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Current Trends Update: Influenza Activity -- United States

Reports of influenza viruses in the United States isolated by the collaborating diagnostic laboratories have increased sharply, with over 200 viruses identified each week since the end of January 1986. Overall, approximately 80% of the reported viruses have been type B, but more type A(H3N2) than type B viruses have been identified among persons older than 64 years. Of type B viruses, 66.3% have been isolated from persons under 19 years old, and 3.2%, from persons over 64 years old. In comparison, 45.9% of type A(H3N2) cases occurred among persons under 19 years old; 26.1% occurred among persons over 64 years old. Forty-five states have now reported type B virus isolates; 29 states, type A(H3N2) isolates; and 28 states, both types.

For the week ending February 22, 18 states reported widespread outbreaks of influenza-like illness, and 15 states and the District of Columbia reported regional outbreaks. For the preceding week, 37 states had reported outbreak status, more than for any week since January 1981. Tallies of patients with influenza-like illnesses seen by the network of family physicians* nationwide averaged 10.9 cases for the reporting week ending February 12, compared with the 11.5 average for the preceding week and the maximum values of 11-12 cases for the two preceding seasons (Figure 1).

The percentage of pneumonia and influenza deaths reported from the 121 U.S. cities for the week ending February 22 was 6.3%, compared with 6.2% for the preceding week (Figure 1). Reported by State and Territorial Epidemiologists; State Laboratory Directors; Statistical Svcs Br, Div of Surveillance and Epidemiologic Studies, Div of Field Svcs, Epidemiology Program Office, WHO Collaborating Center for Influenza, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC. *Cases reported by those members of the American Academy of Family Physicians Research Panel who serve as sentinel physicians for influenza.

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