2024 Volume 47 Issue 7 Pages 1331-1337
Green tea (GT) catechins exhibit antiviral effects in experimental studies. However, we lack clinical evidence on the preventive effects of catechin concentrations in gargling against acute upper respiratory tract infections (URTIs). Therefore, we aimed to investigate the concentration-dependence of GT catechins in gargling on the incidence of URTIs. We conducted an open-label randomized study. The target population consisted of 209 students from the University of Shizuoka and Meiji University, who were randomly assigned to high-catechin (approximate catechin concentration: 76.4 mg/dL), low-catechin (approximate catechin concentration: 30.8 mg/dL), and a control water gargling (catechin concentration: 0 mg/dL) group. All participants gargled water or GT daily for 12 weeks. The symptoms of URTIs were recorded on a daily survey form by participants. The incidences of URTIs occurred in 6 (9.1%), 7 (10.8%), and 11 (15.7%) participants in the high-catechin, low-catechin, and water groups, respectively. Cox proportional hazards analysis, using background factors and prevention status as covariates, revealed a hazard ratio of 0.57 (95% Confidence Interval (CI): 0.21–1.55, p = 0.261) for the high-catechin vs. water group and 0.54 (95% CI: 0.20–1.50, p = 0.341) for the low-catechin vs. water group. Our findings showed the incidence of URTIs in a concentration-dependent GT gargling was not significantly different, partly owing to the low event rates caused by intense precautions against the coronavirus disease 2019 pandemic. Our study would serve as a foundation for the development of an advanced protocol with optimal concentrations and a larger number of participants.
Acute upper respiratory tract infections (URTIs), otherwise known as the common cold, are characterized by excessive nasal discharge, nasal congestion, sneezing, cough, and sore throat, as well as occasional systemic symptoms such as fever, headache, and fatigue.1,2) URTIs spread through droplets and contact infections.3) Most URTIs result from various viruses, including rhinovirus, adenovirus, respiratory syncytial (RS) virus, coronavirus, and influenza virus.3) Currently, the coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2), poses a global challenge. Although most URTI cases are mild, severe instances of influenza or COVID-19 can lead to life-threatening situations, emphasizing the importance of daily preventive measures. Established evidence-based prevention methods include hand hygiene, facemasks, vaccinations, and sanitizing hands with alcohol solutions.3–7) Other techniques, such as gargling the mouth and nose, are reportedly effective in preventing URTIs.8,9) However, these preventive measures are not absolute, and the development of more effective strategies remains a significant public concern.
Green tea (GT), widely consumed throughout Asia, contains catechins, a class of polyphenols. The principal catechins in GT are epigallocatechin-3-gallate (EGCG), epicatechin-3-gallate (ECG), epigallocatechin (EGC), and epicatechin (EC), with EGCG being the most abundant (approximately 60%), followed by EGC (approximately 20%), ECG (approximately 14%), and EC (approximately 6%).10) Notably, EGCG has been extensively studied for its physiological effects, including antioxidant, antiviral, antibacterial, and anti-inflammatory effects.10–15) With regard to the antiviral effects of catechins, inhibitory effects have been reported in vitro studies for various viruses responsible for URTIs, including influenza virus, coronavirus, and others.16–20) For example, EGCG inhibits influenza virus adsorption to host cells and decreases viral titer in a concentration-dependent method at drinking levels (1.25–100 nmol/L).17) In the same report, oral administration of EGCG for BALB/c mice has also been shown to improve survival rates and prolonged average days of death in a dose-dependent method (10, 20, or 40 mg·kg−1·d−1). Against SARS-CoV-2, it also has been reported that EGCG inactivates viruses and inhibits infection of host cells in a concentration-dependent method in the range of 0–20 µg/mL drinking level.18) Other URTI-causing viruses, adenovirus, RS virus, and parainfluenza have been reported that EGCG inhibits infection in vitro.19,20)
Several clinical trials have been conducted to assess whether tea catechins can reduce the incidence of URTIs. A meta-analysis that integrated and analyzed previous clinical trials demonstrated that GT and tea interventions might decrease the incidence of URTIs, including influenza.21) Earlier randomized controlled trials investigating influenza prevention through GT or catechin solution gargling revealed lower morbidity rates in the catechin solution or GT gargling group than in the placebo solution or water gargling group; however, the difference was not significant.22–24) One possible explanation is the lower catechin concentration in the GT used for gargling (Yamada et al.; approximate catechin concentration: 40 mg/dL, Toyoizumi et al.; approximate catechin concentration: 56 mg/dL, Ide et al.; approximate catechin concentration: 37 mg/dL). Furushima et al. compared the risk of developing URTIs in groups consuming different doses of catechin beverages and reported that the incidence of URTIs was similar in the low-dose catechin (57 mg/dL catechins) and placebo groups but lower in the high-dose group (3 times/d of low catechins) than in the placebo group,25) providing some support for this hypothesis. Nevertheless, we lack studies focusing on the catechin concentration in GT gargling. Therefore, we conducted an exploratory clinical trial of GT gargling, categorizing catechin concentrations into high-catechin, low-catechin, and water groups, to investigate the existence of concentration-dependent reduction in URTI incidence.
