Clinical Infectious Diseases MAJOR ARTICLE
Illness Severity and Work Productivity Loss Among Working Adults With Medically Attended Acute Respiratory Illnesses: US Influenza Vaccine Effectiveness Network 2012–2013 Joshua G. Petrie,1 Caroline Cheng,1 Ryan E. Malosh,1 Jeffrey J. VanWormer,2 Brendan Flannery,3 Richard K. Zimmerman,4 Manjusha Gaglani,5 Michael L. Jackson,6 Jennifer P. King,2 Mary Patricia Nowalk,4 Joyce Benoit,6 Anne Robertson,5 Swathi N. Thaker,3 Arnold S. Monto,1 and Suzanne E. Ohmit1 1 Department of Epidemiology, University of Michigan School of Public Health, Ann Arbor; 2Marshfield Clinic Research Foundation, Marshfield, Wisconsin; 3Influenza Division, Centers for Disease Control and Prevention, Atlanta, Georgia; 4University of Pittsburgh Schools of the Health Sciences and University of Pittsburgh Medical Center, Pennsylvania; 5Scott and White Health, Texas A&M University Health Science Center College of Medicine, Temple; 6Group Health Research Institute, Seattle, Washington
Seasonal influenza epidemics cause significant excess morbidity and mortality in the United States each year [1, 2]. The total annual economic burden of influenza in the United States has been estimated to exceed $87 billion; more than $10 billion of which is attributed to direct costs of medical treatment and more than $6 billion to lost productivity from illness [3]. Past estimates of burden, particularly in terms of lost productivity, have typically used nonspecific outcomes such as acute respiratory illness (ARI) or influenza-like illness (ILI) based on symptomatic
Received 10 September 2015; accepted 29 October 2015; published online 12 November 2015. Correspondence: S. E. Ohmit, University of Michigan School of Public Health, 1415 Washington Heights, Ann Arbor, MI 48109 ([email protected]). Clinical Infectious Diseases® 2016;62(4):448–55 © The Author 2015. Published by Oxford University Press for the Infectious Diseases Society of America. All rights reserved. For permissions, e-mail [email protected]. DOI: 10.1093/cid/civ952
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case definitions [4–6]. With the development of real-time reversetranscriptase polymerase chain reaction (RT-PCR) assays, we are now able to identify precisely those illnesses that are potentially vaccine preventable. We can also compare the severity of illnesses that are laboratory-confirmed as influenza to those that are not. To reduce the burden of influenza illness, annual influenza vaccination is currently recommended for all persons aged ≥6 months [7]. Vaccine effectiveness (VE) may vary from year to year due to the age and health status of vaccine recipients, the predominant circulating virus strain, and the antigenic match between vaccine and circulating strains [8–12]. To monitor variation in VE, multiple centers in the United States have collaborated each year since the 2008–2009 season through the US Flu VE Network [8–11]. This network examines the effectiveness of influenza vaccines in preventing medically attended acute respiratory illnesses (MAARI) caused by influenza. Using a test-negative analytic approach, VE is estimated by comparing the vaccination
Downloaded from http://cid.oxfordjournals.org/ at RMIT University Library on January 29, 2016
Background. Influenza causes significant morbidity and mortality, with considerable economic costs, including lost work productivity. Influenza vaccines may reduce the economic burden through primary prevention of influenza and reduction in illness severity. Methods. We examined illness severity and work productivity loss among working adults with medically attended acute respiratory illnesses and compared outcomes for subjects with and without laboratory-confirmed influenza and by influenza vaccination status among subjects with influenza during the 2012–2013 influenza season. Results. Illnesses laboratory-confirmed as influenza (ie, cases) were subjectively assessed as more severe than illnesses not caused by influenza (ie, noncases) based on multiple measures, including current health status at study enrollment (≤7 days from illness onset) and current activity and sleep quality status relative to usual. Influenza cases reported missing 45% more work hours (20.5 vs 15.0; P < .001) than noncases and subjectively assessed their work productivity as impeded to a greater degree (6.0 vs 5.4; P < .001). Current health status and current activity relative to usual were subjectively assessed as modestly but significantly better for vaccinated cases compared with unvaccinated cases; however, no significant modifications of sleep quality, missed work hours, or work productivity loss were noted for vaccinated subjects. Conclusions. Influenza illnesses were more severe and resulted in more missed work hours and productivity loss than illnesses not confirmed as influenza. Modest reductions in illness severity for vaccinated cases were observed. These findings highlight the burden of influenza illnesses and illustrate the importance of laboratory confirmation of influenza outcomes in evaluations of vaccine effectiveness. Keywords. medically attended acute respiratory illness; medically attended influenza; illness severity; work productivity; vaccine effectiveness.
