Risk of COVID-19 infection among frontline healthcare workers during the COVID-19 pandemic

Table 3 Cross-sectional data of clinical infection and seropositivity rates among healthcare workers at various study time periods

Time point	Clinical infection				Nucleocapsid (N) antibody				Spike (S) antibody
	Overall	HR	LR	P value	Overall	HR	LR	P value	Overall	HR	LR	P value
Baseline
T1 (n = 283) [HR (n = 204); LR (n = 79)]	0/283 (0)	0/204 (0)	0/79 (0)	-	3/283 (1.1)	3/204 (1.5)	0/79 (0)	-	0/283 (0)	0/204 (0)	0/79 (0)	-
First wave
T2 (n = 168) [HR (n = 121); LR (n = 47)]	5/168 (3.0)	3/121 (2.5)	2/47 (4.3)	0.56	30/168 (17.9)	21/121 (17.4)	9/47 (19.1)	0.82	13/168 (7.7)	10/121 (8.3)	3/47 (6.4)	0.70
T3 (n = 214) [HR (n = 146); LR (n = 68)]	38/214 (17.8)	32/146 (21.9)	6/68 (8.8)	0.046	73/214 (34.1)	54/146 (37)	19/68 (27.9)	0.36	59/214 (27.6)	46/146 (31.5)	13/68 (19.1)	0.15
Second wave
T4 (n = 156) [HR (n = 110); LR (n = 46)]	6/156 (3.9)	5/110 (4.5)	1/46 (2.2)	0.50	70/156 (44.9)	51/110 (46.4)	19/46 (41.3)	0.72	138/156 (88.5)	98/110 (89.1)	40/46 (86.9)	0.92
T5 (n = 175) [HR (n = 122); LR (n = 53)]	29/175 (16.6)	21/122 (17.2)	8/53 (15.1)	0.77	106/175 (60.1)	77/122 (63.1)	29/53 (54.7)	0.60	172/175 (98.3)	121/122 (99.1)	51/53 (96.2)	0.11
T51 (n = 166) [HR (n = 118); LR (n = 48)]	24/166 (14.5)	17/118 (14.4)	7/48 (14.6)	0.78	100/166 (60.2)	73/118 (61.9)	27/48 (56.3)	0.42	164/166 (98.8)	117/118 (99.2)	47/48 (97.9)	0.52

¹T5 excludes healthcare workers (HCWs) who were vaccinated with COVAXIN^®.

All values are expressed as numbers (n) and percentages and indicate the number of HCWs; the number of participants varies in the different time periods since not all HCWs provided samples for antibodies at all the time points. T1 to T5 indicates the time point at which seroprevalence was assessed and clinical data was collected. Only individuals who had given at least two antibody samples, including one between T1 to T3 and the second sample at T4 or T5 were included in this analysis. Data on clinical infection represents the number of HCWs who developed clinical infection during that period. COVID-19 antibody levels for Nucleocapsid (N) and Spike (S) proteins were measured using Elecsys^® anti-severe acute respiratory syndrome-coronavirus 2. The antibody levels were considered positive if either N or S antibody levels were above the cut-off index (COI) reference limits. The nucleocapsid antibody was considered positive if COI was ≥ 1.0 U/mL and the spike antibody was considered positive if COI was ≥ 0.8 U/mL. HR: High-risk; LR: Low-risk.