This open-label randomized controlled trial consisted of three parallel groups: a high-catechin-concentration GT gargling group (high-catechin group), a low-catechin-concentration GT gargling group (low-catechin group), and a water gargling group as the control (water group). Participants were recruited online via social networking services at the University of Shizuoka (Shizuoka, Japan) and Meiji University (Kanagawa, Japan). The recruitment period lasted from August 20, 2021, to December 7, 2021, with a follow-up period extending from December 1, 2021, to February 28, 2022.
We included men and women who were students at the University of Shizuoka and Meiji University, could gargle for 12 weeks, and could complete the survey form. Exclusion criteria included possible allergy to GT, chronic systemic infection, and inadequate participation as determined by physicians.
The primary endpoint was the incidence of URTIs at 12 weeks. Secondary endpoints included time to URTI onset, the incidence of influenza and COVID-19 at 12 weeks, complications such as pneumonia and encephalopathy or hospitalization, and the number of serious adverse events.
Eligible participants were enrolled and randomly assigned to the high-catechin, low-catechin, or water groups in a 1 : 1 : 1 ratio based on a computer-generated pseudorandom number table. Randomization was conducted using a stratified block substitution method, with the university serving as the stratification factor.
This study received approval from the University of Shizuoka Research Ethics Committee (Protocol Number: 2-16) and was registered at the University hospital Medical Information Network (Registry Number: UMIN000045060). The study was performed in accordance with the Declaration of Helsinki and Ethical Guidelines for Medical and Biological Research Involving Human Subjects. Before commencing the study, participants viewed a remote online video detailing the study and signed a written informed consent form. If the participant was a minor, parental approval was also obtained.
ProceduresBefore participating in the study, participants completed a self-administered questionnaire. The questionnaire items included “sex,” “age,” “height,” “weight,” “public transportation use,” “club affiliation,” and “drinking habits, such as GT, black tea, and oolong tea,” as well as “tea consumption.” Before the intervention, participants received a stainless-steel 400 mL water bottle and a survey form for recording. Those assigned to the high- and low-catechin groups additionally received commercial GT powder “Oi Ocha” (Ito En, Ltd., Tokyo, Japan). Participants gargled thrice daily (morning, noon, and evening) with their assigned gargle solution. Participants received verbal and movie instructions on how to gargle. The gargling method involved rinsing the oral cavity and pharynx with approximately 20 mL of green tea or water for 15 s and then spitting it out in one set, with three sets per gargle. The gargling method in this study was based on the method of Satomura et al.’s report.8) This method is also customarily the common method of gargling in Japan. Participants recorded their daily physical condition on the questionnaire. The questionnaire included the following items: whether the participants gargled, went out, wore a mask, washed hands, used hand sanitizers, were vaccinated against influenza or COVID-19, had common cold, influenza, or COVID-19 infection, and experienced cold symptoms such as fever (≥37.0 °C), cough, sore throat, runny nose, headache, or myalgia and complications such as pneumonia, encephalopathy, or serious adverse events including hospitalization. On vaccine items, participants completed monthly questionnaires in December, January, and February asking “Have you ever been vaccinated, among recent six months until the end of the intervention, against (influenza or COVID-19)?” and checked the box if applicable. Vaccine status shows the final number of applicable respondents. For the COVID-19 vaccine, the number of people who had taken the vaccine at least once was counted. The diagnosis of URTI was defined as the participants’ self-diagnostic check for common cold symptoms on the questionnaire (one or more symptoms including above items. The diagnosis of influenza was defined with positive antigen test, and that of COVID-19 was defined with positive PCR or antigen test examined at medical clinics.
