Table 6.

Subgroup analysis of the association between influenza vaccination and COVID-19 clinical outcomes.

Subgroup Variables	No. of Studies	Adjusted Estimate (95% CI)	p-Value	I2 Value (%)
Hospitalization
Effect Size
OR	10	0.92 (0.78–1.02)	0.40	86.6
RR/HR	5	0.87 (0.71–0.99)	0.03	71.0
Adjusted Factors
age/gender/age and gender	1	1.44 (1.00–2.07)	0.05	89.8
age, gender and comorbidities	7	0.85 (0.71–1.03)	0.49	66.3
COVID-19 test
rt-PCR	10	0.89 (0.78–1.02)	0.09	74.6
Laboratory confirmed/ (not specified)	5	0.88 (0.67–1.12)	0.30	90.0
Continent
Europe	10	0.93 (0.83–1.04)	0.21	75.7
N. America	5	0.80 (0.63–1.02)	0.07	73.1
Intensive Care Unit
Effect Size
OR	9	0.91 (0.76–1.08)	0.26	55.9
HR	2	1.01 (0.99–1.04)	0.43	0.0
COVID-19 test
rt-PCR	9	0.93 (0.84–1.03)	0.14	58.5
Laboratory confirmed/ (not specified)	2	1.12 (0.80–1.57)	0.52	14.1
Adjusted Factors
age, gender and comorbidities	4	0.87 (0.65–1.17)	0.35	17.6
Continent
Europe	5	1.05 (0.95–1.16)	0.32	20.6
N. America	4	0.69 (0.47–1.02)	0.06	10.3
S. America	1	0.93 (0.87–0.99)	0.02	-
Asia	1	0.76 (0.59–0.98)	0.03	-
Mortality
Effect Size
OR	12	0.87 (0.76–0.99)	0.03	60.5
HR	6	0.99 (0.86–1.12)	0.82	46.3
COVID-19 test
rt-PCR	14	0.93 (0.82–1.06)	0.27	80.3
Laboratory confirmed/ (not specified)	4	0.81 (0.65–1.00)	0.05	37.1
Adjusted Factors
age/gender/age and gender	3	1.07 (0.75–1.53)	0.70	59.2
age, gender and comorbidities	8	0.82 (0.61–1.10)	0.19	75.3
Continent
Europe	12	0.94 (0.80–1.09)	0.40	75.3
N. America	3	0.89 (0.76–1.03)	0.12	0.0
S. America	2	0.84 (0.78–0.91)	<0.01	0.0
Asia	1	0.78 (0.63–0.95)	0.02	-