MATERIALS AND METHODS Study Enrollment and Follow-up
The original study enrolled 6766 adults and children seeking care for ARI at ambulatory care facilities, including urgent care clinics affiliated with 5 network centers [9]. The centers included the Group Health Cooperative, Seattle, Washington; Marshfield Clinic Research Foundation, Marshfield, Wisconsin; University of Michigan School of Public Health partnered with the University of Michigan, Ann Arbor, Michigan and the Henry Ford Health Systems, Detroit, Michigan; University of Pittsburgh Schools of Health Sciences partnered with the University of Pittsburgh Medical Center Health System, Pittsburgh, Pennsylvania; and Scott & White Healthcare, Texas A&M Health Science Center College of Medicine, Temple, Texas. The institutional review boards at participating network centers approved the study. Briefly, patients seeking care for ARI, characterized by cough of ≤7 days duration, were interviewed regarding their illness, had throat and nasal swab specimens collected for virus identification, and were asked to complete a follow-up survey 7 days after enrollment. Enrollment data included subject demographic characteristics (age, sex, race/ethnicity), plus subjective assessments of health status, both general
prior to ARI (1 = excellent . . . 5 = poor) and current (0 = worst . . . 100 = best), current activity relative to usual (0 = unable to perform usual activities . . . 9 = able to perform usual activities), current sleep quality relative to normal (0 = worst quality of sleep . . . 9 = normal pre-illness quality of sleep), social position relative to other US households (1 = worst off . . . 9 = best off ) [21], and current smoking status. Respiratory specimens collected at enrollment were combined and tested for influenza virus identification at network laboratories by means of RT-PCR, the most sensitive and specific method of influenza identification currently available [17]. The RT-PCR primers, probes, and testing protocol were developed and provided by the Influenza Division at the Centers for Disease Control and Prevention. Follow-up surveys requested information on timing of return to normal activities, employment status and missed work hours due to illness, and a subjective assessment of the degree to which illness impeded work productivity (0 = no effect on work . . . 10 = completely prevented from working). Measures of illness severity, social position, and work productivity loss ascertained at enrollment or at follow-up were adapted from previously used instruments (Supplementary Table 1) [21–25]. Influenza vaccination for the 2012–2013 season was documented from medical records or immunization registries; subjects without evidence were considered unvaccinated. Subjects were defined as high risk if they had medical record documentation during the year before enrollment of health conditions that increased their risk of influenza complications [7]. Body mass index (BMI; kg/m2) was calculated from self-reported information or medical record–determined weight and height and categorized as under or normal weight, overweight, and obese. Treatment with influenza antiviral medications was defined from the medical record; illnesses in subjects with prescription of antiviral medications within 2 days of study enrollment were considered treated. Analysis Subset: Inclusion Criteria and Variables
There were 4033 adults in the overall study population, of which 1548 (38%) met the criteria for inclusion in this analysis (see Figure 1). Inclusion criteria included completing the followup survey between 7 and 21 days after illness onset, being employed and working at least 20 hours per week, plus having complete data on defined exposures (laboratory-confirmed influenza status and documented influenza vaccination status) and defined outcomes (at enrollment: provided subjective assessments of current health status, current activity status, and current sleep quality; at follow-up: provided information on timing of return to normal activities, the number of work hours missed due to illness, and a subjective assessment of work productivity during illness). Illness severity was represented by subjective assessments of current health status and reported current activity and sleep quality scores, reported at the enrollment visit, plus the timing Illness Severity and Work Productivity
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Downloaded from http://cid.oxfordjournals.org/ at RMIT University Library on January 29, 2016
status of those who test positive for influenza with those who test negative [13, 14]. Generally, these studies have indicated moderate VE (47%–61%), with lower effectiveness against influenza A (H3N2) viruses compared with A (H1N1) and type B strains, and variation in effectiveness by age group [8–11]. Most evaluations of VE have focused on the benefit of primary prevention of influenza illness. Less clear is whether influenza vaccines are effective in reducing the severity of influenza illnesses and the burden of lost productivity among vaccine failures— those infected with influenza despite vaccination. Studies that have examined influenza-related illness severity and the potential benefit of vaccination in modifying illness have generally reported equivocal results or modest reductions in severity among vaccine failures [15–20]. Evaluations of the negative effect of acute respiratory illnesses on work productivity have not directly examined the potential value of vaccination in reducing productivity loss [4] or laboratory-confirmed illnesses as influenza [4–6]. We examined illness severity, time to recovery, and work productivity loss among working adults with MAARI who participated in the US Flu VE Network study during the 2012–2013 influenza season. We compared these outcomes for MAARI subjects with and without laboratory-confirmed influenza and by influenza vaccination status among influenza-positive subjects. The 2012–2013 season was characterized as moderately severe, with simultaneous cocirculation of influenza A (H3N2) and type B strains; overall adjusted VE estimates from the network indicated vaccine was 49% (95% confidence interval [CI], 43, 55) effective in reducing the risk of MAARI caused by influenza [9].
