Table 1.

Association between selected variables and anti-S1 antibody titres among healthcare workers in a research and teaching hospital in Milan, Italy.

Variable	N subjects	Anti-S (U/mL)	Anti-S>7500 U/mL*
		Median	N	%	RR	95% CI
All	3475	1577	530	15.2
Gender
Women	2475	1602	358	14.5	1.00	Reference
Men	1000	1501	172	17.2	1.00	0.87−1.15
p-Value**		0.10		0.12
Age (years)
<35	1162	1976	194	16.7	1.00	Reference
35−44	669	1553	88	13.1	0.67	0.55−0.82
45 + 54	903	1449	134	14.8	0.72	0.60−0.86
55+	741	1208	114	15.4	1.00	0.66−0.94
p-Value**		0.0001		0.22
BMI (kg/m2)
<20	602	1487	75	12.5	1.00	Reference
20−24.99	1612	1556	229	14.2	1.00	0.80−1.24
25−29.99	765	1556	127	16.6	1.20	0.95−1.54
30+	266	2029	60	22.6	1.34	1.02−1.76
p-Value**		0.004		0.006
Cigarette smoking
Never	1908	1756	342	17.9	1.00	Reference
Former	567	1514	83	14.6	0.85	0.71−1.02
Current	804	1218	71	8.8	0.63	0.51−0.77
p-Value**		0.0001		<0.001
Nasopharyngeal swab/symptoms
Never positive	2968	1374	200	6.7	1.00	Reference
Positive <180 days before/No	70	2589	18	25.7	3.67	2.27−5.93
Positive <180 days before/Yes	271	7500	161	59.4	9.00	7.55−10.7
Positive 180+ days before/No	14	7500	12	85.7	11.5	8.1−16.4
Positive 180+ days before/Yes	152	7500	139	91.4	13.3	11.3−15.7
p-Value**		0.0001		<0.001

Abbreviations: BMI, body mass index; CI, confidence interval; LOQ, limit of quantification; RR, risk ratio.

RR were calculated with a multivariable Poisson regression model with robust standard error containing all the variables in the table.

^*Higher measurement limit: 7500 U/mL.

^**p-Values calculated with Kruskal–Wallis (quantitative titres) or chi-squared (titres > LOQ) tests.