Probiotics: Can Probiotics Prevent URTIs?
By Densie Webb, PhD, RD
Today’s Dietitian
Vol. 25 No. 5 P. 12

Research continues to explore their efficacy in younger and older populations, but more studies are needed before RDs can make recommendations.

Upper respiratory tract infections (URTIs), which account for an estimated 10 million outpatient visits annually, carry both economic and social costs.1 URTIs are defined as viral or bacterial infections that affect the mucosal surfaces of the upper part of the respiratory system, including the sinuses and throat, and include conditions such as laryngitis, sinusitis, otitis media, and the common cold.

Several bacteria strains can cause URTIs, but the most common virus is the rhinovirus. Symptoms include a runny nose, sore throat, cough, and sometimes fever. Treatment for URTIs often includes rest, fluids, and over-the-counter pain relievers. If a bacterial infection is suspected, antibiotics may be prescribed.

To date, there has been little clear evidence for effective preventive measures. However, several studies have suggested that some probiotic supplements or fermented dairy products, if taken regularly, may help prevent URTIs, reduce the frequency of infections, lessen the severity of infections, and reduce duration.

Who’s Most at Risk?
While URTIs are common and anyone can become infected, children are the most at risk, as well as individuals who come in close contact with children, such as those working in daycare centers or schools.

An international study of the occurrence of respiratory tract infections (RTIs), including URTIs, found that occurrence generally is greater in males than females. In the same study, researchers found the incidence rate of RTIs was highest in children under age 5 and decreased with increasing age until incidence again increased among older individuals.2

People with asthma and allergic rhinitis are more likely to develop RTIs. Smokers are at increased risk, as are the immunocompromised. Transplant recipients and postsplenectomy patients also are at high risk. Moreover, anatomical anomalies of the upper respiratory tract or nasal polyposis increase the risk of developing RTIs.

The Research
At least 150 published clinical trials have assessed the potential effects of probiotic consumption in preventing URTIs and improving outcomes. The trials have been conducted in various populations in Italy, Japan, France, Belgium, Spain, Germany, China, Sweden, Malaysia, Australia, Italy, Israel, Croatia, Canada, and the United States.3,4 Several of these studies have shown that probiotics can be effective in lowering the incidence of URTIs, as well as alleviating symptoms by shortening the duration of the infection.3 However, studies have varied greatly in the number of participants, the duration of the intervention, the specific probiotic strain or the combination of strains used, the season of the trial (cold or allergy season), whether subjects had received flu vaccines, and the primary outcomes measured. 3 Several meta-analyses have been conducted, with many studies being included in more than one analysis.

Specifically, research has found positive effects of probiotics against URTIs in otherwise healthy children.4,5 A systematic review and meta-analysis of 17 studies conducted in infants and children used probiotic formulations of either Lactobacillus or Bifidobacterium alone or in combination delivered in a range of foods or supplements for four days to nine months. The analysis found that infants and children diagnosed with or reported by a parent or guardian to have acute RTIs (ie, colds, influenza, sinusitis, pharyngitis, acute bronchitis, pneumonia, acute otitis media, or diarrhea) and who received probiotics were less likely to be prescribed antibiotics compared with those who received placebo.

Another meta-analysis of 22 randomized controlled trials (RCTs) and more than 10,000 participants with median ages of 2 to 84 were followed up for a period ranging from one to 12 months. Researchers found that probiotic-fermented dairy products had a significant protective effect against some RTIs, including pneumonia (a lower RTI) and the common cold (a URTI), but no effect on other RTIs.6

With age, changes in diversity and a loss of resilience in the microbiome typically occur, leading to an increased risk of infection.3 Respiratory infections are the most common reason for hospital admittance and a significant cause of death among older adults (defined in most studies as those aged 65 and older). Studies have been conducted to examine the connection between the two. For example, a systematic review of eight RCTs in older subjects (aged 60 and older) in which only single strain probiotics were given in each of the studies concluded that several lactobacilli probiotics were better than placebo at lowering the incidence or the number of older people experiencing acute URTIs.3 However, in the systematic review and meta-analysis of 22 trials mentioned earlier, the probiotic genus and the duration of probiotic supplementation were found to be the most important factors for whether a beneficial effect occurred, not the age of the participants.6

