Influenza Epidemic in Pennsylvania
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Epidemiologicfeatures, 1968-69 Influenza Epidemic in Pennsylvania VICTOR HREHOROVICH, MD, WILLIAM W. DYAL, MS, and WILLIAM D. SCHRACK, Jr., MD ANEW VARIANT of influenza A2 virus was In late November 1968, when isolated cases of first reported from Hong Kong in July 1968. influenza became more frequent in Pennsylvania, Laboratory studies demonstrated that the virus the division of communicable diseases, Pennsyl- had undergone a major antigenic shift within the vania Department of Health, organized a prospec- A2 classification. In the 2 successive months, wide- tive study to determine the impact of the disease spread outbreaks of illness in the Far East were on that State's population. A statewide influenza ascribed to the A2/Hong Kong/68 influenza virus. surveillance system based on institutional ab- In a special session in September 1968, the Public senteeism was organized to (a) delineate the Health Service Advisory Committee on Immuniza- extent of the outbreak in the State, (b) facilitate tion Practices issued a statement that the change early laboratory confirmation of suspected cases, in the virus increased the probability of a sig- (c) elucidate the clinical and epidemiologic char- nificant outbreak of influenza in the United States acteristics of influenza infections, and (d) provide during the winter of 1968-69 (1). current information on the course of the epidemic The introduction of the Hong Kong strain into to concerned medical groups and the general pop- the United States was documented first in early ulation. After the outbreak, the resultant mor- September and was followed by occasional small tality was analyzed by reviewing death certificates outbreaks at military establishments which re- issued throughout the State. ceived personnel from the Orient (2). In October Observations on the epidemiology of the and November increasing numbers of outbreaks 1968-69 influenza epidemic in Pennsylvania are were reported in civilian populations in the reported in this paper. Organization and operation western States (3). of the influenza surveillance system and the laboratory studies are discussed in detail else- Dr. Hrehorovich is chief resident, North Shore where (4, 5). Hospital, Manhasset, N.Y., Mr. Dyal is a statis- tician, Center for Disease Control, Health Services Methods of Study and Mental Health Administration, Atlanta, Ga., and Dr. Schrack is director of the division of Absenteeism. Monitoring of absenteeism was communicable diseases, Pennsylvania Department initiated with the selection of 13 sites for sur- of Health. Tearsheet requests to Victor R. veillance throughout the six geographic areas of Hrehorovich, MD, North Shore Hospital, Man- the State (fig. 1). Several local institutions near hasset, N.Y. 11030. each site agreed to furnish daily attendance fig- November 1972, Vol. 87, No. 9 835 ures. We attempted to enlist at least one industrial for each of the participating schools, the State plant, one secondary school (grades seven average of about 5 percent per day recorded dur- through 12), and one elementary school (kinder- ing the 1967-68 school year for elementary and garten through sixth grade) at each site (4). The secondary schools, exclusive of Pittsburgh and total population under surveillance, excluding Philadelphia, was used. Pittsburgh and Philadelphia (which had separate The epidemic threshold was arbitrarily set at surveillance systems), was 8,079,500. A sample 9 percent, approximately double the baseline of 101,613 people or about 1.2 percent was in- absenteeism for all participating schools. Based cluded in the daily monitoring of absenteeism. on very rough estimates of the usual absenteeism Attendance figures were collected for 42 days in industrial plants, the epidemic threshold for the from December 2, 1968, through January 31, plants was set twice as high as the expected level. 1969. The 19 days for which there were no re- This rationale was expected to lessen the possibil- ports were Saturdays, Sundays, or holidays. In ity of sporadic variations being interpreted as addition, because of the Christmas recess, no unusual events. data on absences were collected from the schools Mortality. Death certificates issued through- from December 23 through January 1. Moreover, out the State were reviewed and analyzed. With not all of the participating institutions were suc- the aid of machine-tabulated punchcards pre- cessful in compiling and reporting daily attend- pared from death certificates, death rates were ance. On the average, for any given day, about computed for age, sex, race, date of death, and 71 percent of the participating plants and 91 cause of death. Cause of death was coded in percent of the participating schools furnished accordance with recommendations of the National attendance reports. Center for Health Statistics (6), and adjusted for To facilitate interpretation