Influenza (flu) infection is the cause of respiratory illness that plagues many in the winter months. Numerous public health interventions have been put forth to help prevent the spread of infection, and additional methods can only help along those lines. There is some evidence that green tea (Camellia sinensis, Theaceae) and its active constituents, catechins and theanine, can prevent the occurrence of influenza in humans, but studies have been inconclusive. This randomized, double-blind, parallel study aimed to further investigate the efficacy of green tea and some of its components in preventing influenza infection.
Healthcare workers from 3 healthcare sites in Higashimurayama, Japan, were recruited for the study during the influenza season of November 9, 2009 to April 8, 2010. Subjects were over 20 years of age and were excluded if they met any of the following criteria: (1) had the flu within the preceding 6 months or 24 hours after entering into the study; (2) were using any medications or supplements that would affect the respiratory system; (3) had tea allergies; (4) were pregnant or lactating; or (5) had an immune, cardiac, respiratory, renal, or hepatic dysfunction.
A total of 197 subjects enrolled, with 98 in the treatment group and 99 in the placebo group. One subject in the treatment group was excluded according to the exclusion criteria (influenza infection within 24 hours after entering the study). Demographic and anthropometric data were collected from 196 subjects (mean age: 42.7 years), as well as tea consumption patterns. Subjects were asked to consume 6 green tea catechin/theanine capsules a day, containing a total of 378 mg catechins (including 270 mg epigallocatechin-3-gallate [EGCG; a proprietary form of EGCG extracted from green tea leaves; THEA-FLAN 90S; ITO EN, Ltd.; Tokyo, Japan] and 210 mg L-theanine [Suntheanine®, produced by fermentation by Taiyo Kagaku Co., Ltd.; Mie, Japan]) or a taste- and appearance-matched placebo for 5 months. Monthly questionnaires collected data on infection occurrence, adverse events, and compliance. Tea or herbal tea consumption was restricted to ≤250 ml per day during the study.
The primary outcome was an influenza infection that was doctor-verified according to the following criteria: fever (≥37.8°C or ≥100°F), plus any 2 of the following symptoms: cough, sore throat, headache, or myalgia. Secondary outcomes were: (1) confirmed infection by viral antigen immunochromatographic assay, and (2) the infection-free time from the beginning of the study.
During the 5-month study period, infection was physician-confirmed in 17 subjects (8.7%) and laboratory-confirmed in 6 subjects (3.1%). No subjects had more than 1 infection during the study period. Doctor-confirmed infection incidence was significantly lower in the catechin/theanine group (4 incidences) compared to the placebo group (13 incidences) (adjusted odds ratio [OR], 0.25; 95% confidence interval [CI], 0.07 to 0.76; P=0.022). A similar pattern was observed in laboratory-confirmed infection, but it did not reach statistical significance. The authors suggest that this may be due to the population size not being large enough, as laboratory detection of viruses has been generally low.
The time to first infection was statistically significantly greater for the catechin/theanine group than the placebo group (adjusted OR, 0.27; 95% CI, 0.09 to 0.84; P=0.023).
In univariate analysis, age was the only variable that correlated with infection; younger age correlated with higher incidence of infection (P=0.027). Other variables included vaccination, preventive measures for maintaining hygiene (e.g., hand washing), smoking, alcohol, the subjects’ sex, and other types of tea consumption.
There were no significant adverse events.
Earlier studies have demonstrated a preventive effect of green tea catechins with combination of L-theanine for influenza infection but with inconclusive evidence; however, the present study shows that a combination of catechins with L-theanine could also lower the incidence of flu infection and thus reduce the prevalence of influenza infection.
The outcome of a statistically significant preventive effect on the incidence of clinically defined influenza infection could be easily perceived; however, the mechanisms of action of green tea catechins and L-theanine on the prevention of influenza infection require further insight. Moreover, the study may overestimate the effect, as a large proportion of the healthcare workers had been vaccinated. Additional large trials are warranted to confirm these interesting results.