Effects of green tea gargling on Coronavirus Disease-2019: A multi-center randomized controlled trial

Abstract Introduction Catechins, a type of flavonoid, and major component of green tea, are expected to serve as a complementary preventive measure against coronavirus disease 2019 (COVID-19). However, to date, clinical evidence has not yet been established. This study aimed to evaluate the preventive effects of green tea gargling against COVID-19 in a multicenter randomized controlled trial. Methods A total of 1,012 adults aged 18–70 years who provided written informed consent and met the eligibility criteria were randomly assigned to either green tea or water gargling groups. Participants gargled three times a day for 12 weeks from December 2023 to February 2025 using commercially available green tea powder according to the package instructions. Background characteristics, incidence of COVID-19, and other preventive measures were assessed using self-administered questionnaires. Group differences were analyzed using multivariate logistic regression and Cox proportional hazards models. Results The incidence of COVID-19 in the full analysis population was 5.4% (27/503) in the green tea gargling group and 5.4% (27/500) in the water gargling group. In the per-protocol set (PPS) population with gargling adherence ≥80%, incidence was 4.4% (17/386) in the green tea group and 5.7% (23/403) in the water group. Multivariate logistic regression adjusted for preventive measures showed an odds ratio (95% confidence interval [CI]) of 0.818 (0.422–1.556), and Cox proportional hazard analysis yielded a hazard ratio (95% CI) of 0.824 (0.439–1.546). Pneumonia and hospitalization occurred in one participant in the water group. No gargling-related adverse events were observed during the 12-week intervention. Conclusions In the PPS population, green tea gargling showed a 22.8% relative reduction in COVID-19 incidence compared with water gargling, although the difference was not statistically significant. Further studies are needed to confirm the preventive effects of optimizing gargling frequency and catechin concentration.