of absences, we the changes in the World Health Organization's selected epidemic threshold levels sufficiently high International Classification of Diseases adapted to preclude their being exceeded by random daily for use in the United States (7). or weekly fluctuations in absenteeism. Since it was Mortality figures for January 1961 through May impossible to obtain baseline absenteeism figures 1969 were reviewed. Age, sex, race, infant, Figure 1. Influenza surveillance sites, Pennsylvania, December 1968-January 1969 836 Health Services Reports Figure 2. Industrial absenteeism in 22 plants, Pennsylvania, December 1968-January 1969 22r * Reported absenteeism at least 201- JulJ 1 ftwo times greater than normal 181- 161- A 0 14[- 0 SLLF Kb CL ._ 12F -o aC 101- a- 8 6 L 4 2 0 I 51 26L I 26 8 1 15 22 29 ± 5 2 19 December 1968 I January 1969 maternal, and disease-specific mortality rates for of December and the first week of January, when 40 causes of death were calculated. Expected about 55 percent of the reporting units registered rates were calculated for each group by fitting a more than 9 percent absenteeism. The curve sug- curve to the observed death rates during nonepi- gests that had there been no Christmas recess, demic periods according to the methods of peak absenteeism would have occurred during the Serfling (8). The epidemic threshold, a curve 1.65 last week of December or the first week of January. standard deviations above the expected mortality, Absenteeism recorded the third week of was also calculated on the basis of previously December probably was influenced by the opening described techniques (9). of the hunting season on December 16, as evi- denced by the progressive decrease in absenteeism Results December 17-19, following the weekly peak on December 16. The greatest difference between School absenteeism. Figure 2 shows the com- secondary and primary school absenteeism was bined elementary and secondary school absen- recorded on that day: secondary school absences teeism. To equalize the weight of data from ranged from 8 to 24.8 percent, for an average different surveillance sites, schools of the same of 13.8 percent; primary school absences were category, in close geographic proximity, and in generally about 3 percent lower, ranging from the same school district are presented as a single 4 to 14.3 percent, for an average of 10.7 percent. reporting unit (4). Thus, the 14 primary schools Otherwise, absenteeism from secondary schools in the Harrisburg area are given the same sta- paralleled that in elementary schools by a slight, tistical weight as the single elementary school in but a very consistent, margin. For example, on Erie. January 3, during a period of significant influenza Only the reporting units that had at least 9 activity, secondary schools reported absenteeism percent absenteeism are plotted. The highest from 7.1 to 26.2 percent, for an average of 11.3 absenteeism was recorded during the third week percent, while primary schools reported absen- November 1972, Vol. 87, No. 9 837 teeism ranging from 5.5 to 27.6 percent, for an reported absenteeism ranging from 0.4 to 11.5 average of 11.1 percent. On January 28, when percent, for an average of 4.7 percent. A slightly most of the influenza activity had subsided, the smaller peak of 56.3 percent was reported Decem- absenteeism reported by secondary schools ranged ber 30. The first 3 days of the hunting season, from 2.2 to 8.9 percent, for an average of 6.4 December 16-18, produced a separate peak (47.4 percent; while primary school absenteeism ranged percent on December 17) distinct from the from 4 to 9 percent, for an average of 6.1 percent. absenteeism curve as a whole. Industrial absenteeism. The expected daily absenteeism at the 22 industrial plants monitored Mortality ranged from 0.2 to 6.6 percent, for an average of 2.9 percent. Excluding days when bad weather The observed number of pneumonia-influenza or the opening of the hunting season precipitated deaths exceeded the epidemic threshold during brief increases, the highest daily absenteeism from the winters of 1962-63, 1964-65, 1967-68, and individual plants during the period of surveillance 1968-69, reflecting periods of known influenza ranged from 0.5 to 15.7 percent, for an average activity. The 1968-69 epidemic was the largest of 6.4 percent. since 1962-63. Weekly pneumonia and influenza Figure 3 shows the proportion of daily reports mortality and the total mortality for the 1968-69 that indicated at least twice the baseline for a epidemic peaked during the first week of January given industrial plant. It appears that more than 1969. The excess deaths, determined by subtract- 10 percent of the plants had excessive absenteeism ing the expected from the observed number, dur- from December 9 to January 17. The peak oc- ing the last five influenza outbreaks in Pennsyl- curred January 6, when 57.9 percent of the plants vania is shown in the following table. Figure 3.