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Table 3 Risk of healthy vaccinee bias and confounding by indication in the included studies, as judged from the baseline characteristics of vaccinated and unvaccinated participants

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

Study Healthy vaccinee biasa Confounding by indicationb Indicated by
Bond et al. (2012) [17] Vaccinated participants have more comorbidities; unvaccinated have worse laboratory values
Campitelli et al. (2011) [7] Vaccinated participants have more comorbidities; unvaccinated patients have worse functional status
Foster et al. (1992) [18] More comorbidities in vaccinated participants
France et al. (2006) [34] More comorbidities in vaccinated participants
Groenwold et al. (2009) [19] More comorbidities, medications and medical visits in vaccinated participants
Hottes et al. (2011) [20] More medical visits in vaccinated participants
Jackson et al. (2002) [21] More comorbidities in vaccinated participants
Jackson et al. (2006) [3] More comorbidities in vaccinated participants
Jackson et al. (2008) [5] No major differences in baseline characteristics between groups
Johnstone et al. (2012) [33] Vaccinated participants have more CAD; unvaccinated have more diabetes and hypertension
Liu et al. (2012) [22] More comorbidities in vaccinated participants
Mangtani et al. (2004) [16] More comorbidities and medications in vaccinated participants
McGrath et al. (2012) [23] Better adherence to dialysis and fewer years with end-stage renal disease in vaccinated participants
Nichol et al. (2008) [24] More comorbidities in vaccinated participants
Nichol et al. (2009) [25] No major differences in baseline characteristics between groups
Ohmit et al. (1995) [26] Unclear data and description
Omer et al. (2011) [27] Some comorbidities different (diabetes) between groups, other not (hypertension)
Örtqvist et al. (2007) [15] More comorbidities in vaccinated participants
Schembri et al. (2009) [28] More comorbidities in vaccinated participants
Sung et al. (2014) [29] More comorbidities in unvaccinated participants
Tessmer et al. (2011) [30] More comorbidities in vaccinated participants
Villa-Corcoles et al. (2007) [31] More comorbidities in vaccinated participants
Wong et al. (2012) [32] More comorbidities and medications in vaccinated participants
  1. green circle/+: low risk of bias; red circle/-: high risk of bias; yellow circle/?: unclear risk of bias; CAD, coronary artery disease
  2. aindicated by: vaccinated participants were healthier (fewer comorbidities) than unvaccinated participants at study entry (cohort studies) or vaccinated controls were healthier (fewer comorbidities) than unvaccinated controls (case–control studies)
  3. bindicated by: vaccinated participants were sicker (more comorbidities) than unvaccinated participants at study entry (cohort studies) or vaccinated controls were sicker (more comorbidities) than unvaccinated controls (case–control studies)