Table 2.

Potential factors impacting the vaccines’ effectiveness identified by the meta-regression.

Potential Impact Factor	OR (%, 95% CI)	p Value
Death as the endpoint (9 studies, 16 estimates)
Interval since the last dose	1.34 (0.08, 22.89)	0.8402
Interval between the two primary injections	9.70 (0.44, 212.51)	0.1491
<3 weeks (Ref.)
≥3 weeks	0.66 (0.11, 4.09)	0.6586
Prior infection	1.05 (0.03, 35.98)	0.9771
Vaccination regimens
Primary vaccination (Ref.)	-	-
Homologous booster vaccination	0.08 (0.01, 0.48)	0.0054
Proportion of elderly	2.32 (0.27, 19.89)	0.4412
Variant of concern
Before delta (Ref.)	-	-
Delta contained	1.58 (0.01, 191.25)	0.8522
Omicron	1.35 (0.04, 40.85)	0.8633
Severe disease as the endpoint (12 studies, 21 estimates)
Interval since the last dose	1.44 (0.71, 2.91)	0.3123
Interval between the two primary injections
<3 weeks (Ref.)	NA
≥3 weeks	NA
Prior infection	NA	NA
Vaccination regimens
Primary vaccination (Ref.)	-	-
Homologous booster vaccination	0.26 (0.07, 0.95)	0.0417
Proportion of elderly	1.62 (0.34, 7.73)	0.5437
Variant of concern
Before delta (Ref.)	-	-
Delta contained	0.65 (0.11, 3.62)	0.6186
Omicron	0.53 (0.1, 2.96)	0.4706