Total enrollment and number of subjects included in analyses of illness severity and work productivity loss: US Influenza Vaccine Effectiveness Network (2012–
of return to normal activities. Work productivity loss was estimated by the total number of missed work hours due to illness and the subjective assessment of work productivity during illness; both were reported as part of the follow-up survey. Statistical Methods
We examined differences in characteristics and outcomes by laboratory-confirmed influenza status among the entire analysis set and separately by influenza vaccination status among influenza-positive subjects (ie, cases). For each influenza and vaccination status group, means and standard deviations or medians and interquartile ranges were estimated for continuous symmetrically distributed or continuous asymmetrically distributed variables, respectively. Proportions were calculated across categorical variables. Statistical significance of differences was assessed by χ2 test for categorical variables or Kruskal–Wallis test for continuous variables. Adjusted associations of influenza status with outcomes and vaccination status among influenza cases with outcomes were modeled in 2 ways. Reported current health status, current activity, current sleep quality, and work productivity scores were modeled using linear regression with estimation of mean differences. The total number of missed work hours due to illness was modeled using negative binomial regression with calculation of percent differences. All outcomes were modeled with the generalized estimating equation method; models were adjusted for the number of days between illness onset and the outcome (measured at either enrollment or follow-up) and for withinsite dependencies by treating site as a correlated cluster. Models 450
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were also adjusted for the following covariates: general health prior to illness, age, sex, race/ethnicity, subjective social status, BMI category, current smoking status, high-risk health status, antiviral treatment, and biweekly calendar time of illness onset. The model of missed work hours was adjusted for expected weekly work hours. Differences in the rate of return to normal activities were examined using Kaplan–Meier plots and tested using the log-rank test; subjects who had not yet returned to normal activities were censored at the time of follow-up survey completion. An alpha of 0.05 was used to determine statistical significance in all tests. Analyses were performed using SAS statistical software (release 9.3; SAS Institute); R statistical program and packages were used for the creation of plots [26]. RESULTS Illness Severity and Work Productivity by Influenza Test Status
The analysis subset included 1548 working adult subjects with MAARI. 588 (38%) subjects tested positive for influenza (ie, cases), including 405 with influenza type A ( primarily A/ H3N2) and 184 with type B; 960 (62%) tested negative (ie, noncases). Distributions and comparisons of characteristics by influenza status are presented in Table 1. The mean age of included subjects was 44 years, 65% were female, 85% were white non-Hispanic, 33% had high-risk health conditions, 46% were classified as obese, 14% were current smokers, and 68% reported their health status prior to illness as excellent/very good. On average, subjects rated their subjective social position as 6.1 on a scale of 1 (worst off ) to 9 (best off ) [21, 27]. As expected, influenza cases were significantly less likely than noncases to have
Downloaded from http://cid.oxfordjournals.org/ at RMIT University Library on January 29, 2016
Figure 1. 2013).
