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Table 2 Baseline characteristics of included studies

From: Frequency and impact of confounding by indication and healthy vaccinee bias in observational studies assessing influenza vaccine effectiveness: a systematic review

Author, year Country Study design Influenza season(s) Age-group (yrs) or risk group Age (yrs), range or mean (± SD) % male Data sources Identification of outcomes Study size (n)
Bond et al., 2012 [17] US Cohort 2005/06 Patients with ESRD V, 60.6 (15.2) V, 52.5 3 ESRD Networks, records of the US Renal Database (USRDS) All-cause death through ESRD death notification form 20,220 (without pneumococcal vaccine)
UV, 57.9 (15.9) UV, 50.8
Campitelli et al., 2010 [7] Canada Cohort 8 seasons between 1996 and 2007 Elderly ≥ 65 V, 75.3 (6.6) V, 40.8 National health surveys data linked to Ontario Health Insurance (OHIP) and Discharge Abstract (CIHI) databases Registered persons database and ICD-9/-10 admission codes V, 14,512
UV, 74.2 (6.7) UV, 40.7 UV, 11,410
Foster et al., 1992 [18] US Case–control (matched) 1989/90 Elderly ≥ 65 V, 65-94+ V, 50.8 Databases of participating hospitals Hospital discharge ICD-9 codes Cases, 721
UV, 65-94+ UV, 47.3 Controls, 1786
France et al., 2006 [34] US Cohort 1995/96-2000/01 Women and their newborns V, 30.8 (5.5) NA Health maintenance organization (Kaiser Permanante and Group Health Cooperative) ICD-9 codes for medically attended acute respiratory illnesses in infants V, 3160
UV, 29.7 (5.5) UV, 37,969
Groenwold et al., 2009 [19] Netherlands Cohort 1995/96-2002/2003 Elderly ≥ 65 V, 75 (median) V, 39.4 GPRD of University Medical Center Utrecht ICPC coding system V, 37,501
UV, 74 (median) UV, 35.2 UV, 13,405
Hottes et al., 2011 [20] Canada Cohort 2000/01-2005/06 Elderly ≥ 65 V, 75 (median) V, 43 Manitoba Immunization Monitoring System (MIMS) and Manitoba health policy database All-cause mortality or hospital admission ICD-9/-10 codes 139,185 (00/01) to 140,735 (05/06)
UV, 73 (median) UV, 44
Jackson et al., 2006 [35] US Cohort 1995/96-2002/3 Elderly ≥ 65 V, 51 % >74 V, 42.7 Health maintenance organization (Group Health Cooperative) All-cause mortality or hospital ICD-9 discharge codes 72,527
UV, 46 % >74 UV, 41.9
Jackson et al., 2002 [21] US Cohort 1992-1996 Patients with nonfatal MI All subjects, 64 (median) among ≥ 65: Health maintenance organization (Group Health Cooperative) Hospital discharge ICD-9 codes, confirmed by chart review V, 1016
UV, 362
V, 47
UV, 59
Jackson et al., 2008 [5] US Case–control (matched) 2000/1-2001/2 Elderly 65-94 Cases, 62 % >74 cases, 51 Health maintenance organization (Group Health Cooperative) ICD-9 code for CA pneumonia and validation using hospital records Cases, 1173 Controls, 2346 (1838 V, 508 UV)
Johnstone et al., 2012 [33] 40 countries Cohort 2003/04-2006/7 Elderly ≥ 65 with VD or diabetes Mean (4 seasons) Range (4 seasons) Clinical databases from two RCTs (ONTARGET- and TRANSCEND-trial) Outcomes adjudicated by independent committee using clinical data 31,546
V, 67-68 V, 72-73
UV, 65-66 UV, 67-70
Liu et al., 2012 [22] Taiwan Cohort 2002-2006 Elderly > 65 with heart disease V, 74.8 (6.3) V, 58.3 National Health Research Institute-released cohort dataset ICD-9 codes for heart diseases V, 2760
UV, 75.7 (7.0) UV, 51.8 UV, 2288
