The transmission risk from infected patients to healthcare workers and/or other patients is a concern during the coronavirus disease 2019 (COVID-19) pandemic.¹ Especially when performing digestive endoscopy, it seems inevitable that healthcare workers will be exposed to either respiratory or gastrointestinal fluids from patients.² Therefore, appropriate infection control during digestive endoscopy based on experience is recommended by the World Endoscopy Organization.³ We read with interest the papers reported by Higashimori et al.⁴ and Nadatani et al.⁵ In these studies, nasal insertion of a thin endoscope and covering of the patient's mouth with a surgical mask prevented the spread of aerosol droplets due to coughing during the procedure. We support the studies’ findings and suggest that this simple infection control measure be implemented for all screening endoscopy cases. Conversely, several reports on gas leakage or release of fluid droplets from biopsy valves of full-sized endoscopes during insertion and withdrawal of endoscopic instruments have been published.^6,7 Gas leakage may be observed even in thin endoscopes. If so, transnasal endoscopy with a surgical mask may be inadequate as an infection control measure, especially when performing procedures requiring instrument traffic, such as biopsies. Therefore, we aimed to visualize the gas leakage from the biopsy valve of the thin endoscope and to prove that simply covering the mouth with a mask is insufficient to prevent COVID-19 infection during endoscopy.