Current Trends Influenza Surveillance Summary -- United States, 1982-1983 Season

Influenza virus types A(H3N2), A(H1N1), and B were isolated during the 1982-1983 U.S. season; H3N2 isolates predominated. The total number of isolates reported to CDC by collaborating laboratories was approximately 1,650, up sharply from the approximately 600 for the preceding season but typical of the numbers reported in the preceding 5 years (Figure 1).

The first outbreaks of influenza activity (type A(H3N2)) occurred in Alaska during October and November, and outbreaks were next reported from Idaho, Montana, Michigan, Minnesota, and upstate New York beginning in late December. This was reflected by an increase in laboratory virus isolations (Figure 2). The peak of activity for the nation occurred in late February and early March, and then declined to low levels in April. Twelve states reported widespread influenza activity during the period of viral circulation in 1982-1983 (Figure 3), compared with the four and 32 states that reported widespread activity during the 1981-1982 and the 1980-1981 seasons, respectively.

Of the isolates obtained by collaborating laboratories, 79% were influenza virus type A(H3N2); 11% were type B; and 10% were type A(H1N1). The H1N1 and type B isolates were detected later in the season (Figure 4), in many regions of the country. Forty-three states reported type A(H3N2) virus isolates; 29, type B; and 24, type A(H1N1); in 14 states, all three types were isolated. Most of the type A(H3N2) strains were closely related to A/Bangkok/79(H3N2). However, some antigenically distinct H3N2 variants were isolated during the season, including A/Philippines/2/82-like strains (1). In contrast, little antigenic variation was detected among the type A(H1N1) and type B viruses isolated during the season in the United States.

Influenza virus type A(H3N2) infection was associated with reports of outbreaks among all age groups, including many nursing home populations, with clinical attack rates up to approximately 60% (2). Influenza A(H1N1) infections were again recognized predominantly among children and young adults, with occasional school outbreaks. Influenza type B virus, which caused several documented outbreaks in schools, was also associated with occasional outbreaks of influenza in nursing homes during the season, and was responsible for some late outbreaks in May (3).

Pneumonia and influenza (P&I) mortality from 121 cities, which increased slightly above the calculated epidemic threshold level in January, did not later return consistently below this threshold in parallel with the decline in other indices of influenza activity (Figure 5). Reported by state and territorial epidemiologists and state laboratory directors; participating physicians in the Ambulatory Sentinel Practice Network-CDC Influenza Project; Div of Surveillance and Epidemiologic Studies, Epidemiology Program Office, Statistical Svcs Activity, WHO Collaborating Center for Influenza, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: National data on influenza activity for the 1982-1983 season were obtained from three established sources: (1) weekly reports of the number of respiratory specimens tested and the number and types of influenza virus isolates identified by 66 collaborating state, county, city, or military laboratories; (2) weekly reports of mortality from 121 cities, including the ratio of P&I deaths to total deaths, an index that has historically reflected seasonal mortality attributable to influenza; and (3) weekly, semi-quantitative estimates from each state health department of the extent of influenza-like morbidity indicated by its individual, statewide surveillance system. Spontaneous reports of unusual cases and outbreaks of influenza from a variety of sources were also received by CDC.

In addition, a pilot program was undertaken in collaboration with the Ambulatory Sentinel Practice Network of North America (ASPN) involving weekly reports to CDC of the number of patients seen with influenza-like morbidity (case definition: fever of 37.8 C (100 F) or greater and at least cough or sore throat) in the offices of approximately 150 primary-care physician-members of the American Academy of Family Physicians' research panel. The physicians also provided a subjective assessment whether an "outbreak" of influenza was occurring among their patients.

Preliminary analysis of results of the ASPN-CDC pilot study of influenza morbidity (Figure 6), suggests that these types of data may provide another useful, and potentially early, indication of the occurrence of influenza epidemics. Comparisons of morbidity reports in several epidemic and non-epidemic years are required, however, to assess the relative specificity and sensitivity of these data.

It is not clear why P&I deaths from 121 cities remained elevated longer than other measures of influenza activity this season. While this elevation in May and June is principally due to excess P&I deaths among persons 65 years of age and older--the group most susceptible to death during periods of influenza virus activity--additional investigation is under way to determine if other factors may have contributed to this observation.

References

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

2. CDC. Impact of influenza on a nursing home population--New York. MMWR 1983;32:32-4.

3. CDC. Late season influenza virus type B activity--United States. MMWR 1983;32:271-2.

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