity. The level of current activity is below the peak of the previous winter when 37 states indicated outbreaks for one week in February.

For the report week ending December 31, the Centers for Disease Control's sentinel physiciansS saw an average of 11.9 patients with cases of influenza-like illness per week per physician; the average was 9.1 for the report week ending January 7 (Figure 1). The maximum averages reported during influenza epidemics of the past three winters were between 11 and 12. The percentage of deaths associated with pneumonia and influenza reported from 121 cities has remained below the epidemic threshold (Figure 1).

Influenza A/Taiwan/86(H1N1)-like virus continues to be the predominant strain of influenza this season and represents 99% of isolates reported from collaborating diagnostic laboratories (Figure 1). Forty-four states and the District of Columbia have now reported isolates of influenza A(H1N1) virusP. Influenza type A(H3N2) has recently been reported from sporadically occurring cases in Colorado and in Texas; only one isolate (from Arizona) had been reported previously this season. There have been no recent reports of type B virus isolates. Reported by G Meiklejohn, MD, School of Medicine, Univ of Health Sciences Center, Denver, Colorado; Influenza Research Center, Baylor College of Medicine, Houston, Texas; State and Territorial Epidemiologists; State Laboratory Directors; WHO Collaborating Center for Influenza, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC. *Colorado, Connecticut, Minnesota, Oregon, Texas, Washington, and Wyoming. **Alabama, Alaska, Arizona, California, Idaho, Iowa, Kansas, Kentucky, Maryland, Massachusetts, Montana, Mississippi, Nebraska, New Mexico, North Carolina, North Dakota, Pennsylvania, South Dakota, Virginia, West Virginia, and Wisconsin. SSentinel physicians are members of the American Academy of Family Physicians who have agreed to report influenza-like activity to CDC. A case was defined as an instance of illness in a patient with fever greater than or equal to 37.8 C (100 F) and at least a cough or sore throat. PLouisiana, North Dakota, Rhode Island, South Carolina, South Dakota, and Wyoming have not reported any influenza isolates so far this season.

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