Supplements to Prevent Colds and Flu: Overview
Sneezing and coughing? Catching a cold or flu can be annoying. It is usually treated with home remedies like steam inhalation, saline rinse, gargles, and herbal teas. Getting a yearly flu vaccine is also recommended by the WHO to stay in good shape during the flu season.But most people don't realize that certain supplements and probiotics can help keep colds and the flu from taking hold. In fact, research reveals that taking probiotics for cold and flu season may help keep these viral infections at bay. In a Cochrane study, probiotics were superior to placebo in decreasing the number of cold and flu episodes, the duration of these episodes, antibiotic use as well as the school absence due to cold.  The evidence supporting the benefit of probiotics for flu prevention is, however, a bit controversial. 
Do Probiotics Help With Colds?
Probiotics Immune System Benefits
A strong immune system is your key defense against all diseases, inclusive of flu and cold. Since probiotics boost your immune system, it is needless to say that a healthy balance of probiotic bacteria may ward off the respiratory viruses from attacking us. 
Probiotics work for colds and flu via live, healthy bacteria that can activate some specialized cells of the immune system like the natural killer cells.  These cells are destined to kill the attacking viruses. Also studied, is the role of probiotics in triggering the production of antibodies to help protect against cold and flu. These are proteins that our body produces in response to the offending agent to help us protect against it. Nevertheless, the production of antibodies in response to probiotics for cold and flu protection has been detected in only animals. 
Probiotics Can Reduce Stress Levels
Diseases like cold and flu are likely to hit you during periods of extreme psychological stress. Weakening of the immune system probably accounts for our increased vulnerability to sickness while we’re stressed. According to a study published in the British Journal of Nutrition, experts revealed that the probiotic strain, Bifidobacterium bifidum may be an effective measure against cold and flu. The participants selected in this study were the academically stressed undergraduate students, suggesting a correlation between stress and viral illnesses. 
Because probiotics can reduce stress and anxiety through several mechanisms, they can also help stave off the cold and flu viruses.
Probiotics Can Also Make The Flu Shot More Effective
Surprisingly, probiotics may also boost the response of your flu shot. A pilot study showed that participants who underwent a 28-day treatment with a lactobacillus strain after receiving the flu vaccine demonstrated enhanced protection against the virus without any side-effects. 
The experts attribute this beneficial response of probiotics to their cold and flu immune response-provoking properties. Perhaps, the gut microbiota intensifies the protective effect of the flu vaccine via specialized gene called TLR5, which is one of the ways in which probiotics can help fight viruses.  This TLR5 gene recognizes a protein called flagellin, which is a component of several mobile microbes, including bacteria. The antibodies produced in response to the flu vaccine depend on the levels of the TLR5 gene as well as on the gut microbiota. As a result, the antibody response to the flu vaccine would not be adequate if a person’s gut lacks healthy bacteria.
Probiotics for Colds and Flu: Summary
Probiotics are found to be helpful during the cold and flu season. You can think of them as supplements to help prevent colds and flu, however, each type of probiotic may affect the flu symptoms differently. But with the best probiotic supplements for cold and flu, you may notice an immediate improvement, whereas lesser probiotics and supplements might not provide you the desired relief from cold and flu.
- Quick M. Cochrane Commentary: Probiotics For Prevention of Acute Upper Respiratory Infection. Explore (NY).2015;11(5):418-20. doi: 10.1016/j.explore.2015.07.012.
- Kang E-J, Kim SY, Hwang I-H, Ji Y-J. The Effect of Probiotics on Prevention of Common Cold: A Meta-Analysis of Randomized Controlled Trial Studies. Korean Journal of Family Medicine. 2013;34(1):2-10. doi:10.4082/kjfm.2013.34.1.2.
- Aziz N, Bonavida B. Activation of Natural Killer Cells by Probiotics. Forum on immunopathological diseases and therapeutics. 2016;7(1-2):41-55. doi:10.1615/ForumImmunDisTher.2016017095.
- Haghighi HR, Gong J, Gyles CL, et al. Probiotics Stimulate Production of Natural Antibodies in Chickens. Clinical and Vaccine Immunology. 2006;13(9):975-980. doi:10.1128/CVI.00161-06.
- Langkamp-Henken B, Rowe CC, Ford AL et al. Bifidobacterium bifidum R0071 results in a greater proportion of healthy days and a lower percentage of academically stressed students reporting a day of cold/flu: a randomised, double-blind, placebo-controlled study. Br J Nutr. 2015 113(3):426-34. doi: 10.1017/S0007114514003997.
- Davidson LE, Fiorino A-M, Snydman DR, Hibberd PL. Lactobacillus GG as an Immune Adjuvant for Live Attenuated Influenza Vaccine in Healthy Adults: A Randomized Double Blind Placebo Controlled Trial. European journal of clinical nutrition. 2011;65(4):501-507. doi:10.1038/ejcn.2010.289.
- Oh JZ, Ravindran R, Chassaing B, et al. TLR5-Mediated Sensing of Gut Microbiota Is Necessary for Antibody Responses to Seasonal Influenza Vaccination. Immunity. 2014;41(3):478-492. doi:10.1016/j.immuni.2014.08.009.