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Take Probiotics This Cold and Flu Season!

Oct 9, 2023 18 min readBy Entegro Health

Every autumn the internet decides probiotics are cold-and-flu armor. The research is more interesting than that, and a good deal more specific.

Woman resting on a sofa under a blanket, holding a mug of tea in soft daylight

It is that time of year again. Somebody in your house is coughing, the office has that particular soundtrack, and your feed fills up with things that promise to keep you upright until spring.

Probiotics are one of those things. And there is real research here — several large analyses pointing in the same direction, which is more than most of the seasonal wellness aisle can say. There are also some numbers that get repeated far past what the studies behind them actually measured. So here is the whole picture, with the receipts.

This article is about research, not treatment. Flourish is a dietary supplement, not a medicine, and nothing here is intended to diagnose, treat, cure or prevent the flu, a cold or any other illness. The things that genuinely prevent influenza are vaccination, handwashing and staying home when you are sick. If you are unwell, call your doctor, not your supplement cabinet.

First, what we are actually talking about

Woman resting on a sofa under a blanket, using a tissue during a cold

We say “cold and flu season” as though it were one thing. It is really a whole category: respiratory tract infections, or RTIs. They affect the lungs, airways, throat and sinuses, and researchers split them in two.

Upper and lower

Upper — the common cold, sore throat, sinusitis, laryngitis. Above the voice box, roughly.
Lower — bronchitis, pneumonia. Down in the airways and lungs.
Some infections do both. COVID-19 and RSV infection can each show up at either end.

That split matters more than it sounds, because almost all of the probiotic research is on upper respiratory infections. Bear that in mind when you see a broad headline.

And yes, the flu is a respiratory infection

Worth stating plainly, because it gets muddled a lot. Influenza is classified as a respiratory tract infection by everyone who classifies such things. The World Health Organization opens its fact sheet with “seasonal influenza is an acute respiratory infection caused by influenza viruses.”¹ The CDC calls it “a contagious respiratory illness.”² In the international disease coding system it lives in chapter ten, diseases of the respiratory system, codes J09 to J11.³

The flu does make you feel wretched all over — fever, aching, the works — and it can occasionally affect other organs. But so can COVID-19 and RSV, and nobody argues those out of the category. The virus sets up shop in your respiratory tract. Everything else is your body’s response to it.

Where your immune system actually lives

Here is a sentence you have read on roughly every supplement website in existence: 80% of your immune system is in your gut.

It is not right, and the true version is more interesting. A 2023 census that counted every immune cell in the human body found the gastrointestinal tract holds about 3% of immune cells and 5% of lymphocytes. Most of them are in bone marrow, lymph nodes and the spleen.⁴ The figure everybody quotes traces back to a 2008 review that used two different numbers in adjacent sentences — gut lymphoid tissue as “almost 70% of the entire immune system,” and about 80% of plasma cells living there.⁵ The industry has been blending them ever since.

The accurate version

Your gut holds roughly 70% of your body’s plasma cells — the cells that manufacture antibodies. Not most of your immune system. Most of your antibody factory. That is a real and important thing to be sitting next to your microbes, and it does not need inflating.

There is also a genuine line of communication between gut and lungs. Researchers call it the gut–lung axis, and the biology behind it — immune signaling, microbial metabolites, shared mucosal tissue — is a live and credible field of study.⁶ Two honest caveats: the main review of it is a summary of mechanisms rather than a clinical trial, and it contains no probiotic intervention data at all. It tells you a route exists. It does not tell you that a supplement travels down it.

The same applies to the mechanism research on probiotics and immune signaling. There is a substantial literature on how particular strains shift cytokine profiles, and it is worth reading — but most of it comes from animal models, and the reviewers themselves note that testing these effects in humans “remain[s] a topical issue.”²² A plausible mechanism is a reason to run a trial. It is not a substitute for one.

If you want the full tour of gut and immunity — what your microbes actually do, what knocks them about, and what recovers on its own — that is its own article. This one stays on respiratory infections.

So what do the trials actually show?

This is the part worth reading slowly, because the answer is genuinely favorable — and genuinely modest, and those two things are both true at once.

