Why are we prone to colds and flus in the winter months?

We tend to get more colds and flus during the winter months due to a combination of factors, mainly because:^1-4

• The cold weather drives people to spend more time in indoors in enclosed spaces. Being in close proximity to a group allows viruses to spread more easily, transmitted via coughing, sneezing, touching contaminated surfaces, and even simply talking face-to-face to one another. Poorly ventilated indoor environments can allow airborne viruses to accumulate in the air, increasing the risk of them being breathed in and causing infection.
• Respiratory viruses survive longer in colder temperatures and lower humidity. The dry air also dries out the mucous membranes in our noses. Nasal mucus plays a critical role in fending off invading pathogens that cause upper respiratory tract infections.
• A lack of regular sunlight exposure, leading to vitamin D deficiency, can weaken immune defences. But also, direct ultraviolet (UV) light from the sun acts as a natural decontaminant.

How does the immune system defend itself?

Our immune system has three lines of defence:^5,6

• The first line of defence against invading pathogens consists of protective physical and chemical barriers, such as mucus secretion. Mucus not only traps pathogens, but it also contains enzymes and proteins that destroy them.
• The second line of defence is referred to as innate immunity, which provides non-specific protection against pathogens. Immune cells like natural killer (NK) cells and their chemical messengers (cytokines) are included in this line of defence.
• The third line of defence is referred to as adaptive immunity. Immune cells in the third line of defence can either kill infected cells (T cells) or produce specific proteins called antibodies (B cells) that target and neutralise specific pathogens. Adaptive immunity is highly specific and has immunological memory, which is why vaccines work – they train the immune system to recognise and respond more effectively to the same invaders in future.

Which nutrients and herbs help to boost immunity?

Some of the most common nutrients and herbs that help to strengthen the three lines of defence, address common nutritional deficiencies associated with increased susceptibility to infections, and/or combat viral infections include:

• Vitamin C (ascorbic acid)^6,7
• Citrus bioflavonoids⁸
• Vitamin A^9,10
• Vitamin D^6,11
• Zinc⁶
• Andrographis paniculata (Andrographis)^12,13
• Ganoderma lucidum (Reishi)^14,15

When is the best time to start taking immune support supplements?

Prevention is key. To help reduce the risk, severity and duration of colds and flus, it is recommended that you start taking general immune support supplements early – in autumn or a few weeks before peak season. Continue to take them daily and consistently throughout the cold and flu season. This is especially true for vitamin D because your body needs time to reach optimal levels, especially if you’re starting from deficiency.

You may benefit from temporarily increased doses of vitamin C, zinc and Andrographis at the first sign of symptoms.

Consult a qualified healthcare practitioner to ensure these supplements are right for you and at what dose.

Who might these nutrients and herbs benefit most?

While nobody is immune to colds or flus, some of us are less able to defend against the viruses that cause them. Certain individuals may experience more frequent infections, severe symptoms or complications, such as pneumonia. In general, those most susceptible include:^2,4,16-18

• Young children, as their immune systems are still developing
• Older adults (60 years or older)
• Pregnant women
• Individuals with impaired or weakened immune function
• Those with cancer or some other chronic medical conditions, such as asthma and diabetes
• People who live or work in densely populated settings
• Those who have prolonged face-to-face contact with others (especially sick people), such as young children, teachers and staff in childcare centres and schools, healthcare workers, and residents in aged care facilities
• Travellers, especially during cold and flu season

There are also specific indications for each of the nutrients and herbs described.

Vitamin C (ascorbic acid) may be best for:

• Smokers, since they have higher vitamin C requirements¹⁹
• People with stress-related disorders, such as those experiencing anxiety and depression²⁰
• Those with poor dietary intake of fruits and vegetables²¹

Citrus bioflavonoids may be best for:

• Those with allergies, since citrus bioflavonoids seem to have antihistamine effects^22,23

Vitamin A may be best for:

• Those with poor dietary intake of retinol (low animal-product intake) or beta-carotene (low vegetable intake)^24,25
• Individuals with recurrent sinus or lung infections, since vitamin A supports the mucosal immune system²⁶
• Those with gastrointestinal disorders that impair fat and fat-soluble vitamin absorption, including coeliac disease, Crohn’s disease and gastrointestinal surgery²⁷

Warning! Pregnant women should avoid high doses of vitamin A due to the associated risk of birth defects.²⁸ Be mindful of vitamin A intake from all sources, including foods, supplements, topical products, and medications (especially isotretinoin/Roaccutane). If you are pregnant or planning to become pregnant, consult a qualified healthcare practitioner before using any supplements.