Table 1. Characteristics of 1548 Working Adults With Medically Attended Acute Respiratory Illness by Laboratory-Confirmed Influenza Status: US Influenza Vaccine Effectiveness Network (2012–2013)
Characteristic Age in years, mean (SD), range, 18–83
Total N = 1548
Influenza Test-Positive Cases n = 588 (38%)
Influenza Test-Negative Noncases n = 960 (62%)
P Value
44.2 (13.1)
45.1 (13.1)
43.7 (13.1)
.07a
Subjective social position, mean (SD), range, 1–9
6.1 (1.4)
6.0 (1.4)
6.1 (1.4)
.28a
Influenza vaccinated, N (%)b,c
750 (48.4)
233 (39.6)
517 (53.9)
Illness Severity and Work Productivity Loss Among Working Adults With Medically Attended Acute Respiratory Illnesses: US Influenza Vaccine Effectiveness Network 2012–2013 Joshua G. Petrie,1 Caroline Cheng,1 Ryan E. Malosh,1 Jeffrey J. VanWormer,2 Brendan Flannery,3 Richard K. Zimmerman,4 Manjusha Gaglani,5 Michael L. Jackson,6 Jennifer P. King,2 Mary Patricia Nowalk,4 Joyce Benoit,6 Anne Robertson,5 Swathi N. Thaker,3 Arnold S. Monto,1 and Suzanne E. Ohmit1 1 Department of Epidemiology, University of Michigan School of Public Health, Ann Arbor; 2Marshfield Clinic Research Foundation, Marshfield, Wisconsin; 3Influenza Division, Centers for Disease Control and Prevention, Atlanta, Georgia; 4University of Pittsburgh Schools of the Health Sciences and University of Pittsburgh Medical Center, Pennsylvania; 5Scott and White Health, Texas A&M University Health Science Center College of Medicine, Temple; 6Group Health Research Institute, Seattle, Washington
Seasonal influenza epidemics cause significant excess morbidity and mortality in the United States each year [1, 2]. The total annual economic burden of influenza in the United States has been estimated to exceed $87 billion; more than $10 billion of which is attributed to direct costs of medical treatment and more than $6 billion to lost productivity from illness [3]. Past estimates of burden, particularly in terms of lost productivity, have typically used nonspecific outcomes such as acute respiratory illness (ARI) or influenza-like illness (ILI) based on symptomatic
Received 10 September 2015; accepted 29 October 2015; published online 12 November 2015. Correspondence: S. E. Ohmit, University of Michigan School of Public Health, 1415 Washington Heights, Ann Arbor, MI 48109 ([email protected]). Clinical Infectious Diseases® 2016;62(4):448–55 © The Author 2015. Published by Oxford University Press for the Infectious Diseases Society of America. All rights reserved. For permissions, e-mail [email protected]. DOI: 10.1093/cid/civ952
448
•
CID 2016:62 (15 February)
•
Petrie et al
case definitions [4–6]. With the development of real-time reversetranscriptase polymerase chain reaction (RT-PCR) assays, we are now able to identify precisely those illnesses that are potentially vaccine preventable. We can also compare the severity of illnesses that are laboratory-confirmed as influenza to those that are not. To reduce the burden of influenza illness, annual influenza vaccination is currently recommended for all persons aged ≥6 months [7]. Vaccine effectiveness (VE) may vary from year to year due to the age and health status of vaccine recipients, the predominant circulating virus strain, and the antigenic match between vaccine and circulating strains [8–12]. To monitor variation in VE, multiple centers in the United States have collaborated each year since the 2008–2009 season through the US Flu VE Network [8–11]. This network examines the effectiveness of influenza vaccines in preventing medically attended acute respiratory illnesses (MAARI) caused by influenza. Using a test-negative analytic approach, VE is estimated by comparing the vaccination
Downloaded from http://cid.oxfordjournals.org/ at RMIT University Library on January 29, 2016
Background. Influenza causes significant morbidity and mortality, with considerable economic costs, including lost work productivity. Influenza vaccines may reduce the economic burden through primary prevention of influenza and reduction in illness severity. Methods. We examined illness severity and work productivity loss among working adults with medically attended acute respiratory illnesses and compared outcomes for subjects with and without laboratory-confirmed influenza and by influenza vaccination status among subjects with influenza during the 2012–2013 influenza season. Results. Illnesses laboratory-confirmed as influenza (ie, cases) were subjectively assessed as more severe than illnesses not caused by influenza (ie, noncases) based on multiple measures, including current health status at study enrollment (≤7 days from illness onset) and current activity and sleep quality status relative to usual. Influenza cases reported missing 45% more work hours (20.5 vs 15.0; P < .001) than noncases and subjectively assessed their work productivity as impeded to a greater degree (6.0 vs 5.4; P < .001). Current health status and current activity relative to usual were subjectively assessed as modestly but significantly better for vaccinated cases compared with unvaccinated cases; however, no significant modifications of sleep quality, missed work hours, or work productivity loss were noted for vaccinated subjects. Conclusions. Influenza illnesses were more severe and resulted in more missed work hours and productivity loss than illnesses not confirmed as influenza. Modest reductions in illness severity for vaccinated cases were observed. These findings highlight the burden of influenza illnesses and illustrate the importance of laboratory confirmation of influenza outcomes in evaluations of vaccine effectiveness. Keywords. medically attended acute respiratory illness; medically attended influenza; illness severity; work productivity; vaccine effectiveness.