Participants in the low- and high-catechin groups used two (approximate mean concentration of total catechins: 30.8 mg/dL, EGCG: 10.9 mg/dL) and five sticks (approximate mean concentration of total catechins: 76.4 mg/dL, EGCG: 28.5 mg/dL), respectively, dissolved in 400 mL of water per day to create the gargle solution. The average concentration of catechins in the gargle solution was measured in advance samples of the powder solution at Ito En’s laboratory using the HPLC methods. Low catechins concentration was determined as similar described in the previous study.22) High catechin concentration was determined to be two to three times higher than low catechin concentration within the healthy safety range. The water used for the gargle solution was obtained from the homes of the participants. Surveys were conducted every four weeks, and participants were interviewed and monitored for any problems with the study at the time of data collection. TT and TI collected the data from December 1, 2021, to April 28, 2022, and analyzed the data for research purposes from April 28, 2022, to October 1, 2022.
StatisticsStatistical analyses were performed using SAS software (version 9.4; SAS Institute, Cary, NC, U.S.A.). No previous studies have observed the incidence of URTIs with GT gargling. We therefore decided on a sample size based on the study by Furushima et al., which observed the incidence of URTIs with GT drinking.25) In the study, the incidence of URTIs was 13.1% in the high-dose catechins group and 26.7% in the placebo group. Therefore, we assumed a 13% incidence of URTIs in the high-concentration GT gargling group and a 27% incidence of URTIs in the water gargling group, with a significance level of 0.05 and a power of 80%, giving a sample size of 127 patients per group (381 in three groups). Considering a drop-out rate of approximately 10%, the sample size was 135 per group (405 in three groups). The Full Analysis Set (FAS), employed for efficacy and safety analyses, was determined based on the intention-to-treat principle. The FAS excluded individuals who did not receive the intervention, had no available data, or refused data usage. A sensitivity analysis of efficacy was conducted in the Per Protocol Set (PPS), which comprised participants who gargled at least 80% of the time.
Continuous variables were reported as mean ± standard deviation (S.D.) or median (interquartile range (IQR)), whereas qualitative variables were expressed as frequency (%). One-way ANOVA was utilized to compare background information and prevention activities for mean values, Kruskal–Wallis tests for median values, and chi-square tests for frequency variables. Covariates with significant differences in background and preventive factors were included in the Cox proportional hazards analysis. The primary endpoint, the incidence of URTIs at 12 weeks, was compared using Fisher’s exact test. The secondary endpoint, the time to onset of URTIs, was evaluated with the log-rank test. The cumulative incidence rate was determined using the Kaplan–Meier method. Hazard ratios and 95% confidence intervals (CIs) were computed through the Cox proportional hazards model, incorporating covariates significant for background and prevention factors. The number of cases of serious adverse events in each group was compared. A p-value of <0.050 was considered statistically significant. For multiple comparisons among the three groups, a p-value of <0.017 obtained using the Bonferroni method was deemed statistically significant.
Participants were recruited from the University of Shizuoka and Meiji University, resulting in 209 enrollees (Fig. 1). We included 70, 69, and 70 participants in the high-catechin, low-catechin, and water groups, respectively. Among the 209 randomized participants, four individuals from the high-catechin group withdrew prior to the commencement of the study, because they could not have participated in this study. Data from four participants in the low-catechin group could not be collected. Consequently, the FAS was conducted on 66, 65, and 70 participants from the high-catechin, low-catechin, and water groups, respectively. PPS analysis was performed on 55 participants from the high-catechin group, 50 from the low-catechin group, and 56 from the water group, after excluding 11 participants in the high-catechin group, 14 in the low-catechin group, 14 in the water group owing to <80% gargling adherence, and one participant in the low-catechin group who dropped out of the study. The participant dropped out owing to personal reasons but not drawbacks in the intervention.
