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Current Trends Influenza -- United States, 1987-88 Season
Influenza A(H3N2), the predominant type of influenza virus isolated in the United States during the 1987-88 season, exhibited antigenic drift from previous epidemic strains (1). Many of the isolates resembled two strains first recognized in China during 1987, A/Sichuan/2/87 and A/Shanghai/11/87. Outbreaks reported during 1987-88 in the United States that were associated with these viruses occurred in all age groups, including residents of nursing homes. Antigenic variants of influenza B also circulated during the 1987-88 season, with most isolates resembling B/Victoria/2/87 (2). The number of influenza B virus isolates increased late in the season when the first outbreaks associated with this virus were reported; at the same time, influenza A(H3N2) declined. Influenza A(H1N1) viruses similar to A/Taiwan/1/86, the predominant influenza virus during the 1986-87 season (3), were the least frequently isolated viruses during the 1987-88 season and were associated with only one possible outbreak, which occurred among college students. The number of influenza A(H1N1) virus isolates also increased late in the season.
Sources for surveillance of influenza were the same as for the 1986-87 season (3) with these exceptions:
The first suspected outbreak of influenza A(H3N2) occurred in cruise ship passengers who were touring Alaska during August (5). In October, a probable outbreak of influenza A(H3N2) occurred among American tourists traveling in the Orient aboard a cruise ship (6). The first reported domestic outbreak of influenza-like illness occurred in November in preschool children in Colorado; influenza A(H3N2) was isolated from a specimen obtained from the index patient (7). Sporadic isolates of influenza B were also reported early in the season from Arizona, Hawaii, and Wisconsin (8). However, the first reported outbreak of influenza B occurred in February in a Connecticut nursing home. Most reported outbreaks of influenza A(H3N2) and influenza B occurred in nursing homes or other long-term-care settings.
According to reports by sentinel physicians, the mean percentage of total weekly patient visits associated with influenza-like illness was 4.8% (Figure 1). Sentinel physicians also reported each week whether an outbreak of influenza is occurring among their patients. Outbreaks were reported primarily during January and February by physicians in the western and central regions of the country and during February and March by physicians from the eastern regions.
Morbidity reports from state epidemiologists indicated that peak influenza activity occurred during February and early March (Figure 1). Widespread or regional outbreaks were reported in 44 states and the District of Columbia (Figure 2). Outbreaks in the western and central regions of the country were reported earlier than those in the eastern regions.
WHO collaborating laboratories tested 26,732 specimens for influenza viruses. Isolates were recovered from 2,532 (9.5%) of these specimens. Nineteen hundred (75%) of the isolates were influenza A(H3N2), 430 (17.0%) were influenza B, and 202 (8.0%) were influenza A(H1N1) (Figure 3). Isolation of influenza A(H3N2) peaked during February, while influenza B and influenza A(H1N1) peaked during late March (Figure 1). Sentinel physicians submitted an additional 420 specimens for testing; 119 (28.3%) of these were positive for influenza viruses. Of the positive specimens, 110 (92.4%) were type A, and nine (7.6%) were type B influenza.
Combining all laboratory reports, influenza A(H3N2) viruses were reported from 49 states and the District of Columbia; influenza B, from 26 states in all regions of the country and the District of Columbia; and influenza A(H1N1), from 19 states primarily in the eastern, central, and southern regions of the country.
The proportion of deaths associated with pneumonia and influenza (P&I) reported from 121 cities exceeded the epidemic threshold for 9 weeks, from the week ending February 20 through the week ending April 16 (Figure 4). Eighty-six percent of the P&I deaths reported occurred in persons greater than or equal to 65 years of age. The 1987-88 season was the fifth year in the last decade that influenza A(H3N2) predominated. In each of the 5 years, excess mortality associated with P&I has occurred.
Preliminary analysis of the data received through ESP indicates the relative proportions of influenza virus types reported through this system were similar to those reported on postcards by the other WHO collaborating laboratories. Of the 661 isolates reported through ESP, 508 (76.9%) were type A(H3N2), 94 (14.2%) were type B, 20 (3.0%) were type A(H1N1), and 39 (5.9%) were type A viruses, not subtyped. Of the ESP isolate reports, 354 (53.6%) were reported from Harris County, Texas, where special influenza studies are conducted by the Influenza Research Center at the Baylor College of Medicine. The mean age of patients from whom isolates were recovered was 27 years for influenza A(H3N2), 20 years for influenza A(H1N1), and 19 years for influenza B. The median number of days between specimen collection and the date the results of virus testing were reported to the state epidemiologist was 27 days. Most reports were then transmitted to CDC within 1 week. Reported by: Participating state and territorial epidemiologists and state laboratory directors. WHO Collaborating Laboratories. Sentinel Physicians of the American Academy of Family Physicians. Participating Veterans Administration Hospitals. Letterman Army Medical Center, San Francisco, California. Hackensack Hospital, Hackensack, New Jersey. Strong Memorial Hospital, Rochester, New York. Vanderbilt Univ, Nashville, Tennessee. Influenza Research Center, Baylor College of Medicine, Houston; 5th Army Medical Laboratory, Fort Sam Houston; USAF School of Aerospace Medicine, Epidemiology Div. Brooks AFB, Texas. Div of Surveillance and Epidemiologic Studies, Epidemiology Program Office; WHO Collaborating Center for Influenza, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC.
Editorial Note: During the 1986-87 season, influenza A(H1N1) was the most frequently isolated influenza virus. Since its reappearance in 1977, A(H1N1) has primarily been associated with morbidity in younger persons. In contrast, influenza A(H3N2)--the predominant strain during the 1987-88 season (3)-- causes morbidity in all age groups and mortality in the elderly. In 1986-87, only 2.3% of all influenza isolates were from persons greater than or equal to 65 years of age, while in 1987-88, 20.7% of the influenza A(H3N2) isolates reported by WHO collaborating laboratories reporting through the postcard system were from persons in this age group (Table 1) (3). The excess mortality associated with P&I is consistent with an increased occurrence of influenza in the elderly (Figure 4).
The 1987-88 influenza epidemic was associated with strains that exhibited antigenic drift from the strain that had been included in the vaccine. However, because these variations were not recognized until the fall of 1987, the trivalent influenza vaccine could not be modified to include the new variant. As a result, the efficacy of the vaccine, at least in certain high-risk persons, may have been reduced.
Efforts to improve influenza control are emphasizing rapid detection and reporting of influenza viruses--including those circulating in the Far East--in time to consider incorporating these viruses into the influenza vaccine. In addition, surveillance in the United States augmented by laboratory support enhances the monitoring of influenza, often before outbreaks occur, and can contribute to influenza control by enabling the use of antiviral agents in locations where influenza A is circulating.
The ESP and Sentinel Physician Surveillance have expanded options for epidemiologic surveillance of influenza. The ESP for influenza surveillance was first operated during the 1987-88 influenza season and provided data not reported by the postcard system. Specimen collection dates and additional case-specific information permit more detailed epidemiologic analysis than the postcard reporting system, thereby enhancing surveillance of both morbidity and viral isolation. The results of the Sentinel Physician Surveillance Network, a pilot study in progress for several years, have demonstrated the feasibility of a relatively inexpensive method for rapid confirmation of influenza in specimens collected by family physicians and have provided prompt feedback to these physicians (9).
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