Table 4. Univariable analysis of non-meteorological covariates with time to infection of premises in the largest cluster (n = 3153), northwest of Sydney, during the 2007 equine influenza outbreak in Australia.

Factor	Category	No.	Hazard ratio	(95% CI)	P-valuea
Premises attributes
Area (acres)	>15.2	788	0.99	(0.85, 1.15)	<0.001
	5.1–15.2	788	1.94	(1.69, 2.23)
	4.8–5.1	789	2.09	(1.83, 2.40)
	<4.8	788	1.00
Horse density	>1.00	776	1.50	(1.29, 1.74)	<0.001
(horses acre−1)	0.40–1.00	799	2.51	(2.18, 2.89)
	0.20–0.40	787	1.85	(1.63, 2.17)
	<0.20	791	1.00
Number of horses	>5	662	3.28	(2.82, 3.82)	<0.001
	3–5	902	2.48	(2.14, 2.88)
	2	787	2.08	(1.79, 2.43)
	1	802	1.00
Length of shared fence	>300	742	1.45	(1.29, 1.63)	<0.001
with other horse	1–300	725	1.64	(1.47, 1.84)
premises (m)	0	1686	1.00
Vaccination statusb	Yes	490	0.28	(0.04, 2.13)	0.137
	No	2663	1.00
Spatial covariates
Elevation (m)	>115	785	0.72	(0.63, 0.82)	<0.001
	45–115	777	0.66	(0.58, 0.76)
	25–45	786	1.02	(0.90, 1.15)
	<25	805	1.00
Human population	>500	1059	1.05	(0.94, 1.18)	<0.001
density (people km−2)	1–500	954	1.29	(1.48, 1.44)
	0	1140	1.00
Distance to nearest	>2.2	787	1.23	(1.08, 1.41)	0.021
main road (km)	1.1–2.2	789	1.14	(1.00, 1.31)
	0.4–1.0	788	1.11	(0.97, 1.27)
	<0.4	786	1.00

^a P-values derived from likelihood ratio (LRT) tests comparing univariable to null Cox regression models.

^bTime-changing covariate.