MATERIALS AND METHODS Study Enrollment and Follow-up
The original study enrolled 6766 adults and children seeking care for ARI at ambulatory care facilities, including urgent care clinics affiliated with 5 network centers [9]. The centers included the Group Health Cooperative, Seattle, Washington; Marshfield Clinic Research Foundation, Marshfield, Wisconsin; University of Michigan School of Public Health partnered with the University of Michigan, Ann Arbor, Michigan and the Henry Ford Health Systems, Detroit, Michigan; University of Pittsburgh Schools of Health Sciences partnered with the University of Pittsburgh Medical Center Health System, Pittsburgh, Pennsylvania; and Scott & White Healthcare, Texas A&M Health Science Center College of Medicine, Temple, Texas. The institutional review boards at participating network centers approved the study. Briefly, patients seeking care for ARI, characterized by cough of ≤7 days duration, were interviewed regarding their illness, had throat and nasal swab specimens collected for virus identification, and were asked to complete a follow-up survey 7 days after enrollment. Enrollment data included subject demographic characteristics (age, sex, race/ethnicity), plus subjective assessments of health status, both general
prior to ARI (1 = excellent . . . 5 = poor) and current (0 = worst . . . 100 = best), current activity relative to usual (0 = unable to perform usual activities . . . 9 = able to perform usual activities), current sleep quality relative to normal (0 = worst quality of sleep . . . 9 = normal pre-illness quality of sleep), social position relative to other US households (1 = worst off . . . 9 = best off ) [21], and current smoking status. Respiratory specimens collected at enrollment were combined and tested for influenza virus identification at network laboratories by means of RT-PCR, the most sensitive and specific method of influenza identification currently available [17]. The RT-PCR primers, probes, and testing protocol were developed and provided by the Influenza Division at the Centers for Disease Control and Prevention. Follow-up surveys requested information on timing of return to normal activities, employment status and missed work hours due to illness, and a subjective assessment of the degree to which illness impeded work productivity (0 = no effect on work . . . 10 = completely prevented from working). Measures of illness severity, social position, and work productivity loss ascertained at enrollment or at follow-up were adapted from previously used instruments (Supplementary Table 1) [21–25]. Influenza vaccination for the 2012–2013 season was documented from medical records or immunization registries; subjects without evidence were considered unvaccinated. Subjects were defined as high risk if they had medical record documentation during the year before enrollment of health conditions that increased their risk of influenza complications [7]. Body mass index (BMI; kg/m2) was calculated from self-reported information or medical record–determined weight and height and categorized as under or normal weight, overweight, and obese. Treatment with influenza antiviral medications was defined from the medical record; illnesses in subjects with prescription of antiviral medications within 2 days of study enrollment were considered treated. Analysis Subset: Inclusion Criteria and Variables
There were 4033 adults in the overall study population, of which 1548 (38%) met the criteria for inclusion in this analysis (see Figure 1). Inclusion criteria included completing the followup survey between 7 and 21 days after illness onset, being employed and working at least 20 hours per week, plus having complete data on defined exposures (laboratory-confirmed influenza status and documented influenza vaccination status) and defined outcomes (at enrollment: provided subjective assessments of current health status, current activity status, and current sleep quality; at follow-up: provided information on timing of return to normal activities, the number of work hours missed due to illness, and a subjective assessment of work productivity during illness). Illness severity was represented by subjective assessments of current health status and reported current activity and sleep quality scores, reported at the enrollment visit, plus the timing Illness Severity and Work Productivity
•
CID 2016:62 (15 February)
•
449
Downloaded from http://cid.oxfordjournals.org/ at RMIT University Library on January 29, 2016
status of those who test positive for influenza with those who test negative [13, 14]. Generally, these studies have indicated moderate VE (47%–61%), with lower effectiveness against influenza A (H3N2) viruses compared with A (H1N1) and type B strains, and variation in effectiveness by age group [8–11]. Most evaluations of VE have focused on the benefit of primary prevention of influenza illness. Less clear is whether influenza vaccines are effective in reducing the severity of influenza illnesses and the burden of lost productivity among vaccine failures— those infected with influenza despite vaccination. Studies that have examined influenza-related illness severity and the potential benefit of vaccination in modifying illness have generally reported equivocal results or modest reductions in severity among vaccine failures [15–20]. Evaluations of the negative effect of acute respiratory illnesses on work productivity have not directly examined the potential value of vaccination in reducing productivity loss [4] or laboratory-confirmed illnesses as influenza [4–6]. We examined illness severity, time to recovery, and work productivity loss among working adults with MAARI who participated in the US Flu VE Network study during the 2012–2013 influenza season. We compared these outcomes for MAARI subjects with and without laboratory-confirmed influenza and by influenza vaccination status among influenza-positive subjects. The 2012–2013 season was characterized as moderately severe, with simultaneous cocirculation of influenza A (H3N2) and type B strains; overall adjusted VE estimates from the network indicated vaccine was 49% (95% confidence interval [CI], 43, 55) effective in reducing the risk of MAARI caused by influenza [9].
