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

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

green circle/+: low risk of bias; red circle/-: high risk of bias; yellow circle/?: unclear risk of bias; CAD, coronary artery disease

^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)

^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)