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Update: Influenza Activity -- United States

Influenza outbreaks in schools and colleges, associated with influenza types A(H1N1) or B, have occurred in all regions of the United States this year. However, there has been no consistent elevation of deaths attributable to pneumonia and influenza in the 121 reporting cities through mid-February, suggesting that older populations have been only slightly affected by the outbreaks.

A cumulative total of 590 influenza virus isolates has been reported this season through February 10, 1984; 381 (65%) were identified as type A(H1N1); 171 (29%), as type B; and 38 (6%), as type A(H3N2) (Figure 3). Results of virus laboratory testing are reported according to three broad age categories--patients less than 31 years of age, patients 31-65 years of age, and patients over 65 years of age. Of the 7,125 specimens tested by February 10 for which ages of patients were reported, 85%, 10%, and 5%, respectively, were from patients in these age brackets. Ninety-seven percent of the A(H1N1) isolates have been from persons in the youngest age group, representing 5% of specimens tested from these persons. In contrast, 0.5% of specimens from persons over 65 years of age were positive for A(H1N1) virus. In 1982-1983, however, when activity was predominantly caused by type A(H3N2) virus, 62% of A(H3N2) isolates were from persons in the youngest age group, and the proportion of specimens positive for influenza type A(H3N2) increased from 7% in the under-31-years group to 20% in the over-65-years group. While findings for the current season may change, the preliminary results are consistent with the paucity of reported outbreaks of influenza among the elderly this year, despite the prevalence of type A(H1N1) outbreaks in younger age groups. Reported by State Epidemiologists and Laboratory Directors; Statistical Svcs Br, Div of Surveillance and Epidemiologic Studies, Epidemiology Program Office, Statistical Svcs Activity, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC.

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