Fig. 1.

Study timeline First vaccine doses were administered to HCW in December 2020. Second doses were administered 21-days later, in January 2021, and the third doses during August 2021. IgG anti-S measurements began 0–5 days prior to the second dose (time 1), and then one month (time 2), three months (time 3), six months (time 4), nine months (time 5), and 12 months (time 6) after the second dose. Measurements of Quant anti-S were done at time points 5 and 6, after we identified very high IgG anti-S levels which were higher than the assay's upper limit following the third dose.