Table 3.

Risk factors associated with COVID-19 reinfection in Hong Kong identified by Cox regression

	COVID-19 reinfection
Risk factor	Crude HR	95% CI of Crude HR	P value	Coefficient of multivariate Cox regression	Adjusted HR	95% CI of adjusted HR	P value
Sex
• Male	1	—		Reference	1	—
• Female	1.12	1.11, 1.14	<0.001	0.11	1.12	1.11, 1.13	<0.001
Age (in years)
0–9	1	—		Reference	1	—
10–19	1.22	1.18, 1.27	<0.001	0.18	1.19	1.15, 1.24	<0.001
20–29	1.27	1.23, 1.31	<0.001	0.16	1.17	1.13, 1.21	<0.001
30–39	1.24	1.20, 1.28	<0.001	0.11	1.12	1.09, 1.16	<0.001
40–49	1.04	1.01, 1.08	0.009	−0.08	0.92	0.89, 0.95	<0.001
50–59	0.85	0.82, 0.87	<0.001	−0.31	0.73	0.71, 0.76	<0.001
60–69	0.83	0.80, 0.86	<0.001	−0.39	0.68	0.66, 0.70	<0.001
70–79	1.03	0.99, 1.06	0.160	−0.31	0.73	0.70, 0.76	<0.001
> = 80	2.11	2.04, 2.18	<0.001	−0.18	0.83	0.80, 0.87	<0.001
Chronic diseases
• No	1	—		Reference	1	—
• Yes	1.18	1.16, 1.19	<0.001	0.17	1.18	1.16, 1.20	<0.001
RCH residents
• No	1	—		Reference	1	—
• Yes	6.08	5.96, 6.20	<0.001	1.91	6.78	6.61, 6.95	<0.001
Vaccinated doses at censoring
• 0–2	1	—		Reference	1	—
• 3+	0.94	0.93, 0.96	<0.001	−0.003	0.997	0.982, 1.012	0.689
Additional vaccination after primary infection
• No	1	—		Reference	1	—	<0.001
• Yes	0.89	0.88, 0.90	<0.001	−0.22	0.80	0.79, 0.81	<0.001

HR = Hazard Ratio, CI = Confidence Interval.