The Cochrane review

Cochrane reviews are the closest thing evidence-based medicine has to a referee. The current one on probiotics and upper respiratory infections pooled 23 randomized trials and about 6,950 people. Here is what it found, with the certainty ratings the reviewers themselves assigned — because those are the part that usually goes missing.⁷

What Cochrane found, and how confident it is

Fewer people got at least one infection — risk ratio 0.76. Low-certainty evidence.
Fewer people got three or more — risk ratio 0.59. Moderate certainty.
Episodes ran about 1.22 days shorter. Low certainty.
Fewer courses of antibiotics prescribed — risk ratio 0.58. Moderate certainty.
No increase in side effects — risk ratio 1.02. Low certainty.

Cochrane’s own vocabulary is precise about this. Low certainty means “may reduce.” Moderate means “likely reduces.” The headline result — fewer people catching anything — is the low-certainty one.

Two scope limits are worth knowing, and they are easy to miss. The review covers upper respiratory infections only, so it says nothing about pneumonia, bronchitis, COVID-19 or RSV. And it deliberately excluded studies where most participants had been vaccinated against influenza.⁷ Which is a slightly awkward fact for an article with the word “flu” in the title, so we will say it plainly rather than leave it out.

An independent check

The most useful counterweight is a 2022 analysis run by the US Army Research Institute of Environmental Medicine — no supplement company anywhere near it. It pooled 42 reports covering 9,179 adults and found probiotics reduced the risk of getting at least one respiratory infection (risk ratio 0.91), along with total sick days, duration and severity. The authors’ own summary word was “modestly.” They rated most of the underlying studies as having “some concerns” about bias.⁸

Two findings from that analysis that rarely get quoted, and should be. The benefit was not seen in physically active populations at all. And the effect on how long illnesses lasted was largest when the probiotic was delivered in fermented dairy — which is a point about the food it arrives in, not about liquids in general.⁸

A separate 2014 review found illness episodes shorter by almost a day. It is a real finding, and it comes with two honest asterisks: the statistical scatter between studies was very high, and the work was funded by a consortium of probiotic manufacturers including Yakult, Danone and DuPont.⁹

The direction is consistent. The size is small. Both halves of that sentence are the finding.

The most rigorous test was negative

The strongest study design here is to deliberately infect volunteers with a cold virus and see who gets ill. When researchers did exactly that with 334 people, giving half of them a probiotic, illness rates came out at 56% versus 50% — no significant difference.¹⁰

We include this because leaving it out would make everything above look stronger than it is. It is the single most controlled test in the field, and it did not find a benefit.

What about children?

Two young children playing together in a green garden on a summer day

A 2016 analysis of 23 randomized trials in 6,269 children found that children taking probiotics were about 11% less likely to have at least one respiratory infection (risk ratio 0.89).¹¹

Read that as a relative figure, because that is what it is. An 11% relative reduction is not eleven fewer children in a hundred — it is a modest shift in the odds, and the confidence interval runs from about 4% to 18%. The same review found no significant difference in how long each illness lasted.¹¹ Fewer episodes, not shorter ones.

The study people usually cite for children is a 2009 trial in 326 three- to five-year-olds. Six months of two probiotic strains cut the incidence of fever, coughing and runny nose against placebo, and the numbers were large — reductions of 53% to 73%.¹² Three things you should know before you weigh it: the symptoms were reported by parents rather than lab-confirmed, the runny-nose benefit was not statistically significant for the single strain on its own, and the study was run by the company that made the strains. Effect sizes that large have not turned up again in any of the pooled analyses since.

And older adults?

Older couple harvesting fresh lettuce together in a vegetable garden

There is a 2015 trial in 100 healthy adults aged 60 to 74, and it deserves a careful reading because it is often described as more positive than it was.

The trial measured how many days participants had symptoms of infection over four months. On that — its main question — it found no significant difference: 5.1 days on the probiotic against 6.6 on placebo, which with this many people is well inside the range of chance.¹³ The trial did not meet its endpoint.

What it did find was that a subset of 44 participants who gave samples showed measurably higher secretory IgA — a frontline antibody — in saliva and stool. Within that same subset, an additional analysis the researchers ran after the fact suggested fewer respiratory infections.¹³ That is a hypothesis worth following up, not a result to build a claim on, and the authors themselves stuck to the careful phrasing: the strain “may be a safe effective way to stimulate immune responses.” The trial was run by employees of the company that makes the strain.

The gap between the two findings — a measurable change in an immune marker, no measurable change in whether people got sick — is probably the fairest one-line summary of this entire field.

