It seems that cold and flu season has become more stressful for us all in recent years as we carry on in the shadow of a lingering pandemic. Contagious illness is difficult to avoid, especially in the colder months when we are spending more time indoors in close quarters with less fresh air and lower levels of vitamin D due to less sun exposure. A dry and cold environment challenges the immune system and helps some viruses to thrive. Also some viruses are seasonal and make their presence known in the winter, specifically Influenza virus and Respiratory Syncytial Virus (RSV).

So what can we do to prevent getting sick? We know that there are general precautions we can take to decrease viral transmission like regular hand-washing, covering your face when coughing or sneezing, staying home when sick and vaccinations. And of course, getting adequate sleep and nutrition is essential for maintaining a healthy immune system, which is our first-line defense against infectious disease.

There are many botanical medicines, vitamins and minerals that have been used historically for illness prevention. We have decided to take a look at a few supplements that have good evidence to back up their use: Echinacea, Green Tea, Ginseng, Vitamin D, Zinc, and Vitamin C.


A meta-analysis that studied the effect of the herb Echinacea on preventing the common cold, found that different preparations made from the 3 most commonly used species of Echinacea (E. Angustifolia, E. Purpurea, E. Pallida) decreased the odds of contracting the common cold by 58%.1 Of those people who caught a cold, cold symptoms resolved quicker among those taking echinacea preventively.

A recent study showed that taking 800 mg of Echinacea Purpurea extract three times a day significantly prevented corona-enveloped virus infections.2 In fact, this daily dose of echinacea reduced the risk of SARS-CoV-2 (“COVID-19”) infection by 63% when compared to placebo.2

Green Tea (Camellia sinesis)

Several studies have shown that daily consumption of a standardized green tea extract lowers the risk of viral respiratory infections, and decreases the duration and severity of the illness if a person should become sick.3 Green tea extract contains catechins and L-theanine, two highly researched therapeutic components of green tea.3

Drinking green tea may decrease the risk of influenza in children and may dramatically lower a woman’s risk of dying from influenza.3

Ginsengs: Asian Ginseng (Panax ginseng) & American ginseng (Panax quinquefolius)

Asian ginseng (Panax ginseng) has shown to decrease the risk of viral respiratory infections by almost half (at a dose of 1 gram three times a day).3 Supplementing with only 100 mg of Panax ginseng daily has shown to provide additional protection and increased antibodies against the flu in those who have received the influenza vaccine.3

Taking 400 mg daily of American ginseng (Panax quinquefolius) may lower the risk of viral respiratory infection (specifically influenza and RSV) and decrease the amount of infections per season.3

Vitamin D

Vitamin-D plays a significant role in promoting healthy immune function.

Research shows that low vitamin-D levels are associated with an increased risk of respiratory tract infections, including influenza and COVID-19, and that supplementation with vitamin-D may reduce the risk of contracting these illnesses (especially in people who have frequent respiratory infections).4–6

Major studies have shown that individuals with low vitamin-D levels (<30 mg/mL) may be 80% more likely to contract COVID-19 than those with higher vitamin D levels. Additionally, people with low vitamin-D levels who do become infected with COVID-19 have an increased risk of hospitalization and/or mortality.4

Dosage recommendations can range (from 400 IU to 4000 IU daily), depending on current vitamin-D levels, age and other individual factors.5 One dosing strategy is to supplement high doses of vitamin-D (i.e 10,000 IU daily) to quickly raise serum concentrations, followed by 5,000 IU daily. The goal should be to raise serum vitamin-D concentrations above 40-60 ng/mL (100-150 nmol/L). It is best to start supplementation in early autumn in order to ensure an adequate vitamin-D level in winter.5


Many studies have agreed that supplementation of zinc is helpful in reducing the risk of pneumonia, the common cold and the incidence of respiratory tract infections, specifically in the elderly and in children.7 Recently some indirect evidence suggests that zinc may potentially reduce the risk, duration and severity of COVID-19 as well.8

Taking at least 75 mg of zinc acetate or gluconate may shorten the duration of colds by approximately 33%.9 It is important to begin taking zinc within 24 hours of symptom onset to experience the maximum benefit. 9 Taking zinc with a meal may reduce the potential side effect of gastrointestinal upset and nausea. Always consult with your physician before adding zinc to your supplement regimen, as long term use can cause a copper deficiency.