Studies are lacking in people with overweight and obesity who have increased risks of viral URTIs. However, in one study, 220 participants aged 30 to 65 with BMIs between 25 and 34.9 taking a propriety blend of supplemental probiotics (four types of lactobacilli and bifidobacteria) for six months experienced a 27% reduction in URTI symptoms compared with the control group. A subset of the population younger than 45 or with a BMI less than 30 kg/m2 experienced even greater reductions in symptoms with probiotics. The reduction was seen within two weeks of probiotic use. Gut microbiome diversity remained stable throughout the study in those taking probiotics.7

Monique Richard, MS, RDN, LDN, FAND, IFNCP, RYT, an integrative dietitian nutritionist, owner of Nutrition-In-Sight, and a national media spokesperson for the Academy of Nutrition and Dietetics, says, “It’s important to weigh the pros and cons of supplements vs fermented foods. As nutrition experts, we prioritize food because of the synergistic benefits we know are so significant, so I encourage clients to consume fermented foods (as tolerated, accessible, willing, and appropriate) and foods with active live cultures. There will be certain clients for whom a supplement may be more appropriate, but there are more unknowns with a supplement as far as strength, interactions, possible side effects, effectiveness, etc, whereas with food, there are almost always multiple benefits pretty well understood.”

A recent comprehensive review on the topic of probiotics and URTIs published by the Cochrane Library in 2022 as a follow-up to a Cochrane Library review in 2015 examined 24 RCTs with almost 7,000 participants ranging in age from 1 month to 85 years.8,9 Most of the studies administered one or two strains (Lactobacillus plantarum HEAL9, Lactobacillus paracasei 8700:2 or N1115) and 109 or 1,011 colony-forming units (CFUs)/day for more than three months. The review concluded that probiotics may reduce the number of people diagnosed with at least one URTI in the time period specified in the studies by about 24% and would likely reduce the number of people diagnosed with at least three URTIs by 41% and reduce the duration of acute URTIs by 1.22 days. However, the findings were “very uncertain” on whether treatment with probiotics resulted in a decrease in the number of people absent from childcare centers, schools, or work due to acute URTIs.

Probiotic Delivery Systems and Strains
One important factor in choosing a probiotic is the format or delivery system. Probiotics can be sold as capsules, tablets, or part of fermented foods and drinks.3 However, most studies have examined the potential benefits of consuming yogurt and other fermented dairy products. These have found that consumption of a probiotic fermented dairy product has the potential to prevent RTIs.6

The positive effects of fermented drinks or yogurts with probiotics are attributed to not only the added probiotic but also the ferments or metabolites that result from the proprietary fermentation process from the starter cultures used in yogurt, most commonly Streptococcus thermophilus and Lactobacillus bulgaricus.3 In at least two studies, the fermented drink Actimel (DanActive in the United States) containing Lacticaseibacillus paracasei subsp. paracasei CNCM I-1518, formerly known as Lactobacillus casei DN-114 001, was given to subjects. Changes in nomenclature of probiotic bacteria, as was pointed out in the study using Actimel, present an obstacle to comparing findings of different studies. Both studies reported at least one statistically significant benefit of the probiotic drink on URTIs in older people.10,11

In the recent Cochrane review, probiotics in studies were more likely to be given in milk-based foods to children, powdered form to adults, and in milk-based foods or capsules to seniors.

The type of probiotic strain used in studies, as well as the concentration or CFUs, are important considerations in assessing clinical trials, as well as for consumers choosing a probiotic product. In clinical studies, most strain selections are based on the positive findings from previous trials as well as support from manufacturers that provide products for study.3

In the studies included in the Cochrane review, most used one or two probiotic strains—Lactobacillus plantarum HEAL9 or Lactobacillus paracasei (8700:2 or N1115). Contrary to the systematic review and meta-analysis in infants and children, the systematic review and meta-analysis of 22 RCTs previously cited found that consumption of probiotic fermented dairy products had a protective effect when Lactobacillus and milk were given, but no significant effect was found in studies that used Bifidobacterium-containing dairy drinks or yogurt.6

For consumers, some, but certainly not all, products provide information on the label about its probiotic strains. Fewer provide the CFUs on the product label. But according to Benjamin Mullish, PhD, NIHR, an academic clinical lecturer in the department of metabolism, digestion, and reproduction at Imperial College in London, “We are at too early a phase for us to be able to use any clinical or biological rationale to select one probiotic rather than another in most cases.”

