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

Table 4 Seropositivity among COVISHIELD™ vaccinated and unvaccinated healthcare workers at T4 or T5, n (%)

Antibody positive		High-risk areas (n = 143)	Low-risk areas (n = 59)	Inference1
Nucleocapsid +	Spike +	95 (66.4)	34 (57.6)	Infection with vaccination
Nucleocapsid -	Spike +	45 (31.5)	22 (37.3)	Vaccination
Nucleocapsid +	Spike -	0 (0)	1 (1.7)	Infection
Nucleocapsid -	Spike -	3 (2.1)	2 (3.4)	Not infected or vaccinated

¹The inference must be made cautiously since all infections and vaccinations do not necessarily produce an antibody response.

All values are expressed as number (n) of healthcare workers (HCWs) and percentage, (+) indicates positive for nucleocapsid or spike antibody while (-) indicates negative for nucleocapsid or spike antibody. Coronavirus disease 2019 (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. Since COVAXIN^® can elicit nucleocapsid and spike antibodies and COVISHIELD™ elicits antibodies only to the spike protein, HCWs who were vaccinated with COVAXIN^® were excluded from this analysis to distinguish the antibody response between natural infection and vaccination. Only those who provided at least two samples for antibodies with the 2^nd sample given during the 2^nd wave of COVID-19 (T4 or T5) were included in this analysis. The 11/143 (7.7%) and 5/59 (8.5%) from high-risk (HR) and low-risk (LR) groups were unvaccinated. In the sero-study group, clinical infection occurred in 31.5% (45/143) and 23.7% (14/59) in the HR and LR groups respectively (P = 0.23) during the entire study period.