Mangtani et al., 2004 [16] UK Cohort 1989/90-1998/99 Elderly ≥ 65 Not reported Not reported General practice research database (GPRD) ICD-9 codes for acute respiratory illnesses; all respiratory-related deaths Person-years: influenza season, 419,748 summer, 692,415
McGrath et al., 2012 [23] US Cohort 1997-1999 and 2001 Patients with ESRD Mean (4 seasons) Range (4 seasons) Medicare claims from the US Renal Data System (USRDS) All-cause death through ICD-9 codes; Medicare claims from the USRDS 107,465 (1997) to 126,699 (2001)
V, 62.3-63.9 V, 52.2-53
UV, 60.3-61.7 UV, 50.4-51.6
Nicol et al., 2008 [24] US Cohort 2002/03-2005/06 Adults (students) V, 25.2 (7.9) V, 25.5 Internet-based survey Self-reported occurrences of ILI and health care use 12,795
UV, 23.3 (6.3) UV, 29.4
Nicol et al., 2009 [25] US Cohort 2006/07 Adults, 50-64 Not reported V, 24 Internet-based survey Self-reported occurrences of ILI and health care use 479
UV, 16
Ohmit et al., 1995 [26] US Case–control (matched) 1990/91-1991/92 Elderly ≥ 65 not reported not reported Admission and discharge data of 21 participating hospitals ICD-9 hospitalization code for pneumonia/influenza Cases, 1557 Controls, 3401
Omer et al., 2011 [27] US Cohort 2004/05-2005/06 Women and their newborns V, 18.3 % < 19 NA Georgia Pregnancy Risk Assessment Monitoring System (PRAMS) PRAMS database (questionnaire or interview) V, 578
UV, 3590
UV, 14.5 % < 19
Örtqvist et al., 2007 [15] Sweden Cohort 1998/99-2000/01 Elderly ≥ 65 V, 51 % >74 V, 41.3 Population register (via national identification number) ICD-9/-10 codes and cause of death register 260,155
UV, 51 % >74 UV, 39.0
Schembri et al., 2009 [28] UK Cohort 1998-2006 Adults > 40 with COPD V, 27 % > 69 V: 42.8 The Health Improvement Network database (THIN), covering data of general practices Diesease classification codes of THIN databases (“Read codes”) V, 9679
UV, 31,062
UV, 12 % > 69 UV: 42.8
Sung et al., 2014 [29] Taiwan Cohort 2000-2007 Elderly ≥ 55 with COPD V, 20 % >74 V: 58.7 Reimbursement claims from National Health Insurance Research Database (NHIRD) ICD-9 codes for acute MI or angina pectoris with invasive therapy V, 3027
UV, 17 % >74 UV: 60.8 UV, 4695
Tessmer et al., 2011 [30] Germany Cohort 2002-2006 Adults with pneumonia (CAP) V, 67.6 (14.5) *1 V: 57.2 National Community Acquired Pneumonia Competence Network (CAPNET) CAPNET database entries V, 1721
UV, 3279
UV, 55.7 (19.0) UV: 52.5
Vila-Córcoles et al., 2007 [31] Spain Cohort 2002-2005 Elderly ≥ 65 V, 51 % > 74 V, 44.3 Databases of Primary Health Care Centres (PHCC) ICD-9 codes of PHCC and Civil Registry Offices V, 6051
UV, 5189
UV, 38 % > 74 UV, 43.6
Wong et al., 2012 [32] Canada Cohort 2000/01-2008/09 Elderly ≥ 65 V, 75.5 (6.6) V, 43.8 Ontario Health administrative databases ICD-9/ -10 codes of databases and registered persons database 1,297,051 (00/01) to 1,527,364 (08/09)
UV, 74.5 (6.8) UV, 43.6
  1. SD standard deviation, V vaccinated, UV unvaccinated, ESRD end-stage renal disease, ICD international classification of disease, CHD chronic heart disease, GPRD general practice research database, ICPC international classification of primary care, MI myocardial infarction, CA(P) community acquired (pneumonia), VD vascular disease, ONTARGET-trial Ongoing Telmisartan Alone and in Combination With Ramipril Global EndPoint Trial, TRANSCEND-trial Telmisartan Randomized Assessment Study in ACE Intolerant Subjects with Cardiovascular Disease, ILI influenza-like illness, COPD chronic obstructive pulmonary disease