Influenza Activity -- United States, 1993-94 Season

From mid-November through December 1993, influenza activity in the United States increased. This report summarizes surveillance information regarding influenza activity in the United States from October 3, 1993, through January 1, 1994.

As of January 1, 1994, influenza viruses had been reported in 46 states; four states had not documented the presence of influenza this season (Delaware, Mississippi, New Hampshire, and Rhode Island). Nearly all (99%) influenza isolates reported to CDC have been influenza type A; one influenza type B virus was isolated in New York.

Of the 750 influenza type A viruses reported from the World Health Organization (WHO) collaborating laboratories, 409 were subtyped as type A(H3N2) and 16 as type A(H1N1); 325 were not subtyped. As of January 10, 208 influenza isolates had been received by the WHO Collaborating Center for Influenza at CDC for antigenic analysis. Of the 68 influenza type A(H3N2) virus isolates completely analyzed, all were antigenically related to the A/Beijing/32/92(H3N2) strain included in the 1993-94 influenza vaccine. The ratio of specimens positive for influenza virus to total specimens submitted for respiratory virus testing increased from less than 0.01 in previous weeks to 0.04 the week ending November 20 and to 0.18 the week ending December 25.

Weekly reports by state and territorial epidemiologists also had indicated increasing levels of influenza-like illness (ILI). The number of states reporting sporadic * activity ranged from five to eight per week during October and from 17 to 20 per week during November. Regional activity associated with laboratory-confirmed influenza was first reported from Wyoming and Montana during the week ending November 13 (1). However, no more than three states reported regional activity during any week until the week ending December 4, when regional activity was reported by six states and the District of Columbia. For the week ending December 18, the number of states reporting regional activity increased to 12. Widespread activity was first reported in Florida and Oregon for the week ending December 11.

For the week ending January 1, 13 states reported sporadic activity, 18 reported regional activity, and 12 reported widespread activity (Figure_1). Most reported outbreaks of laboratory-confirmed influenza type A were in schools, but outbreaks also occurred in nursing homes and other facilities housing older adults.

Of total deaths reported through CDC's 121-city mortality surveillance system, the proportion of deaths associated with pneumonia and influenza exceeded the epidemic threshold ** for 2 consecutive weeks beginning December 19. This is the first time this season that these proportions have been elevated for 2 consecutive weeks in addition to other indices of increased influenza activity; they indicate the occurrence of epidemic influenza-associated mortality. Reported by: Participating state and territorial epidemiologists and state public health laboratory directors. World Health Organization collaborating laboratories. Sentinel Physicians Influenza Surveillance System of the American Academy of Family Physicians. Influenza Br and Epidemiology Activity, Office of the Director, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Although sharp increases in influenza activity were reported in some areas during December, national influenza activity is expected to continue increasing during January based on observations of other seasons in which influenza type A(H3N2) predominated.

Compared with periods of predominant influenza type A(H1N1) or type B activity, influenza type A(H3N2) activity is associated with higher morbidity and mortality among the elderly. For example, approximately 80%-90% of influenza-related deaths occur among persons aged greater than 64 years, and influenza A(H3N2) epidemics often are characterized by an increase in the proportion of deaths attributed to pneumonia and influenza (2).

Although antigens contained in the 1993-94 influenza vaccine closely match circulating influenza viruses, the antiviral agents amantadine and rimantadine -- which are effective against influenza A -- should be considered as an adjunct to vaccination. Because of differences in the pharmacokinetic properties of these two drugs, the dosage recommendations and the potential for adverse reactions vary with such factors as patient's age, presence of certain underlying health conditions, and the potential for adverse drug interactions.

Influenza surveillance data are updated every other week throughout the influenza season, and summaries are available by computer to subscribers of the Public Health Network and to health-care providers and the public through the CDC Voice Information System at (404) 332-4555 or by facsimile at (404) 332-4565.

References

1. CDC. Update: influenza activity -- United States and Europe, 1993- 94 season. MMWR 1993;42: 909-11.

2. Lui K-J, Kendal AP. Impact of influenza epidemics on mortality in the United States from October 1972 to May 1985. Am J Public Health 1987;77:712-6.

* Levels of activity are 1) sporadic sporadically occurring ILI or culture-confirmed influenza, with no outbreaks detected; 2) regional outbreaks of ILI or culture-confirmed influenza in counties having a combined population of less than 50% of the state's total population; and 3) widespread outbreaks of ILI or culture-confirmed influenza in counties having a combined population of 50% or more of the state's total population. 

** The epidemic threshold is 1.645 standard deviations above the seasonal baseline calculated using a periodic regression model applied to observed percentages since 1983. This baseline was calculated using a robust regression procedure.

Figure_1


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