Table 1 presents the background information of the study participants. A total of 201 participants were included in FAS, with 120 from the University of Shizuoka and 81 from Meiji University. Among them, 141 (70.1%) were women with a mean age ± S.D. of 20.9 ± 1.91 years and a mean BMI of 20.47 ± 2.69. Of the participants, 99 (49.3%) used public transportation to school, 105 (55.2%) participated in club activities, and 121 (60.2%) had tea-drinking habits. In the FAS, tea consumption habits were significantly lower in the low-catechin group (high-catechin group, 63.6%; low-catechin group, 47.7%; water group, 68.6%; one-way ANOVA, p = 0.037). PPS analysis revealed no differences in background factors among the groups.
| FAS | High-catechin N = 66 | Low-catechin N = 65 | Water N = 70 | p-Value |
|---|---|---|---|---|
| Participants (Shizuoka/Meiji) | 40/26 (60.6%) | 40/25 (61.5%) | 40/30 (57.1%) | 0.859a) |
| Sex (women/men) | 48/18 (72.7%) | 44/21 (67.7%) | 49/21 (70.0%) | 0.820a) |
| Age (years) | 21.0 (1.6) | 21.0 (2.0) | 20.9 (2.1) | 0.949b) |
| BMI (kg/m2) | 20.3 (2.3) | 21.0 (3.4) | 20.1 (2.2) | 0.134b) |
| Use of public transportation | 30 (45.5%) | 31 (47.7%) | 38 (54.3%) | 0.562a) |
| Belonging to the clubs | 38 (57.6%) | 34 (52.3%) | 33 (47.1%) | 0.477a) |
| Tea drinking habits | 42 (63.6%) | 31 (47.7%) | 48 (68.6%) | 0.037a)* |
| Tea consumption (mL/d) | 200 (0–500) | 0 (0–500) | 200 (0–600) | 0.076c) |
| PPS | High-catechin N = 55 | Low-catechin N = 50 | Water N = 56 | p-Value |
| Participants (Shizuoka/Meiji) | 35/20 (63.6%) | 31/19 (62.0%) | 37/19 (66.1%) | 0.908a) |
| Sex (women/men) | 39/16 (70.9%) | 35/15 (70.0%) | 39/17 (69.6%) | 0.989a) |
| Age (years) | 20.8 (1.6) | 21.0 (1.9) | 20.9 (2.2) | 0.791b) |
| BMI (kg/m2) | 20.3 (2.3) | 21.0 (3.6) | 19.9 (2.0) | 0.108b) |
| Use of public transportation | 27 (49.1%) | 23 (46.0%) | 28 (50.0%) | 0.913a) |
| Belonging to the clubs | 32 (58.2%) | 26 (52.0%) | 24 (42.9%) | 0.267a) |
| Tea drinking habits | 34 (61.8%) | 27 (54.0%) | 36 (64.3%) | 0.535a) |
| Tea consumption (mL/d) | 200 (0–500) | 150 (0–500) | 200 (0–500) | 0.636c) |
Public transportation was defined as a bus or train. Tea was defined as green, black, or oolong tea. Age and BMI were expressed as mean (S.D.). Tea consumption was expressed as median (IQR). Other factors are expressed as frequencies (%). a) p-Values based on the chi-squared test. b) p-Value based on one-way ANOVA. c) p-Value based on Kruskal–Wallis tests. p < 0.050 is denoted by *.
Table 2 displays the preventive measures against infectious diseases undertaken during the study period. The proportion of going out was high in each group, at approximately 90%; nonetheless, preventive activities such as mask-wearing, handwashing, and hand sanitizer usage were similarly high (over 80%). FAS analysis indicated that the overall proportion of hand sanitizer usage was 89.1% (high-catechin group 93.7%; low-catechin group 84.3%; water group 89.3%; p = 0.003). Furthermore, PPS analysis revealed that the overall proportions of handwashing and sanitizer use were 97.3 and 89.1%, respectively (high-catechin group: 98.3 and 93.9%, low-catechin group: 95.3 and 84.2%, water group: 98.3 and 88.7%; p = 0.049, 0.007, respectively).