Probiotics and the flu shot

A 2017 meta-analysis looked at whether taking probiotics or prebiotics around the time of a flu vaccination changed how well people responded to it. Pooling nine randomized trials in 623 adults, it found that people who did were more likely to reach protective antibody levels against some influenza strains.¹⁴

Here is what that does and does not mean. The review measured antibody levels in the blood. It did not count a single case of influenza — the authors list this as their first limitation. Results were also inconsistent across strains: three of the six strain-and-outcome combinations they tested came back with no effect at all. And the analysis pooled probiotics together with prebiotics, which are different things.¹⁴

One thing this is definitely not

This is a finding about people who got vaccinated. It is not a reason to skip a flu shot, and no probiotic is a substitute for one. If the vaccine is the thing doing the work, the study is an argument for getting it, not for replacing it.

What about RSV?

You may see a probiotic strain cited in connection with respiratory syncytial virus. The study behind that is real, and it is in mice — oral doses lowered the amount of virus in mouse lungs, and the authors described it as “prophylactic potential.”¹⁵ It was also conducted by employees of the company that owns the strain. Interesting science. Not a human result, and not a reason to buy anything.

The unglamorous things that actually work

Hands lathering with soap in warm daylight beside a window

We sell a probiotic, so you can take the following as slightly against interest, which is rather the point.

The flu vaccine is the single most effective thing you can do about the flu. Washing your hands properly beats almost every supplement ever marketed for this season. Sleep, movement and eating actual plants do more for how your immune system behaves than anything you can buy in a bottle. And staying home when you are ill protects everyone who would otherwise have caught it from you.

None of that is exciting and all of it is better evidenced than the rest of the shelf. A probiotic is something you might add alongside them. It is not a replacement for any of them.

Where Flourish fits

Two women hugging, one holding a bottle of Flourish Junior

Flourish is a living liquid probiotic, delivered in the liquid it was fermented in rather than freeze-dried into a capsule. Each tablespoon carries 11 strains from 5 families, grown together as one culture on non-GMO sugarcane molasses or sweet sorghum, with every batch third-party tested and cleared for delivery of billions of live CFUs per dose.

Because nothing is dried out and reconstituted, the bottle also carries the metabolites the culture produced while it grew — short-chain fatty acids, bioactive peptides, amino acids and enzymes among them.

On the liquid-versus-capsule question, here is the honest state of it. One laboratory study compared eight commercial probiotic products in simulated stomach conditions and found the liquid ones tended to survive better than the freeze-dried ones.¹⁶ That is a test tube, not a person, and Flourish was not one of the eight products tested. We think the format is a good one. We are not going to tell you it has been proven in people, because it has not.

The most important caveat on this page

Probiotic effects are strain-specific. This is not a technicality — it is the central finding of the field. A systematic review looking directly at the question found strong evidence that efficacy is specific both to the strain and to the condition, and warned that pooling different probiotics together produces misleading conclusions.¹⁷ The international consensus definition says the same.¹⁸

Which means: the strains in Flourish have not been tested in trials for respiratory infections. Every study on this page tested a different, specifically named strain. Their results belong to those strains. We are showing you the field, not making a claim about this bottle.

Who Flourish is for

Flourish is a fermented food-grade product made for adults and for children old enough to eat and drink normally.

If you are pregnant or nursing, immunocompromised, seriously unwell or in hospital, or considering a probiotic for an infant or very young child, talk to your doctor or your child’s pediatrician first. Probiotic-associated infections are rare but they are documented, and the people most likely to experience them are those whose health is already compromised.¹⁹ The FDA has not approved any probiotic product for use in infants of any age, and in 2023 the agency warned about the risk of invasive infection in preterm infants given probiotic products.²⁰

What we’d claim for Flourish is digestive and microbiome support, taken consistently — which is why consistency is key. What we wouldn’t claim is that it will stop you catching anything this winter.

Flourish is a dietary supplement made to support digestive and microbiome health. It is not a treatment for the cold, the flu or any other illness, and it is not a substitute for vaccination. Follow the dosage on the label. Check with your doctor before starting a new supplement, and with your pediatrician before giving one to a child.

Bottles of Flourish Original and Flourish Junior resting on a pale stone surface

Ready to try a living probiotic?

Eleven living strains. One daily shot. For the whole family — grown-ups and older kids alike.

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And since taking it regularly is the part that actually matters, that’s what Subscribe & Save is for: 5% off all Subscribe & Save orders, plus free shipping on subscriptions of 2+ Flourish.

