Table 2. Study characteristics of included clinical studies.

Author, year	Study design	Sample size	Characteristics of participants	Virus	Intervention	Comparator	Quality assessment	Outcome
Turner, 2017	Randomized, double-blind, placebo controlled, parallel trial	115	Healthy adult volunteers serum neutralizing antibody titer of ≤1:4	Rhinovirus 100 tissue culture infectious dose 50 (TCID50) of virus by intranasal drops	Bifidobacterium animalis subsp. lactis Bl-04 (Bl-04) a minimum of 2 × 109 cfu of Bl-04 mixed with 1 g of sucrose for 33 days	1 g of sucrose	6	CXCL8↓, CXCL10, G-CSF concentration, virus titer↓, NO, total symptom score
Tapiovaara, 2016	Randomized, double-blind, placebo-controlled, pilot study	50	Healthy subjects aged 18–65 years	Human rhinovirus (HRV)39 100–300 tissue culture infectious dose (TCID)50	Juice enriched with live or heat-inactivated L. rhamnosus GG for 6 weeks	Control juice	4	Virus titer↓, NO, total symptom score↓
Luoto, 2014	Randomized, double-blind, placebo-controlled trial	68	Preterm infants gestational age between 32 + 0 and 36 + 6 weeks birth weight greater than 1500 g	Rhinovirus	Prebiotics (galacto_x0002_oligosaccharide and polydextrose mixture, 1:1) probiotic (L. rhamnosus GG, ATCC 53103), between days 3 and 60 of life	Microcrystalline cellulose	6	The incidence of RTIs↓, rhinovirus infections↓, virus load↓,
Ettorre, 2020	Randomized, double blind, placebo-controlled, pilot trial	59	Healthy volunteers aged 18–65 years	Rhinovirus immunotype 39 100–300 tissue culture infectious dose (TCID)50	Live or heat-inactivated L. rhamnosus GG for 6 weeks	Carrier juice	4	Diarrhea and other symptoms↓, estimated risk of developing respiratory failure↓, prevalence of patients transferred to ICU and mortality↓
Kumpu, 2015	Randomized, placebo-controlled, double-blind, experiment	213	Healthy adults (30–59 years old) from Japan female:male = 121:92	Human influenza virus A/H1N1 (A/PR/8/34)	Lactococcus lactis ssp. lactis JCM5805 for 10 weeks	Placebo beverage	5	Occurrence and severity of cold symptoms↓, number of subjects with rhinovirus infection↓
Tetsu, 2015	Randomized, placebo-controlled, double-blind, experiment	213	Healthy adults (30–59 years old) from Japan female:male = 121:92	Human influenza virus A/H1N1 (A/PR/8/34)	Lactococcus lactis ssp. lactis JCM5805 for 10 weeks	Placebo beverage	5	Major symptoms of an influenza-like illness↓, IFN-α↑, IGSF15↑

Quality assessment according to the Cochrane Risk Assessment Scale.

Abbreviations: IFN, interferon; TCID, tissue culture infective dose.

↑, the effect in intervention group was greater than control group; ↓, the effect in intervention group was smaller than control group.