| FAS | High-catechin | Low-catechin | Water | p-Value |
|---|---|---|---|---|
| Gargling adherence | 89.5% (14.7) | 87.5% (15.3) | 88.2% (16.8) | 0.761a) |
| Going out | 92.2% (8.9) | 89.0% (11.9) | 89.6% (11.2) | 0.204a) |
| Mask | 92.7% (9.0) | 89.2% (12.8) | 91.0% (10.9) | 0.202a) |
| Hand wash | 98.1% (5.3) | 95.5% (9.3) | 97.5% (6.3) | 0.102a) |
| Hand sanitizer | 93.7% (9.7) | 84.3% (19.6) | 89.3% (15.8) | 0.003a)* |
| Flu vaccine | 14 (21%) | 22 (34%) | 24 (35%) | 0.162b) |
| COVID-19 vaccine | 61 (92%) | 64 (98%) | 67 (97%) | 0.179b) |
| PPS | High-catechin | Low-catechin | Water | p-Value |
| Gargling adherence | 95.0% (5.5) | 94.5% (5.3) | 95.4% (5.0) | 0.725a) |
| Going out | 92.3% (8.9) | 88.5% (12.8) | 90.4% (10.1) | 0.204a) |
| Mask | 92.9% (9.0) | 88.7% (13.9) | 91.7% (10.0) | 0.152a) |
| Hand wash | 98.3% (5.0) | 95.3% (10.1) | 98.3% (5.7) | 0.049a)* |
| Hand sanitizer | 93.9% (9.4) | 84.2% (19.9) | 88.7% (16.4) | 0.007a)* |
| Flu vaccine | 13 (24%) | 18 (36%) | 17 (30%) | 0.382b) |
| COVID-19 vaccine | 50 (91%) | 49 (98%) | 54 (96%) | 0.208b) |
Vaccination status is expressed as frequencies (%). Other factors are expressed as the mean proportion (S.D.). The COVID-19 vaccination frequency represents the number of people who received one or two vaccinations. a) p-Value based on one-way ANOVA. b) p-Value based on the chi-square test. p < 0.05 is denoted by *.
In the FAS analysis, the primary endpoint, URTIs, was observed at 12 weeks (Table 3) in 6 participants (9.1%) in the high-catechin group, 7 participants (10.8%) in the low-catechin group, and 11 participants (15.7%) in the water group (Fisher’s exact test: overall, p = 0.482; high-catechin group vs. water group, p = 0.304; low-catechin group vs. water group, p = 0.455; high-catechin group vs. low-catechin group, p = 0.779). In the PPS analysis, URTIs were observed in 5 participants (9.1%) in the high-catechin group, 5 participants (10%) in the low-catechin group, and 8 participants (14.3%) in the water group (overall, p = 0.670; high-catechin group vs. water group, p = 0.557; low-catechin group vs. water group, p = 0.565; high-catechin group vs. low-catechin group, p = 1.000). None of the participants with URTIs were infected with influenza. COVID-19 infection rates were consistent in the FAS and PPS analyses: one in the high-catechin group and two in the water group, but none in the low-catechin group.
| FAS | High-catechin | Low-catechin | Water | p-Value |
|---|---|---|---|---|
| URTIs | 6 (9.1%) | 7 (10.8%) | 11 (15.7%) | 0.482 |
| Flu | None (0%) | None (0%) | None (0%) | — |
| COVID-19 | 1 (1.5%) | None (0%) | 2 (2.9%) | 0.775 |
| PPS | High-catechin | Low-catechin | Water | p-Value |
| URTIs | 5 (9.1%) | 5 (10%) | 8 (14.3%) | 0.670 |
| Flu | None (0%) | None (0%) | None (0%) | — |
| COVID-19 | 1 (1.8%) | None (0%) | None (0%) | 0.652 |
p-Value based on Fisher’s exact test.