A few questions we hear a lot

Will taking a probiotic stop me getting a cold?

No, and nobody can honestly tell you otherwise. The pooled evidence suggests a modest reduction in the chance of getting at least one upper respiratory infection over a season — Cochrane rates that finding as low certainty, and the most controlled test of it came back negative.^7,10

Is the flu a respiratory infection?

Yes. The WHO, the CDC and the international disease classification all place it there.^1,2,3 It produces whole-body symptoms, but the infection itself is respiratory.

Is 80% of my immune system really in my gut?

No. About 3% of your immune cells are in your gastrointestinal tract. What is true — and more interesting — is that roughly 70% of your antibody-producing plasma cells live there.^4,5

Should I take a probiotic instead of getting a flu shot?

No. The one study connecting probiotics to influenza vaccination is a study of people who were vaccinated, and it measured antibody levels rather than actual illness.¹⁴ Nothing in this literature supports skipping the vaccine.

Do the studies on this page apply to Flourish?

Not directly, no. Probiotic effects are strain-specific, and every trial above used a different named strain from the ones in this bottle.^17,18 We would rather tell you that than let you assume otherwise.

Can my kids take it?

Flourish Junior is made for children old enough to eat and drink normally. For infants and very young children, ask your pediatrician — the FDA has not approved any probiotic for use in infants of any age.²⁰

Does the "balance" of my gut bacteria matter?

“Balancing your gut flora” is a phrase the supplement world likes rather more than the research world does — there is no scientifically defined balanced microbiome, and the related term “dysbiosis” has been criticized in the literature as circular.²¹ What is real is that your gut microbes influence immune signaling. That is worth caring about without the tidy metaphor.

About this article

Written by: Kelsy Armstrong, Entegro Health | Originally published: October 2023

Important information

This article is for general education and isn’t a substitute for professional medical advice, diagnosis, or treatment. Probiotic effects are specific to the strain and to the outcome studied; evidence for one product does not transfer to another. Nothing here is a treatment for the cold, influenza or any other infection, and nothing here is a substitute for vaccination. If you are unwell, please speak to a qualified health professional. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Sources

