ABSTRACT The anecdotal report describes two examples of COVID-19-positive healthcare professionals (HCPs) who had worked as nurses during pre-symptomatic period and subsequently presented a mild clinical course of COVID-19. The nurses' responsibilities were almost the same and worked in the general ward with no aerosol generating medical procedures. Two HCPs were expected to have similar infectiousness, but the number of secondary transmission by each HCP were different. Eleven close contacts from HCP1 were notified, and all of them tested negative for SARS-CoV-2. However, 13 of 35 close contacts of HCP2 tested positive for SARS-CoV-2. While working, mask-wearing style differed between the two HCPs. HCP1 wore a KF94 mask appropriately and kept wearing it while working.HCP2 wore a surgical mask while working, but often pulled it down to her chin or removed it. It was strongly suspected that the difference of mask wearing contributed to the SARS-CoV-2 transmission. However, other factors such as talkative behavior, exposure time, ventilation in rooms, and hand hygiene performance rates, could also have affected the transmission of COVID-19.It is crucial that healthcare workers wear a mask adequately and continuously, and they maintain proper hand hygiene while working during the COVID-19 pandemic.