Figures 2a and b illustrate the relationship between the cumulative incidence rate and observation time, according to the Kaplan–Meier method. In the FAS analysis, the median time to onset was 26.5, 27, and 30 d in the high-catechin, low-catechin, and water groups, respectively (log-rank test: overall, p = 0.455, high-catechin group vs. water group, p = 0.236, low-catechin group vs. water group, p = 0.403, high-catechin group vs. low-catechin group, p = 0.736). In the PPS analysis, the median time to onset was 29, 7, and 17 d in the high-catechin, low-catechin, and water groups, respectively (overall, p = 0.637; high-catechin group vs. water group, p = 0.372; low-catechin group vs. water group, p = 0.517; high-catechin group vs. low-catechin group, p = 0.843).
(a) FAS analysis set, (b) PPS analysis set. p-Value based on log-rank test.
Table 4 presents the results of the Cox proportional hazards analysis. In the FAS analysis, the hazard ratio (95% CI) was 0.56 (0.21–1.54) for the high-catechin group vs. the water group and 0.62 (0.24–1.64) for the low-catechin group vs. the water group (p = 0.262, 0.336). The PPS analysis results exhibited a similar tendency, with a hazard ratio of 0.59 (0.19–1.83) for the high-catechin group vs. the water group and a hazard ratio of 0.66 (0.21–2.10) for the low-catechin group vs. the water group (p = 0.361, 0.483). No serious adverse events, such as pneumonia, encephalopathy, or hospitalization, could be definitively attributed to the intervention.
| FAS | Hazard ratio | 95%CI | p-Value |
|---|---|---|---|
| Group (reference = water) | |||
| High-catechin | 0.56 | 0.21–1.54 | 0.262 |
| Low-catechin | 0.62 | 0.24–1.64 | 0.336 |
| Tea drinking habits (reference = no habits) | 0.77 | 0.34–1.76 | 0.540 |
| Hand sanitizer | 1.00 | 0.97–1.02 | 0.734 |
| PPS | Hazard ratio | 95%CI | p-Value |
| Group (reference = water) | |||
| High-catechin | 0.59 | 0.19–1.83 | 0.361 |
| Low-catechin | 0.66 | 0.21–2.10 | 0.483 |
| Hand wash | 0.99 | 0.93–1.05 | 0.637 |
| Hand sanitizer | 1.01 | 0.97–1.04 | 0.782 |
Abbreviation: CI, confidence interval.
This is the first clinical study investigating the effects of GT gargling on the prevention of URTIs, with a focus on catechin concentration. Most previous studies examining the preventive effect of GT gargling on URTIs have been between a control group and catechin groups,22–24) and the only study that confirmed a dose-dependent effect was a drinking intervention.25) The results of this study are proposed to be informative with respect to the determination of antiviral concentrations of catechins in humans. However, since this study used a special intervention method of gargling rather than ingestion, it is necessary to take this into consideration when determining the concentration. As there is no clear evidence of exact catechin concentrations in GT gargling for preventing URTIs, we determined low catechin concentrations (approximate mean concentration of total catechins: 30.8 mg/dL, EGCG: 10.9 mg/dL) in this study exploratory, following previous studies to evaluate only influenza incidents,22–24) and high catechin concentration (approximate mean concentration of total catechins: 76.4 mg/dL, EGCG: 28.5 mg/dL) was designed exploratory three times the low catechin concentration for safety reasons. It is not known how much catechins are ingested by GT gargling, but a safe intake level for adults is 338 mg EGCG/d.26) Even if all the highly concentrated GT used in this study was ingested, it would still be 114 mg EGCG/d, so it was deemed safe. As a result in this study, none of the participants observed serious adverse events, suggesting that high catechin GT gargling is a safe intervention in this study’s concentrations. Further clinical studies should be conducted to explore the use of higher GT concentrations based on this study’s results.
