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Epidemiologic Notes and Reports Influenza -- United States

Update on Activity

During the 1987-88 influenza season in the United States, peak activity occurred in late February and early March. During the 4-week period February 14-March 12, 1988, widespread or regional outbreaks of influenza-like illness were reported from 28 to 30 states each week. For the week ending February 20, 30 states reported outbreaks, the highest number for any single week this season. Surveillance conducted throughout the country also showed a peak in the percentage of patients seen with influenza-like illness during the week ending February 20: an average of 8.1% of patients seen that week had an influenza-like illness, compared with the overall seasonal average of 4.8%. Correlating with these indicators, the proportion of deaths attributed to pneumonia and influenza (P&I) first exceeded the epidemic threshold* on the week ending February 20, peaked during the week ending March 5, and remained above the threshold on April 2 (Figure 1).

Influenza type A(H3N2) has been the predominant influenza strain this season, representing 85% of all influenza virus isolates reported in the United States by the World Health Organization Collaborating Laboratories as of March 26. Influenza A(H3N2) isolates have been confirmed in the District of Columbia and in all states except New Hampshire and Rhode Island. Many states have reported outbreaks of influenza-like illness in nursing homes, often with isolation of influenza A(H3N2) virus from specimens collected either from the nursing-home residents or from residents of nearby communities.

During the latter part of the 1987-88 season, influenza types A(H1N1) and B have been isolated more frequently, but as of March 26, these viruses still represented only 6% and 9% of isolates, respectively. Influenza A(H1N1) has been isolated in 16 states,** and influenza B, in 26 states.*** Although these isolates have primarily been associated with sporadically occurring cases, three culture-confirmed outbreaks of influenza B have been reported with onset dates during late February or early March. Two occurred in nursing homes in Connecticut, and the third in a pediatric long-term care wing of a New York hospital. Characterization of Antigenic Variants of Influenza A(H3N2) Viruses

Earlier, CDC reported circulation of two type A(H3N2) viruses, A/Sichuan/2/87 and A/Victoria/7/87, that were antigenically distinct from viruses circulating from 1985 through the spring of 1987, such as A/Leningrad/360/86 and A/Mississippi/1/85 (2). Viruses with reaction patterns that were intermediate between A/Sichuan/2/87 and A/Leningrad/360/86 were also described. These intermediate viruses have now been characterized and found to resemble two reference strains: A/Sydney/1/87 and A/Shanghai/11/87 (Table 1). These viruses are inhibited at higher titers than is A/Sichuan/2/87 virus with antiserum prepared against A/Leningrad/360/86 and at lower titers with A/Sichuan/2/87 antiserum. The A/Shanghai/11/87 virus is distinct from the A/Sydney/1/87 virus as evidenced by the lower hemagglutination-inhibition titers with antisera to A/Caen/1/84, A/Mississippi/1/85, and A/Sydney/1/87. Since the fall of 1987, 158 viruses collected in the United States have been characterized; seven (5%) are A/Sichuan/2/87-like, 19 (12%) are A/Victoria/7/87-like, 24 (15%) are A/Sydney/1/87-like, and 108 (68%) are A/Shanghai/11/87-like. Reported by: Participating State and Territorial Epidemiologists and State Laboratory Directors. Participating Physicians of the American Academy of Family Physicians. WHO Collaborating Laboratories. WHO Collaborating Center for Influenza, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC. Editorial Note: Cocirculation of two or more closely related antigenic variants in a single season is not uncommon. This season, four variants of influenza type A(H3N2) are circulating as well as types A(H1N1) and B.

Of the influenza viruses currently in circulation, influenza A(H3N2), which emerged in 1968, has been associated with the greatest excess P&I deaths and total excess mortality. Excess mortality has occurred during each of 11 influenza A(H3N2) epidemics. Thus, the elevation of the percentage of P&I deaths observed this season is consistent with observations during other influenza A(H3N2) epidemics.

Weekly reports of deaths in 121 cities in the United States are used to determine preliminary estimates of influenza-related mortality during the influenza season. The percentage of deaths attributed to P&I is calculated each week and compared with a ratio of P&I deaths to total deaths that would be expected in the absence of an influenza epidemic (1). Data from the National Center for Health Statistics (NCHS) on all deaths in the United States are used to determine final estimates of excess P&I deaths and total excess mortality; these statistics are not available until approximately 2 years after the epidemic period. The P&I ratio from 121 cities offers a useful method for evaluating the impact of influenza during epidemics, and death rates calculated with use of the 121-city data and the final NCHS data show similar trends. However, the 121-city data cannot be used to project accurately total influenza-related deaths for this season or to make other than general comparisons to past influenza seasons. References

  1. 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.

  2. Centers for Disease Control. Antigenic variation of recent influenza A(H3N2) viruses. MMWR 1988;37:38-40,46-47. *The epidemic threshold for the 1987-88 influenza season was estimated at 1.645 standard deviations above the values projected on the basis of a periodic regression model applied to observed P&I deaths for the previous 5-year period, but excluding the observations during influenza outbreaks (1). **Alabama, Arkansas, Connecticut, Georgia, Illinois, Louisiana, Maine, Massachusetts, Nebraska, New Jersey, New York, North Carolina, South Carolina, Texas, Vermont, and Virginia. ***Alabama, Alaska, Arizona, Arkansas, California, Connecticut, Delaware, Hawaii, Illinois, Iowa, Maine, Massachusetts, Montana, Nebraska, Nevada, New Mexico, New York, Ohio, Pennsylvania, Tennessee, Texas, Utah, Virginia, Washington, West Virginia, and Wisconsin.

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