Update: Influenza -- United States, 1989-90

CDC's influenza monitoring systems indicate that the level of influenza activity for the 1989-90 season (October 1-April 30) in the United States is declining. This report summarizes data for October 1989 through February 1990, and includes weekly reports from 63 World Health Organization (WHO) Collaborating Laboratories, 150 sentinel physicians, the 55 state and territorial health departments, and the 121 Cities Pneumonia and Influenza Mortality Reporting System (1,2). WHO Collaborating Laboratories

In September 1989, the first influenza virus isolated in the United States this season (an A/Shanghai/11/87-like (H3N2) virus) was isolated from a Wisconsin student who became ill within 48 hours of returning from West Africa (3). Additional viruses were not isolated until the week ending November 18, when A/Shanghai/11/87-like (H3N2) viruses were reported from Arizona, Hawaii, Montana, and Washington. From the weeks ending November 25 through December 16, the total number of specimens submitted for influenza testing and the number positive increased from 562 and two (0.4%) to 1081 and 63 (5.8%). From January 13 to February 3, the largest number of specimens (mean: 2021 per week) were submitted for influenza testing, and the largest number of influenza viruses were isolated (mean: 467 (23%)). Submission of viral culture specimens began to decline the week ending February 10.

As of February 24, WHO Collaborating Laboratories reported the isolation of 2785 influenza viruses; 2777 (99.7%) were type A and eight (0.3%) were type B. Of the influenza A isolates that were subtyped, 99% were influenza A(H3N2); 17 influenza A(H1N1) isolates were reported. Domestic isolates that were antigenically characterized were similar to the components of the 1989-90 influenza vaccine (4). Influenza Sentinel Physicians

From October 1 through November 18, an average of 3% of patient visits to 150 sentinel physicians were for influenza-like illness; from November 19 through December 16, the average was 4.2%. For the week ending December 23, the percentage increased to 6.4% and reached a season high of 8.9% the week ending December 30. The percentage stabilized at approximately 8% through January, then decreased to 6.5% during February. State and Territorial Health Departments

For the week ending December 2, Montana's state health department became the first to report sustained regional* influenza activity and 2 weeks later was the first to report widespread influenza activity. During the week ending January 27, 38 states reported widespread or regional activity. By February 24, only two states reported widespread activity, although the number reporting regional activity (19) remained comparable to that in early January. 121 Cities

During the week ending January 6, the proportion of deaths attributable to pneumonia and influenza (P&I) first exceeded the epidemic threshold (Figure 1). The P&I ratio peaked during the week ending February 3 but remained above the epidemic threshold through March 3. Reported by: State and territorial health department epidemiologists and state public health laboratory directors. WHO Collaborating Laboratories. Sentinel Physicians Influenza Surveillance System of the American Academy of Family Practice. Div of Surveillance and Epidemiologic Studies, Epidemiology Program Office; Epidemiology Activity, Biometrics Activity, Influenza Br, Div of Viral and Rickettsial Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The predominance of influenza A(H3N2) during the 1989-90 epidemic exceeds that for recent influenza seasons in the United States. The only comparable season during the past decade was 1984-85, when influenza A(H3N2) isolates accounted for 97.3% of total subtyped influenza isolates. The number of isolates, the percentage of patients with influenza-like illness seen by sentinel physicians, and the activity levels reported by state and territorial health departments have not indicated exceptionally high levels of influenza morbidity during the 1989-90 season; however, the P&I ratio reflects the excess mortality in the elderly historically attributable to influenza A(H3N2).

In the 1988-89 season, predominant influenza A activity in the winter was superseded by influenza B during March and April. Although a similar trend has not been observed so far in 1989-90, this pattern demonstrates the importance of continued monitoring of influenza activity, including culturing of patients with suspected influenza, throughout the influenza season to guide prophylaxis and treatment decisions (4).

References

1. CDC. Influenza--United States, 1986-87 season. MMWR 1988;37:466-70,475.

2. CDC. Influenza--United States, 1987-88 season. MMWR 1988;37:497-503.

3. CDC. Update: influenza activity--worldwide, 1988-89. MMWR 1989;38:817-8. 4. ACIP. Prevention and control of influenza: part 1, vaccines. MMWR 1989;38:297-8,303-11. *Levels of activity are: 1) sporadic--sporadically occurring influenza-like illness or culture-confirmed influenza, with no outbreaks detected; 2) regional--outbreaks of influenza-like illness or culture-confirmed influenza in counties having a combined population of less than 50% of the state's total population; 3) widespread--outbreaks of influenza-like illness or culture-confirmed influenza in counties having a combined population of greater than or equal to 50% of the state's total population.

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