The FAS and PPS analysis demonstrated similar results, suggesting the robustness of our results. A statistically significant difference was not observed in the incidence of URTIs, as the primary endpoint, among the three groups. These results are consistent with previous examinations.22–24) This could be for two main reasons. One is the sample size. This study was initially planned before the COVID-19 pandemic; however, the pandemic made it impossible to hold in-person information sessions, limiting recruitment to posters and online platforms. These problems made it difficult to obtain cooperation from the participants, and overall participation was compelled to be lower than the estimated target number. The target number of participants for this study was 405, whereas the final number was approximately half of the estimated total number. Therefore, Type II errors may have occurred owing to the insufficient sample size.27) The other reason is likely owing to the high preventive activities, such as hand sanitizer,4) wearing masks,5) and hand washing,6) of the participants during the study period. The COVID-19 pandemic significantly influenced the smaller event rates. Satomura et al. conducted a study before the COVID-19 pandemic and reported that the incidence of URTIs in the water-gargling group was 30.1%,8) whereas, in the present study, the incidence was 15.7%, a difference of approximately 15%. This reduction in the baseline incidence of URTIs owing to the effect size of other preventive activities during the COVID-19 pandemic may have underestimated the effect size of GT gargling. In general, several factors, such as the living environment, could influence the results. However, the influence of familial infection or the living area between two universities was considered to be smaller than the preventive activity in this study. The influenza vaccination coverage of the participants in this study was 35% in the water gargling group, more than double the 14.3% in the water gargling group of the participants in Satomura et al.8) On the other hand, other non-vaccine preventive activities, such as masks, were considerably higher at approximately 90% (Table 2). These findings indicate that preventive activities were actively conducted during the study period. There were also few cases of influenza in the country during the study period.28) These may have suppressed the baseline incidence in the present study due to the high impact of high preventive activities. Age is also known as a factor affecting the baseline risk of URTIs, and epidemiological studies have reported an inverse relationship between age and the incidence of URTIs.3) Therefore, the younger population is expected to be the higher incidence of URTIs. Although the participants in this study were recruited within university students and the mean age was 20.9 years, younger than the mean age of 34.7 years (water gargling group) of the participants in Satomura et al.8) The incidence of URTIs in the water gargling group in this study was lower in spite of younger age population, probably because of the intense preventive activities conducted in universities at the COVID-19 pandemic.
In the study, the participants were uniform in young adult age. This allowed us to observe the effects of GT gargling without taking into account the variability of age. However, it is not clear whether the effects of GT gargling are influenced by aged population. In the literature, only the study by Yamada et al. reported the effects of GT catechin solution gargling on influenza prevention in an elderly population.29) Their results suggest that the effects of GT gargling in elderly, but the study design was not randomized, prospective cohort. Therefore, a randomized controlled study like ours will be required to clarify the strong clinical evidence.
This study has several limitations. The causes of URTIs encompass viruses, bacteria, and complications of underlying diseases. In this study, URTIs, including influenza and COVID-19, were defined as URTIs, and the presence of URTIs among study participants was determined based on their subjective judgment of clinical symptoms unless an antigen or PCR test was performed. This subjective judgment varies among individuals, and some participants may have had URTIs but did not report them owing to mild symptoms. Thus, it is unclear if this judgment represents the exact number of URTIs. Furthermore, because green tea is commonly consumed in Japan, we did not restrict GT consumption during the study period for ethical reasons. Daily consumption of green tea has been found in epidemiological studies to provide population protection against influenza and COVID-19.30,31) Daily tea habit was used as an adjustment factor for the hazard ratio (Table 4), but the actual amount of tea consumed in this study was not known. Consequently, catechins taken from sources other than intervention may have influenced our results. Finally, the most important limitation of this study is the large impact of preventive activities during the exceptional COVID-19 pandemic. Therefore, the present results are limited and should be interpreted with caution.
We investigated the preventive effect of GT gargling on URTIs by focusing on catechin concentration. However, these differences were not statistically significant. Further clinical trials are required to assess the maximum effects of GT gargling on catechin concentration.
We express our sincere gratitude to the students at the University of Shizuoka and Meiji University who participated in this study for their cooperation. We also thank Dr. Yoriyuki Nakamura, Mr. Shohei Makinose, Mr. Hirotomo Nakamura, Mr. Shimpei Oishi, Ms. Ibuki Sugiyama, Mr. Yuya Tanaka, and Mr. Atsunori Makita from the University of Shizuoka, as well as Ms. Miho Naoi from Meiji University for their collaboration in conducting this study.
TT, TI, DF, and TN declare they have no competing interests. HY received scholarship donations from Ito En, Ltd. to establish the Department of Tea & Health Sciences. The funder had no role in the study design, data collection and analysis, decision to publish, or manuscript preparation. This study was supported by the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (KAKENHI) under Grant Number: 20K10382 (HY).