Total enrollment and number of subjects included in analyses of illness severity and work productivity loss: US Influenza Vaccine Effectiveness Network (2012–
of return to normal activities. Work productivity loss was estimated by the total number of missed work hours due to illness and the subjective assessment of work productivity during illness; both were reported as part of the follow-up survey. Statistical Methods
We examined differences in characteristics and outcomes by laboratory-confirmed influenza status among the entire analysis set and separately by influenza vaccination status among influenza-positive subjects (ie, cases). For each influenza and vaccination status group, means and standard deviations or medians and interquartile ranges were estimated for continuous symmetrically distributed or continuous asymmetrically distributed variables, respectively. Proportions were calculated across categorical variables. Statistical significance of differences was assessed by χ2 test for categorical variables or Kruskal–Wallis test for continuous variables. Adjusted associations of influenza status with outcomes and vaccination status among influenza cases with outcomes were modeled in 2 ways. Reported current health status, current activity, current sleep quality, and work productivity scores were modeled using linear regression with estimation of mean differences. The total number of missed work hours due to illness was modeled using negative binomial regression with calculation of percent differences. All outcomes were modeled with the generalized estimating equation method; models were adjusted for the number of days between illness onset and the outcome (measured at either enrollment or follow-up) and for withinsite dependencies by treating site as a correlated cluster. Models 450
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were also adjusted for the following covariates: general health prior to illness, age, sex, race/ethnicity, subjective social status, BMI category, current smoking status, high-risk health status, antiviral treatment, and biweekly calendar time of illness onset. The model of missed work hours was adjusted for expected weekly work hours. Differences in the rate of return to normal activities were examined using Kaplan–Meier plots and tested using the log-rank test; subjects who had not yet returned to normal activities were censored at the time of follow-up survey completion. An alpha of 0.05 was used to determine statistical significance in all tests. Analyses were performed using SAS statistical software (release 9.3; SAS Institute); R statistical program and packages were used for the creation of plots [26]. RESULTS Illness Severity and Work Productivity by Influenza Test Status
The analysis subset included 1548 working adult subjects with MAARI. 588 (38%) subjects tested positive for influenza (ie, cases), including 405 with influenza type A ( primarily A/ H3N2) and 184 with type B; 960 (62%) tested negative (ie, noncases). Distributions and comparisons of characteristics by influenza status are presented in Table 1. The mean age of included subjects was 44 years, 65% were female, 85% were white non-Hispanic, 33% had high-risk health conditions, 46% were classified as obese, 14% were current smokers, and 68% reported their health status prior to illness as excellent/very good. On average, subjects rated their subjective social position as 6.1 on a scale of 1 (worst off ) to 9 (best off ) [21, 27]. As expected, influenza cases were significantly less likely than noncases to have
Downloaded from http://cid.oxfordjournals.org/ at RMIT University Library on January 29, 2016
Figure 1. 2013).
Table 1. Characteristics of 1548 Working Adults With Medically Attended Acute Respiratory Illness by Laboratory-Confirmed Influenza Status: US Influenza Vaccine Effectiveness Network (2012–2013)
Characteristic Age in years, mean (SD), range, 18–83
Total N = 1548
Influenza Test-Positive Cases n = 588 (38%)
Influenza Test-Negative Noncases n = 960 (62%)
P Value
44.2 (13.1)
45.1 (13.1)
43.7 (13.1)
.07a
Subjective social position, mean (SD), range, 1–9
6.1 (1.4)
6.0 (1.4)
6.1 (1.4)
.28a
Influenza vaccinated, N (%)b,c
750 (48.4)
233 (39.6)
517 (53.9)
