PROS1 and COVID-19: We next examined the kinetics of anti-S protein titres in four previously infected HCWs at baseline and 1, 2, 3, 4, 5, 6, and 7 weeks after the first dose of BNT162b2 mRNA vaccine (Figures 1A and 4A, B; note that individual #1 had moderate COVID-19 symptoms and individuals #2, 3, and 4 had mild symptoms).