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Update: Influenza Activity -- United States

Morbidity reports show a continued increase in the number of outbreaks of influenza-like illness in the United States. In most cases for which laboratory results have been obtained, influenza type A(H1N1) has been isolated. The greatest spread of type A(H1N1) virus has occurred in the South Atlantic and South Central states. Preliminary results indicate that, in January, separate school outbreaks of influenza types A(H1N1) and B occurred in Hennepin County, Minnesota. The influenza type B outbreak was the first reported in the country this season. Following an almost total absence of type A(H3N2) virus isolates after the outbreaks in Alaska in November/December 1983, isolates have recently been reported from patients with sporadic cases in Arizona, New Mexico, and Pennsylvania.

Thus far in the 1983-1984 season, isolates of type A(H1N1) virus have been reported from the District of Columbia and 20 states: Alabama, Alaska, Arizona, Arkansas, California, Colorado, Florida, Georgia, Hawaii, Iowa, Louisiana, Maine, Massachusetts, Minnesota, Nevada, New Mexico, New York, North Carolina, South Carolina, and Texas; with associated outbreaks reported from Arizona, Arkansas, Georgia, Iowa, Louisiana, Minnesota, New Mexico, New York, North Carolina, South Carolina, and the District of Columbia. Isolates of type B virus have been reported from 13 states: Arizona, Arkansas, California, Colorado, Hawaii, Minnesota, Nevada, New Mexico, New York, Oregon, Texas, Washington, and West Virginia; with associated outbreaks reported from Minnesota, and possibly Texas. Isolates of type A(H3N2) virus have been reported from Alaska, Arizona, New Mexico, Pennsylvania, and Tennessee; associated outbreaks were reported from Alaska at the end of 1983. Reported by C Hedburg, Hennepin County Health Dept, D Peterson, MS, D Stiepan, J Braun, AG Dean, MD, State Epidemiologist, Minnesota Dept of Health; State Epidemiologists and Laboratory Directors; Div of Field Svcs, Epidemiology Program Office, Statistical Svcs Activity, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Since 1977, type A(H1N1) virus has twice caused epidemics in children and young adults--in 1977-1978 and 1978-1979. Although related viruses have been isolated in each succeeding year, they have not caused widespread outbreaks again until this year. Several factors probably contribute, including waning levels of immunity among those infected during the 1977-1979 outbreaks and evolution of new variant strains (1). When multiple virus types circulate (i.e., type A(H1N1), type A(H3N2), and type B viruses), the relative prevalence of the different virus strains can alter during the course of the season and may vary between regions. Outbreaks caused by several virus types are also possible. Continued laboratory diagnosis is especially important under current circumstances.

Reference

  1. CDC. Antigenic analysis of recent influenza virus isolates. MMWR 1983;32:195-6, 201.



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