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Current Trends Influenza -- United States, 1983-1984 Season

During the 1983-1984 influenza season, school and college outbreaks of type A(H1N1) strains began and increased sharply after January 1 and peaked in February. Type B strains were also isolated in all regions of the country from about February to April, largely in schools and colleges; a few outbreaks were reported in older age groups. Type A(H3N2) virus activity was generally sporadic, despite an early outbreak in Alaska. Preliminary analysis suggests there was little excess mortality associated with influenza.

National data on influenza activity for the 1983-1984 season were obtained from four major sources: (1) weekly reports of the number of respiratory specimens tested and the number and types of influenza virus isolates identified by 61 collaborating state, county, city, or military laboratories; (2) weekly reports of mortality from 121 cities, an index that has historically reflected seasonal mortality attributable to influenza; (3) weekly semi-quantitative estimates from each state health department of the extent of influenza-like morbidity indicated by its individual statewide surveillance system; and (4) weekly returns from approximately 125 primary-care physician members of the American Academy of Family Physicians research panel, who recorded the number of patients seen in their offices with influenza-like illnesses* and provided their assessment of whether an "outbreak" of influenza was occurring among their patients. In addition, spontaneous reports of unusual influenza cases and outbreaks from a variety of sources were also received by CDC.

Before the usual influenza season, sporadic isolates of influenza A(H3N2) virus were obtained in August (Tennessee); of influenza B virus, in September (Tennessee); and of influenza A(H1N1) virus, in October (Alabama). The first influenza outbreaks confirmed by virus isolation occurred during November and December in Alaska, where type A(H3N2) virus was active among young adults. Immediately after the new year, however, reports of isolations of type A(H1N1) virus from sporadic cases and outbreaks in schools and colleges were received, particularly from the South Atlantic and South Central regions. Rapid increases in the number of type A(H1N1) virus isolations were noted (Figure 1) as the virus became active across the country (Figure 2).

Also during November and December, sporadic type B influenza virus isolates were recovered in California, Texas, and West Virginia, primarily from children or young adults. Early in 1984, type B influenza virus activity increased in association with school outbreaks and persisted at fairly constant, relatively moderate levels from February through April, remaining active into May (Figure 1). This contrasted with the shorter period of type A(H1N1) virus activity that peaked sharply in February. Some nursing-home outbreaks of type B influenza were reported. By the end of the season, many regions had experienced both type A(H1N1), and type B virus activity, although early on, type B virus isolates were not identified in the southeastern states, where type A(H1N1) virus was active in outbreaks; conversely, type A(H1N1) isolates were not at first identified in Washington and Oregon, when type B virus was causing outbreaks (Figure 2). Influenza A(H3N2) viruses were isolated at low levels throughout the season, with at least one associated nursing-home outbreak.

Forty-seven states reported type A(H1N1) isolates; 40 states reported type B isolates; and 15 states reported type A(H3N2) isolates. Of the 2,130 isolates obtained by the collaborating laboratories, 50.6% were influenza type A(H1N1); 44.7% were type B; and 4.7% were type A(H3N2). The number of isolates reported was the largest total since the 1976-1977 season (Figure 3).

Twenty-two state epidemiologists reported widespread influenza activity during the period of viral circulation in 1983-1984 (Figure 4); in comparison, during the 1981-1982 and 1982-1983 seasons, widespread activity was reported from only four and 12 states, respectively. However, during the 1980-1981 season, 32 states reported widespread activity. Morbidity reports by family physicians exhibited temporal and geographic trends consistent with other indicators of influenza activity. For example, the earliest and largest rise in visits for influenza-like illness was in the southern regions (Figure 5), which corresponded with the large numbers of type A(H1N1) outbreaks there.

Preliminary analysis of pneumonia and influenza mortality from 121 cities did not indicate a consistent, statistically significant rise above the rates expected in the absence of an epidemic. Failure to detect excess mortality would correspond with this season's paucity of reported outbreaks of influenza among the elderly, the group that usually is most vulnerable to severe influenza infection. Reported by State and Territorial Epidemiologists and State Laboratory Directors; Other collaborating laboratories; Participating physicians of the American Academy of Family Physicians, Statistical Svcs Br, Div of Surveillance and Epidemiologic Studies, Epidemiology Program Office, Computer Systems Office, Office of the Centers Director, Statistical Svcs Activity, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC. *Case definition: fever of 37.8 C (100 F) or higher with cough or sore throat.

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