Chronic heavy alcohol consumption is associated with a decreased frequency of lymphocytes, a type of white blood cells that is important for the immune system, and an increased risk of both bacterial and viral infections1,2. Chronic heavy alcohol consumption also increases the severity of the infection when someone is already sick. This is because alcohol in large quantities works as an immunosuppressant.
Moderate alcohol consumption
The effect of moderate alcohol consumption (one glass per day for women and two for men) on viral infections is less clear. But looking at observational studies, moderate alcohol consumption is associated with reduced inflammation and improved responses to vaccination, compared to abstaining1. So, in contrast to heavy chronic alcohol consumption, moderate alcohol consumption may trigger the immune response rather than suppress it. For example, moderate intake of alcohol in people exposed to rhinoviruses have been associated with a decreased risk of the common cold3. So there seems to be a kind of J-shaped association between alcohol consumption and immune system function.
A review1 from 2016 looked at the evidence to support such a J-shaped relation by examining the experimental and cell studies, and found that moderate alcohol consumption indeed has a very different effect on the immune system than heavy alcohol consumption1. But the authors conclude that the molecular mechanisms of the dose-dependent effects of alcohol on the immune system remain poorly understood due to a lack of systematic studies that examine the effect of multiple doses and different time courses.
To date, no studies have been published that examine the relation between alcohol consumption and the immune system taking into account COVID-19.
Apart from restraining from excessive alcohol consumption, it is important to have a healthy lifestyle to maintain a good functioning immune system. Having healthy habits, such as being active, eating a balanced diet, not smoking and getting enough sleep, can keep your immune system strong.
1 Barr, T., Helms, C., Grant, K., & Messaoudi, I. (2016). Opposing effects of alcohol on the immune system. Progress in Neuro-Psychopharmacology and Biological Psychiatry, 65, 242-251.
2 Diaz, L. E., Montero, A., Gonzalez-Gross, M., Vallejo, A. I., Romeo, J., & Marcos, A. (2002). Influence of alcohol consumption on immunological status: a review. European journal of clinical nutrition, 56(3), S50-S53.
3. Cohen, S., Tyrrell, D. A., Russell, M. A., Jarvis, M. J., & Smith, A. P. (1993). Smoking, alcohol consumption, and susceptibility to the common cold. American Journal of Public Health, 83(9), 1277-1283.