Researchers in Germany have conducted a study demonstrating the potential antiviral effects of commonly consumed herbal teas on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) – the pathogen that causes coronavirus disease 2019 (COVID-19).
The team – from the University of Duisburg-Essen – found that aqueous infusions of sage and perilla elicited potent antiviral activity against the virus in different human cell lines.
After just 30 minutes of treating the cells in both therapeutic and prophylactic regimens, significant antiviral activity was observed.
The researchers call for future clinical studies to investigate whether herbal teas based on perilla and sage might help to prevent or treat SARS-CoV-2 infection when combined with standard treatments.
"It will be very interesting to identify the compounds responsible for the antiviral activity of aqueous perilla and sage infusions," they say. "Given the urgency, such inexpensive and broadly available substances might provide help during the pandemic - especially in low-income regions."
A pre-print version of the paper is available on the bioRxiv* server, while the article undergoes peer review.
Aqueous plant infusions have been used for thousands of years
In traditional medicine, aqueous plant infusions have been used for thousands of years across various different cultures to prevent or treat respiratory infections.
Plant members of the Lamiaceae family such as sage (Salvia officinalis) and perilla (Perilla frutescens) are particularly popular herbs used in the preparation of teas.
During the current pandemic, many people have used herbal products - usually in the form of teas - to try to protect themselves.
"Therefore, we wondered how effective herbal teas actually are against SARS-CoV-2," say Trilling and colleagues.
What did the researchers do?
The researchers tested the effects of aqueous infusions of sage and perilla on Vero E6 cells that had been infected with SARS-CoV-2 for one hour. As a control, the researchers included coriander in the experiments, since this herb is not a member of the Lamiaceae family and is not widely used as a medicinal substance.
The infusions were prepared by boiling the plants' leaves and simmering them at a temperature of 60°C for 2 hours.
The team says this short-term treatment with either herb was enough to inhibit the replication of SARS-CoV-2 significantly.
Next, Trilling and colleagues treated infected cells using two different doses of the virus and visualized them using immunofluorescence microscopy.
The number of infected cells was clearly reduced following treatment, even when a high viral dose (0.5 plaque-forming units [PFU] per cell) was used.
Since the plant components were extracted by boiling the leaves in water, the team concluded that the antiviral activity is induced by water-soluble heat-stable compound(s).
Standard teabags may contain sufficient levels of antiviral compounds
The researchers say the amount of herbal material that was used to prepare the infusions suggests that a 1/10 dilution of the infusions already matches the concentration of herbal teas prepared from standard tea bags.
"This indicates that herbal teas prepared from commercially available tea bags or dried herb leaves might contain a sufficient concentration of the antiviral compound(s)," they write.
To adapt the experiment to reflect the conditions of tea consumption more realistically, the researchers reduced the treatment time to 30 minutes.
A significant reduction in viral replication was still observed following treatment with either herbal infusion.
Next, the researchers compared the effects of infusions prepared from fresh and dried leaves.
The dried sage leaves retained most of the antiviral activity, says the team. On the other hand, the dried perilla leaves were less effective, although significant inhibition of viral replication was still observed for the 1/10 dilutions.
Testing prophylactic effects
To test whether the herbs might elicit prophylactic effects, Vero E6 cells were treated with different dilutions of the herbal infusions for one hour before the herbal components were removed and the cells infected with SARS-CoV-2.
"By removing the herbal infusions before infection, we aimed to primarily assess antiviral effects based on cellular responses and not on direct virucidal elimination of infectious virus particles," the researchers explain.
The results of six different experiments using two distinct SARS-CoV-2 isolates showed that the infusions significantly reduced infectivity, particularly the perilla infusion.
"Perilla and sage teas might not only be suitable for the treatment of SARS-CoV-2 infections but also for prevention of infections," write Trilling and colleagues.
Testing the infusions in another cell line
Considering that cells differ in terms of the mode of SARS-CoV-2 entry, the infusions were tested in another cell line called Caco-2.
Again the team observed potent inhibition of SARS-CoV-2 replication in cells that had been treated with sage and perilla infusions for one hour. Visualization of the cells by fluorescence microscopy revealed a clear reduction in the number of cells that had been infected with SARS-CoV-2.
The team says the findings suggest that the Lamiaceae plants perilla and sage contain water-soluble heat-stable components that induce potent therapeutic and prophylactic antiviral activity against SARS-CoV-2 in different cell lines.
"Obviously, the consumption of herbal teas cannot and should not replace NPIs or clinically approved drugs," say the researchers.
"However, given their inexpensive and universal availability, they might contribute to prevent or relieve some of the hardness and suffering of the COVID-19 pandemic," they conclude.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.