A Randomized Controlled Study on the Effects of Gargling with Tea Catechin Extracts on the Prevention of Influenza Infection in Healthy Adults

Abstract

Experimental studies have revealed that tea catechins prevent influenza virus infection ; however, the clinical effects have been inconclusive. At the onset of the influenza season, a randomized, double-blind, placebo-controlled study was conducted from December 2005 to March 2006 in Japan. A total of 404 healthy volunteers, 20-65 years of age, were enrolled and randomly assigned to two groups : the catechin group gargling with tea catechin extract solution (approximately 400 μg/mL catechins) or the placebo group gargling without tea catechin extracts. In both groups, gargling was performed three times daily for 90 days. All participants were inoculated with the influenza vaccine before participating in the study. The primary outcome measure was the incidence rate of influenza infection during the study identified by a rapid assay for influenza virus antigens. On an intention to treat basis, 195 participants in the catechin group and 200 in the placebo group who started the intervention were included in the analysis. Of the participants, 6 (1.5%) were infected with influenza. The incidence rate of influenza infection in the catechin group (1.0%, 2 participants) was half that in the control group (2.0%, 4 participants), but not significant between the two groups. We could not find significant effects of gargling with tea catechin on prevention of influenza in the healthy adults inoculated with the influenza vaccine of the 2005-2006 season. However, the effects in more susceptible groups, i.e., those not vaccinated against the influenza virus, children, elderly or immunosuppressed people remain inconclusive.