Table 4. Relative vaccine effectiveness of fifth dose BA.1/ancestral mRNA bivalent vaccines against hospitalisation, compared with fourth dose monovalent mRNA vaccines, 21 September 2022–23 January 2023 (n = 884).

Characteristic	rVE (95% CI)	rOR (95% CI)	P-value
Univariable logistic regression model
Fifth dose of bivalent mRNA vaccine	46.2 (21.1–63.0)	0.538 (0.370–0.789)	0.001
Multivariable logistic regression model
Fifth dose of monovalent mRNA vaccine	46.7 (18.0–65.1)	0.533 (0.349–0.820)	0.004
Time between 4th dose and admission	NA	1.000 (0.997–1.003)	> 0.9
Vaccine branda		0.882 (0.611–1.283)	0.5
Age		1.006 (0.974–1.038)	0.7
Sex (male)		1.329 (0.925–1.910)	0.12
CCI		1.005 (0.895–1.123)	> 0.9
IMD		1.024 (0.961–1.092)	0.5
LTCF resident		0.953 (0.525–1.646)	0.9
Respiratory disease		0.807 (0.551–1.173)	0.3
Prevalenceb		1.006 (0.998–1.013)	0.13
Matched conditional logistic regression modelc
5th dose of monovalent mRNA vaccine	48.8 (19.8–67.3)	0.512 (0.327–0.802)	0.003
Time between fourth dose and admission	NA	1.000 (0.997–1.003)	0.8
Vaccine branda		0.909 (0.620–1.335)	0.6
Prevalenceb		1.006 (0.999–1.014)	0.10

CI: confidence interval; CCI: Charlson comorbidity index; IMD: index of multiple deprivation; LTCF: long-term care facility; NA: not applicable; rOR: relative odds ratio; rVE: relative vaccine effectiveness.

^a Vaccine brand of fifth dose of BA.1/ancestral mRNA bivalent mRNA vaccine or fourth dose of monovalent mRNA vaccine (before last dose, any vaccine combination is considered), where 1 = SpikeVax and 0 = Comirnaty.

^b Prevalence was calculated on a daily basis.

^c 1:4 nearest neighbour propensity score matching with replacement (propensity score estimated using logistic regression on age, sex, CCI score, IMD, LTCF residency and respiratory disease), 150 test-positive cases were matched to 600 test-negative controls with no match found for 123 controls.