More than one year of COVID-19 closure and restriction, the world is transiting towards the endemic mode of living with the disease. Many countries in Europe, UK, and America has already dropped most, if not all, of the COVID-19 restrictions due to high vaccination rates. Relaxations of borders for vaccinated travellers without quarantine are also happening or soon to roll out in many countries. Similarly, Singapore is also transitioning now, with 83% of the population inoculated against COVID-19.
However, such opening-up may not be good news for anyone who has not acquired immunity towards the disease, either naturally or artificially (through vaccine). The high-risk groups are unvaccinated elderly, especially those with underlying medical conditions such as diabetes and cardiovascular diseases. Even for vaccinated individuals with weakened immunity, any form of infection can be severe, let alone the highly contagious coronavirus. As I have pointed out in my blog post at the start of the pandemic, vaccine alone cannot be a magic bullet that ends the COVID-19 disease (See “Is vaccine the answer to the coronavirus pandemic?”).
Building a robust immune system naturally through a healthy diet and lifestyle is more important to be impervious to the disease. Here are some foods that can help to strengthen the immune system for living with COVID-19.
Medicinal mushrooms
Many mushrooms have medicinal values against infections, inflammatory disorders, and even cancers. Commonly available medicinal mushrooms include Shiitake, Maitake, Cordyceps, monkey head, LingZhi, Turkey tail, and many more are known to be good for the immune system. It has been suggested that extracts from medicinal mushrooms can be potentially effective against pneumonia and severe lung inflammation due to COVID-19 infection [1]. Although such a claim has not been proven in human clinical trials, it is still worth adding them to the diet.
Shiitake 
Monkey head 
Ling Zhi
Anti-inflammatory and antiviral spices
Turmeric, pepper, garlic, ginger, and onion are some spices with anti-inflammatory and antiviral properties. A study that uses Curcuma longa (active constituent of turmeric) with piperine (from pepper) to treat COVID-19 symptoms showed that it can improve recovery and reduce the severity of the disease [2]. Allicin, the active constituent of garlic, has been considered a targeted drug for this coronavirus [3]. Ginger is also being tested for its potential effect in improving COVID-19 symptoms [4]. Similarly, onion has been suggested as a phytopharmaceutical candidate for reducing respiratory complications of COVID-19 [5].
Ginger 
Garlic 
Turmeric powder 
Onion
Antioxidant foods
Oxidative stress can be triggered by viral infections and increases the risk of severe complications [6]. We need antioxidants to reactive oxygen species that can be harmful to healthy tissues and worsen inflammation. Among the natural antioxidant compounds studied for their effects on COVID-19 include resveratrol (from grapes, berries, and red wine), catechins and epigallocatechin gallate (EGCG, from green tea), quercetin (from berries, cherries, broccoli, and citrus fruits), and many other flavonoids from fruits and vegetables [7]. Honey is also an anti-inflammatory and antioxidant food that should be considered [8]. Hence, adding honey, green tea, fresh fruits, and vegetables into your diet is sensible.
Honey 
Green tea 
Berries 
Red wine
Probiotic and prebiotic
Having a healthy gut microflora is essential for a robust immune system. Hence, it is crucial to add fermented food to the diet to increase the good bacteria population. The healthful, functional, nutritional, and immune benefits of probiotics and prebiotics are unquestionable. Natural foods with prebiotics include whole grains, fruits and vegetables, and plant-based proteins (tofu, chickpeas, lentils, beans, etc.). Whereas sources of probiotics are such as miso, sauerkraut, tempeh, and kefir [9].
Tempeh 
Miso 
Sauerkraut
Conclusion
Lockdown and travel restrictions will soon end with the world living with COVID-19 in endemic mode. It is time to strengthen the immune system, even if you have been vaccinated. Consider adding medicinal mushrooms, spices, antioxidant-rich foods, probiotics and prebiotics to the diet. They can help reduce the chance of having more severe complications in any infections.
References
[1] G. Hetland, E. Johnson, S.V. Bernardshaw, B. Grinde, Can medicinal mushrooms have prophylactic or therapeutic effect against COVID-19 and its pneumonic superinfection and complicating inflammation?, Scand J Immunol 93(1) (2021) e12937.
[2] K.S. Pawar, R.N. Mastud, S.K. Pawar, S.S. Pawar, R.R. Bhoite, R.R. Bhoite, M.V. Kulkarni, A.R. Deshpande, Oral Curcumin With Piperine as Adjuvant Therapy for the Treatment of COVID-19: A Randomized Clinical Trial, Front Pharmacol 12 (2021) 669362.
[3] R.K. Pandey, R.K. Pandey, S.S. Shukla, P. Pandey, A review on corona virus and treatment approaches with Allium sativam, Future Journal of Pharmaceutical Sciences 7(1) (2021) 159.
[4] O. Safa, M. Hassaniazad, M. Farashahinejad, P. Davoodian, H. Dadvand, S. Hassanipour, M. Fathalipour, Effects of Ginger on clinical manifestations and paraclinical features of patients with Severe Acute Respiratory Syndrome due to COVID-19: A structured summary of a study protocol for a randomized controlled trial, Trials 21(1) (2020) 841-841.
[5] W. Dorsch, J. Ring, Anti-inflammatory substances from onions could be an option for treatment of COVID-19-a hypothesis, Allergo J Int 29(8) (2020) 284-285.
[6] B.V. Chernyak, E.N. Popova, A.S. Prikhodko, O.A. Grebenchikov, L.A. Zinovkina, R.A. Zinovkin, COVID-19 and Oxidative Stress, Biochemistry (Mosc) 85(12) (2020) 1543-1553.
[7] L.R.L. Diniz, C.d.S.M. Bezerra Filho, B.C. Fielding, D.P. de Sousa, Natural Antioxidants: A Review of Studies on Human and Animal Coronavirus, Oxidative Medicine and Cellular Longevity 2020 (2020) 3173281.
[8] K.S. Hossain, M.G. Hossain, A. Moni, M.M. Rahman, U.H. Rahman, M. Alam, S. Kundu, M.M. Rahman, M.A. Hannan, M.J. Uddin, Prospects of honey in fighting against COVID-19: pharmacological insights and therapeutic promises, Heliyon 6(12) (2020) e05798.
[9] J. Hu, L. Zhang, W. Lin, W. Tang, F.K.L. Chan, S.C. Ng, Review article: Probiotics, prebiotics and dietary approaches during COVID-19 pandemic, Trends Food Sci Technol 108 (2021) 187-196.