World Health Organization. Influenza (seasonal) fact sheet — “seasonal influenza is an acute respiratory infection caused by influenza viruses.” Source
Centers for Disease Control and Prevention. About Influenza — “flu is a contagious respiratory illness caused by influenza viruses.” Source
World Health Organization. ICD-10 International Statistical Classification of Diseases, Chapter X (Diseases of the respiratory system), block J09–J18, “Influenza and pneumonia.” Source
Sender, R., Weiss, Y., Navon, Y., et al. (2023). The total mass, number, and distribution of immune cells in the human body. PNAS, 120(44), e2308511120 — the GI tract holds about 3% of immune cells and 5% of lymphocytes, while housing roughly 70% of the body’s plasma cells. Source
Vighi, G., Marcucci, F., Sensi, L., Di Cara, G., & Frati, F. (2008). Allergy and the gastrointestinal system. Clinical & Experimental Immunology, 153(Suppl 1), 3–6 — the origin of both the “70%” and “80%” figures, in adjacent sentences. Source
Enaud, R., Prevel, R., Ciarlo, E., et al. (2020). The gut-lung axis in health and respiratory diseases: a place for inter-organ and inter-kingdom crosstalks. Frontiers in Cellular and Infection Microbiology, 10, 9 — a review of mechanisms; contains no probiotic intervention data. Source
Zhao, Y., Dong, B. R., & Hao, Q. (2022). Probiotics for preventing acute upper respiratory tract infections. Cochrane Database of Systematic Reviews, 2022(8), CD006895 — 23 trials, 6,950 participants; all effect estimates and GRADE certainty ratings quoted above; excluded populations more than 50% vaccinated against influenza. Source
Coleman, J. L., Hatch-McChesney, A., Small, S. D., et al. (2022). Orally ingested probiotics, prebiotics, and synbiotics as countermeasures for respiratory tract infections in nonelderly adults: a systematic review and meta-analysis. Advances in Nutrition, 13(6), 2277–2295 — 42 reports, 9,179 subjects; RR 0.91; “modestly” reduce incidence, duration and severity; no benefit in physically active populations. Source
King, S., Glanville, J., Sanders, M. E., Fitzgerald, A., & Varley, D. (2014). Effectiveness of probiotics on the duration of illness in healthy children and adults who develop common acute respiratory infectious conditions: a systematic review and meta-analysis. British Journal of Nutrition, 112(1), 41–54 — episodes shorter by 0.77 days, I² = 80%; funded by the Global Alliance for Probiotics. Source
Turner, R. B., Lehtoranta, L., Hibberd, A., et al. (2021). Effect of Bifidobacterium animalis spp. lactis Bl-04 on rhinovirus-induced colds: a randomized, placebo-controlled, single-center, phase II trial in healthy volunteers. EClinicalMedicine, 43, 101224 — 56% vs 50% illness, not significant. Source
Wang, Y., Li, X., Ge, T., et al. (2016). Probiotics for prevention and treatment of respiratory tract infections in children: a systematic review and meta-analysis of randomized controlled trials. Medicine (Baltimore), 95(31), e4509 — RR 0.89 (95% CI 0.82–0.96) for at least one RTI; no significant difference in episode duration (P = 0.19). Source
Leyer, G. J., Li, S., Mubasher, M. E., Reifer, C., & Ouwehand, A. C. (2009). Probiotic effects on cold and influenza-like symptom incidence and duration in children. Pediatrics, 124(2), e172–e179 — parent-reported symptoms; rhinorrhea reduction not significant for the single strain (P = .68); authors affiliated with the strain manufacturer, Danisco. Source
Lefevre, M., Racedo, S. M., Ripert, G., et al. (2015). Probiotic strain Bacillus subtilis CU1 stimulates immune system of elderly during common infectious disease period: a randomized, double-blind placebo-controlled study. Immunity & Ageing, 12, 24 — primary endpoint not met (5.1 vs 6.6 days, P = 0.2015); secretory IgA increased and the respiratory-infection finding came from a post-hoc analysis in 44 of 100 subjects; authors affiliated with the strain manufacturer, Lesaffre. Source
Lei, W. T., Shih, P. C., Liu, S. J., Lin, C. Y., & Yeh, T. L. (2017). Effect of probiotics and prebiotics on immune response to influenza vaccination in adults: a systematic review and meta-analysis of randomized controlled trials. Nutrients, 9(11), 1175 — 9 of 20 trials pooled, 623 participants; outcomes were seroprotection and seroconversion rates, not clinical influenza; three of six strain-outcome comparisons were null. Source
Eguchi, K., Fujitani, N., Nakagawa, H., & Miyazaki, T. (2019). Prevention of respiratory syncytial virus infection with probiotic lactic acid bacterium Lactobacillus gasseri SBT2055. Scientific Reports, 9(1), 4812 — a mouse study; authors employed and funded by Megmilk Snow Brand. Source
Fredua-Agyeman, M., & Gaisford, S. (2015). Comparative survival of commercial probiotic formulations: tests in biorelevant gastric fluids and real-time measurements using microcalorimetry. Beneficial Microbes, 6(1), 141–151 — eight commercial products tested in vitro; liquid formulations tended to survive better than freeze-dried ones. Source
McFarland, L. V., Evans, C. T., & Goldstein, E. J. C. (2018). Strain-specificity and disease-specificity of probiotic efficacy: a systematic review and meta-analysis. Frontiers in Medicine, 5, 124 — “strong evidence was found supporting the hypothesis that the efficacy of probiotics is both strain-specific and disease-specific.” Source
Hill, C., Guarner, F., Reid, G., et al. (2014). Expert consensus document: the ISAPP consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology & Hepatology, 11(8), 506–514. Source
Hempel, S., Newberry, S., Ruelaz, A., et al. (2011). Safety of probiotics used to reduce risk and prevent or treat disease. AHRQ Evidence Report/Technology Assessment, No. 200 — “participants with compromised health are most likely to experience adverse events associated with probiotics.” Source
U.S. Food and Drug Administration (26 October 2023). FDA raises concerns about probiotic products sold for use in hospitalized preterm infants — “The FDA has not approved any probiotic product for use as a drug or biological product in infants of any age.” Source
Brüssow, H. (2020). Problems with the concept of gut microbiota dysbiosis. Microbial Biotechnology, 13(2), 423–434. Source
Azad, M. A. K., Sarker, M., & Wan, D. (2018). Immunomodulatory effects of probiotics on cytokine profiles. BioMed Research International, 2018, 8063647 — a review of mechanism, largely from animal models; the authors note that investigating these effects in humans “remain[s] a topical issue.” Source