Vitamin C

Vitamin-C has important anti-inflammatory, immunomodulating, antioxidant, antithrombotic (anti-blood clot) and antiviral properties.10 It is well understood that a person’s vitamin-C requirement increases if they are critically or acutely ill compared to when they are healthy.

While there is some evidence that supplementing with vitamin-C may decrease your risk of catching a cold or pneumonia, the evidence more strongly supports the benefits of prophylactic vitamin-C on supporting cold symptoms rather than preventing the incidence of a cold. Cold symptoms have been shown to be less severe, resolve more quickly, and occur less frequently with 500 mg BID of oral vitamin-C.10 Research suggests that prophylactic vitamin-c supplementation prevents respiratory illness during brief periods of severe physical exertion as seen in high-performance athletes.

Vitamin-C supplementation has been shown to shorten hospital stays for pneumonia patients and reduce the duration of mechanical ventilation in critically ill patients.10 Additionally, those with the highest baseline levels of vitamin-C may have a 30% decreased risk of pneumonia.10

Most of the research demonstrating improvement of COVID-19 outcomes by using vitamin-C has studied intravenous administration of vitamin-C, not oral supplementation. However, there are dozens of ongoing trials currently investigating the impact of vitamin-c on COVID-19.

Most people can tolerate up to 3 grams a day of oral vitamin-C before experiencing side effects like diarrhea. Always consult with your physician before starting high dose vitamin C as it may be harmful to people with certain health conditions (glucose-6-phosphate deficiency (G6PD), haemochromatosis, thalassemia or history of kidney stones/dysfunction).

I invite you to schedule an appointment to help determine your specific needs, doses and prevention strategies.

In good health,

Dr. Gili Schwerd, ND


1. Shah SA, Sander S, White CM, Rinaldi M, Coleman CI. Evaluation of echinacea for the prevention and treatment of the common cold: a meta-analysis. Lancet Infect Dis. 2007. doi:10.1016/S1473-3099(07)70160-3

2. Kolev E et al. Echinacea Purpurea For the Long-Term Prevention of Viral Respiratory Tract Infections During Covid-19 Pandemic: A Randomized, Open, Controlled, Exploratory Clinical Study. Front Pharmacol. 2022;(Apr 26;13:856410.). doi:10.3389/fphar.2022.856410

3. Yarnell E. Herbs for Viral Respiratory Infections. Altern Complement Ther. 2018. doi:10.1089/act.2017.29150.eya

4. Teshome A, Adane A, Girma B, Mekonnen ZA. The Impact of Vitamin D Level on COVID-19 Infection: Systematic Review and Meta-Analysis. Front Public Heal. 2021. doi:10.3389/fpubh.2021.624559

5. Zhu Z, Zhu X, Gu L, Zhan Y, Chen L, Li X. Association Between Vitamin D and Influenza: Meta-Analysis and Systematic Review of Randomized Controlled Trials. Front Nutr. 2022. doi:10.3389/fnut.2021.799709

6. Grant WB, Lahore H, McDonnell SL, et al. Evidence that vitamin d supplementation could reduce risk of influenza and covid-19 infections and deaths. Nutrients. 2020. doi:10.3390/nu12040988

7. Rondanelli M, Miccono A, Lamburghini S, et al. Self-Care for Common Colds: The Pivotal Role of Vitamin D, Vitamin C, Zinc, and Echinacea in Three Main Immune Interactive Clusters (Physical Barriers, Innate and Adaptive Immunity) Involved during an Episode of Common Colds – Practical Advice on Dosages . Evidence-based Complement Altern Med. 2018. doi:10.1155/2018/5813095

8. Arentz S, Hunter J, Yang G, et al. Zinc for the prevention and treatment of SARS-CoV-2 and other acute viral respiratory infections: a rapid review. Adv Integr Med. 2020. doi:10.1016/j.aimed.2020.07.009

9. Hemilä H. Zinc lozenges and the common cold: a meta-analysis comparing zinc acetate and zinc gluconate, and the role of zinc dosage. JRSM Open. 2017. doi:10.1177/2054270417694291

10. Holford P, Carr AC, Jovic TH, et al. Vitamin C—An adjunctive therapy for respiratory infection, sepsis and COVID-19. Nutrients. 2020. doi:10.3390/nu12123760