TABLE II.
Recent Meta-analyses Including Randomized Controlled Trials of Healthy Adults to Assess the Effects of Probiotics, Prebiotics, and/or Synbiotics on the Incidence, Duration, and/or Severity of Respiratory Tract Infections (RTI), or Immune Responses to Vaccinations Against RTI-causing Viruses
| Reference | Studies included | Results |
|---|---|---|
| Probiotics | ||
| King et al. 20142,4 | 20 RCTs (8 in healthy non-elderly adults; excluded athletes) Population: Children and adults Intervention: Bifidobacterium or Lactobacillus probiotic Outcome: RTI incidence and duration |
Probiotics reduced duration of illness episodes by 0.8 d [95% CI: 0.04 d, 1.5 d]; (9 studies, n = 3,350) Probiotics reduced days of illness by 0.3 d/person [95% CI: 0.1 d, 0.4 d]; (10 studies, n = 2,647) Probiotics reduced work/school days missed by 0.2 d [95% CI: 0.03 d, 0.3 d]; (10 studies, n = 2,647) |
| Hao et al. 201523 | 13 RCTs (3 in healthy non-elderly adults; excluded athletes) Population: Children and adults Intervention: Any probiotic Outcome: RTI incidence and duration |
Probiotics reduced odds of experiencing ≥1 RTI episodes by 47% [95% CI: 24%, 63%]; (7 studies, n = 1,927) Probiotics did not reduce the rate of RTI episodes (rate ratio = 0.83 [95% CI: 0.7, 1.05]; (5 studies, n = 1,608) Probiotics reduced duration of RTI episodes by 1.9 d [95% CI: 1.7 d, 2.0 d]; (3 studies, n = 831) |
| Lei et al. 20173,4, Yeh et al. 20183,5 | 13 RCTs (5 in healthy non-elderly adults) Population: Healthy and hospitalized adults Intervention: Any probiotic Outcome: Immune responses to influenza vaccination |
Probiotics increased odds of achieving seroprotectiona against H3N2 (OR = 2.7 [95% CI: 1.2, 5.7]), but not H1N1 (OR = 1.7 [95% CI: 0.8, 3.8]) or B-strain (OR = 1.2 [95% CI: 0.6, 2.3]); (3 studies, n = 155) Probiotics increased odds of achieving seroconversionb against H3N2 (OR = 3.5 [95% CI: 1.4, 8.5]) and B-strain (OR = 2.2 [95% CI: 1.2, 4.1]) but not H1N1 (OR = 1.9 [95% CI: 0.7, 5.4]); (4 studies, n = 362) Probiotics increased antibody titers following H1N1 (mean difference from placebo = 4.7 [95% CI: 0.5, 8.9] and H3N2 (16.9 [95% CI: 0.9, 32.8]), but not B-strain (3.0 [95% − 0.8, 6.8]) vaccination; (7 studies, n = 476) |
| Prebiotics | ||
| Lei et al. 20173,4, Yeh et al. 20183,5 | 6 RCTs (1 in healthy non-elderly adults) Population: Healthy and hospitalized adults Intervention: Any prebiotic Outcome: Immune responses to influenza vaccination |
Prebiotics increased odds of achieving seroprotectiona against H3N2 (OR = 3.1 [95% CI: 1.2, 7.7]) and H1N1 (OR = 1.9 [95% CI: 1.1, 3.3]), but not B-strain (OR = 0.8 [95% CI: 0.5, 1.5]); (3 studies, n = 215) Prebiotics did not increase odds of achieving seroconversionb against H3N2 (OR = 1.3 [95% CI: 0.2, 8.0]), H1N1 (OR = 1.0 [95% CI: 0.5, 1.8]) or B-strain (OR = 1.8 [95% CI: 0.9, 2.6]); (2 studies, n = 191) Prebiotics increased antibody titers following H1N1 (mean difference from placebo = 35.5 [95% CI: 0.3, 70.0], but not H3N2 (18.7 [95% CI: −13.2, 50.6]) or B-strain (20.7 [95% CI: −9.1, 50.4]) vaccination; (5 studies, n = 213) |
| Synbiotics | ||
| Chan et al. 20205,8 | 16 RCTs (2 in healthy non-elderly adults) Population: Healthy, any age Intervention: Any synbiotic Outcome: RTI incidence |
Synbiotics reduced the rate of RTI by 16% [95% CI: 4%, 27%]; (9 studies, n = 2,845) Synbiotics reduced the risk of RTI by 16% [95% CI: 5%, 26%]; (7 studies, n = 7,273) |
CI, confidence interval; d, days; OR, odds ratio; RCT, randomized controlled trial; RTI, respiratory tract infection.
a
Seroprotection is defined as an antibody titer superior to an established threshold for clinical protection against the virus.21
b
Seroconversion is defined as achieving a certain fold increase (e.g., 4-fold) in specific antibody titers after vaccination.21