Vitamin D may be best for:

• People with limited or less effective sun exposure, including individuals staying indoors most of the time, those wearing full body covering (for work or cultural reasons) and people with naturally darker skin²⁹
• Elderly adults, since skin production of previtamin D3 declines with age³⁰
• People with autoimmune conditions or frequent infections³¹

Zinc may be best for:

• Vegetarians/vegans, since plant-based diets are higher in phytic acid which inhibits zinc absorption³²
• Older age, since zinc deficiency is common in the elderly^33,34
• People with slow healing wounds³⁵

Andrographis may be best for:

• Those experiencing acute cold and flu symptoms^13,36
• Individuals seeking herbal immune support

Reishi may be best for:

• Those with phlegm, cough and wheeze (asthma)¹⁵

Consult a qualified healthcare practitioner for personalised advice before using any supplements, especially if you are pregnant, breastfeeding, administering to children, taking medication, or have a diagnosed medical condition.

What else can you do to strengthen your immunity?

* To learn about our top 5 tips to stay healthy during cold and flu season, refer to the Eagle Immune Health patient handout.

References

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2. Healthline. Scientists finally figure out why you're more likely to get sick in cold weather [Internet]. San Francisco: Healthline; 2022 [cited 2025 Jun 12]. Available from: https://www.healthline.com/health-news/scientists-finally-figure-out-why-youre-more-likely-to-get-sick-in-cold-weather

3. Healthline. 8 ways dry air can affect your health [Internet]. San Francisco: Healthline; 2020 [cited 2025 Jun 12]. Available from: https://www.healthline.com/health/dry-air

4. Healthline. You’re more likely to get sick in the winter — but not for the reasons you think [Internet]. San Francisco: Healthline; 2019 [cited 2025 Jun 12]. Available from: https://www.healthline.com/health-news/sick-in-rainy-weather-reasons

5. Department of Health, Disability and Ageing. About immunisation [Internet]. Canberra: Australian Government; 2025 [cited 2025 Jun 12]. Available from: https://www.health.gov.au/topics/immunisation/about-immunisation

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15. American Herbal Pharmacopoeia. Reishi mushroom - Ganoderma lucidum: standards of analysis, quality control, and therapeutics. Santa Cruz: American Herbal Pharmacopoeia; 2000.

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17. National Institutes of Health. Flu and older adults [Internet]. Washington: U.S. Department of Health and Human Services; 2022 [cited 2025 Jun 12]. Available from: https://www.nia.nih.gov/health/flu/flu-and-older-adults

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23. Lee NK, Choi SH, Park SH, Park EK, Kim DH. Antiallergic activity of hesperidin is activated by intestinal microflora. Pharmacology. 2004 Aug;71(4):174-180. DOI: 10.1159/000078083

24. Healthline. 20 foods high in vitamin A [Internet]. San Francisco: Healthline; 2024 [cited 2025 Jun 16]. Available from: https://www.healthline.com/nutrition/foods-high-in-vitamin-a

25. Healthline. Benefits of beta carotene and how to get it [Internet]. San Francisco: Healthline; 2023 [cited 2025 Jun 16]. Available from: https://www.healthline.com/health/beta-carotene-benefits

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27. Andrès E, Lorenzo-Villalba N, Terrade JE, Méndez-Bailon M. Fat-Soluble Vitamins A, D, E, and K: review of the literature and points of interest for the clinician. J Clin Med. 2024 Jun;13(13):3641. DOI: 10.3390/jcm13133641

28. Azaïs-Braesco V. Vitamin A in pregnancy: requirements and safety limits. Am J Clin Nutr. 2000 May;71(5 Suppl):1325S-1333S. DOI: 10.1093/ajcn/71.5.1325s

29. Healthdirect. Vitamin D deficiency [Internet]. Canberra: Australian Government; 2024 [cited 2025 Jun 16]. Available from: https://www.healthdirect.gov.au/vitamin-d-deficiency

30. MacLaughlin J, Holick MF. Aging decreases the capacity of human skin to produce vitamin D3. J Clin Invest. 1985 Oct;76(4):1536-1538. DOI: 10.1172/JCI112134

31. Wimalawansa SJ. Infections and autoimmunity-the immune system and vitamin D: a systematic review. Nutrients. 2023 Sep;15(17):3842. DOI: 10.3390/nu15173842

32. Craig WJ. Nutrition concerns and health effects of vegetarian diets. Nutr Clin Pract. 2010 Dec;25(6):613-620. DOI: 10.1177/0884533610385707

33. Cabrera ÁJR. Zinc, aging, and immunosenescence: an overview. Pathobiol Aging Age Relat Dis. 2015;5:25592. DOI: 10.3402/pba.v5.25592

34. Haase H, Rink L. The immune system and the impact of zinc during aging. Immun Ageing. 2009 Jun;6:9. DOI: 10.1186/1742-4933-6-9

35. Lin PH, Sermersheim M, Li H, Lee PHU, Steinberg SM, Ma J. Zinc in wound healing modulation. Nutrients. 2017 Dec;10(1):16. DOI: 10.3390/nu10010016

36. Hu XY, Wu RH, Logue M, Blondel C, Lai LYW, Stuart B, et al. Andrographis paniculata (chuān xīn lián) for symptomatic relief of acute respiratory tract infections in adults and children: a systematic review and meta-analysis. PLoS One. 2017;12(8):e0181780. DOI: 10.1371/journal.pone.0181780