How Probiotics Might Work
The potential of probiotics to prevent URTIs may occur either through the direct action of microbiota components on pathogens or through a number of interactions with the immune system. Manipulation of the microbiome with probiotics potentially may protect against respiratory infections via a gut-lung connection—a sort of crosstalk between the gut microbiota and the lungs.

Several mechanisms have been suggested for how probiotics may boost the immune system and help prevent and reduce the severity of URTIs, including the following:

• Probiotics promote the production of immune cells in the lower intestinal tract.

• Probiotics suppress inflammatory immune cells.

• Probiotics stimulate components of both innate and acquired immunity. Two studies found an increase in natural killer cells and IFN-gamma in the groups receiving probiotics,12,13 while two other studies found no significant differences.10,14

“At the moment, we are still at a very nascent stage of trying to pick exactly which bacteria—and by which specific mechanisms—[probiotics] exert their effects. It may be that a further consideration is ‘host microbiome interactions.’ In other words, different probiotics may be suited for different patient groups,” Mullish says.

Bottom Line
In short, the research isn’t there yet. “We just don’t know enough right now about all the details of the gut microbiome and the interaction of specific strains with an individual’s own unique makeup, how effective it is, or how long it lasts, but the research is evolving and it’s exciting to keep learning,” Richards says.

The findings of several studies suggest that under certain conditions, in certain age groups, some probiotic bacteria in specific delivery systems may help reduce the incidence, length, and severity of URTIs. However, comparison of findings is difficult because of the wide variety of probiotic strains used and their CFUs (which isn’t always stated), the changes in bacteria nomenclature over time, the length of time a probiotic has been administered, and the differences in the delivery system (eg, supplements, yogurt, or fermented drinks).

With that in mind, researchers have stated that future RCTs should be designed to assess outcomes commonly reported in previous clinical studies to make comparisons possible and incorporate longer interventions to allow time for a potential improvement in the immune system to occur. More studies especially are needed in older populations.3,9 However, studies conducted to date provide support for further trials to assess the potential role of probiotics in preventing viral URTIs, particularly in older adults and people with overweight and obesity.7

— Densie Webb, PhD, RD, is a freelance writer, editor, and industry consultant based in Austin, Texas.


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7. Mullish BH, Marchesi JR, McDonald JAK, et al. Probiotics reduce self-reported symptoms of upper respiratory tract infection in overweight and obese adults: should we be considering probiotics during viral pandemics? Gut Microbes. 2021;13(1):1-9.

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11. Turchet P, Laurenzano M, Auboiron S, Antoine JM. Effect of fermented milk containing the probiotic Lactobacillus casei DN-114001 on winter infections in free-living elderly subjects: a randomized, controlled pilot study. J Nutr Health Aging. 2003;7(2):75-77.

12. Makino S, Ikegami S, Kume A, Horiuchi H, Sasaki H, Orii N. Reducing the risk of infection in the elderly by dietary intake of yoghurt fermented with Lactobacillus delbrueckii ssp. bulgaricus OLL1073R-1. Br J Nutr. 2010;104(7):998-1006.

13. Lefevre M, Racedo SM, Ripert G, et al. Probiotic strain Bacillus subtilis CU1 stimulates immune system of elderly during common infectious disease period: a randomized, double-blind placebo-controlled study. Immun Ageing. 2015;12:24.

14. Fonollá J, Gracián C, Maldonado-Lobón JA, et al. Effects of Lactobacillus coryniformis K8 CECT5711 on the immune response to influenza vaccination and the assessment of common respiratory symptoms in elderly subjects: a randomized controlled trial. Eur J Nutr. 2019;58(1):83-90.