Table 1.

Studies that evaluated the association between breastfeeding and infection risk in childhood.

Author, Year of Publication	Type of Infection	Country/Continent	Type of Study	Cases, n	Length of Follow-Up/Age of Enrolled Children/Data of Literature Search	Results
Zhu, Q. et al., 2012 [31]	Hand, foot, and mouth disease	China	Cross-sectional	372	Age of children: 6 months–6 years	Prolonged exclusive breastfeeding (OR 0.401) was a protective factor for the incidence of fever.
Netzer-Tomkins, H. et al., 2016 [32]	Neonatal fever	Israel	Retrospective case–control	140	Age of infant: <1 month	Hospitalized children had a 2.5-fold increased risk of not being exclusively or predominantly breastfed (OR 2.49).
Boccolini, C.S. et al., 2011 [33]	Pneumonia	Brazil	Ecological	642,792	Age of infants: <1 year	Breastfeeding prevalence among children between 9 and 12 months old and exclusive breastfeeding prevalence among children under 6 months old were associated with a lower rate ratio of hospitalization for pneumonia (rate ratio 0.62 and 0.52, respectively).
Nascimento, R.M.D. et al., 2021 [34]	Pertussis-like illness	Brazil	Case–control	267	Age of infants: <6 months	The protective effect of breastfeeding was of 74%. Children younger than six months, who were exclusively breastfed and with mothers vaccinated against pertussis during pregnancy were 5 times less likely to develop pertussis-like illness, corresponding to a protection of 79%.
Jang, M.J. et al., 2020 [35]	RSV bronchiolitis	Korea	Retrospective study	411	Age of infants: <1 year	The OR for oxygen therapy was significantly higher in the artificial-milk-formula-fed group than in the breast milk feeding group (adjusted OR 3.807).
Gómez-Acebo, I. et al., 2021 [36]	Bronchiolitis	Spain	Cohort	969	Length of follow-up: 1 year of life	At 4 months, exclusive breastfeeding reduced the number of episodes of bronchiolitis by 41% (IR 0.59) and mixed feeding by 37% (IR 0.63). An early swap to mixed breastfeeding before months 2 or 4 was associated with a reduced number of episodes of bronchiolitis when compared with infant formula alone.
Jansen, S. et al., 2020 [37]	Respiratory infections	Indonesia	Retrospective case–control	100	Age of infants: 7–12 months	Non-breastfed infants were at 14 times greater risk of contracting respiratory infections.
Wang, J. et al., 2017 [38]	Respiratory infections	UK	Cohort	4040	Length of follow-up: 2 years of life	Breastfeeding for >6 months was protective against bronchiolitis (OR 0.72).
Pandolfi, E. et al., 2019 [39]	Respiratory infections	Italy	Case–control	496	Age of infants: <6 months	Exclusive breastfeeding at symptom onset was associated with a higher risk of viral respiratory infection in the first 6 months of life (OR 3.7), but protection increased with breastfeeding duration (OR 0.98).
Vereen, S. et al., 2014 [40]	Respiratory infections	USA	Cross-sectional	629	Median infant age: 3 months	Breastfeeding (ever vs. never) was associated with decreased relative odds of a lower versus upper acute viral respiratory tract infection in the first year of life (OR 0.64).
Tromp, I. et al., 2017 [41]	Respiratory infections	The Netherlands	Cohort	5322	Length of follow-up: 4 years of life	Breastfeeding for ≥6 months was significantly associated with a reduced risk of lower respiratory tract infection up to 4 years of age (OR 0.71).
Zivich, P. et al., 2018 [42]	Respiratory infections and diarrhea	Democratic Republic of Congo	RCT	931	Length of follow-up: 6 months of life	Implementation of Baby-Friendly Hospital Initiative steps 1–9 was associated with a decreased incidence of reported diarrhea (IRR 0.72) and respiratory illness (IRR 0.48), health facility visits due to diarrhea (IRR 0.60) and respiratory illness (IRR 0.47) in the first 6 months of life.
Guo, C. et al., 2020 [43]	Respiratory infections and diarrhea	China	Longitudinal	41	Length of follow-up: 1 years of life	Breastfeeding was significantly associated with a lower incidence of respiratory infections and diarrhea in children born from vaginal delivery or cesarean section.
Yamakawa, M. et al., 2015 [44]	Respiratory infections and diarrhea	Japan	Longitudinal	43,367	Length of follow-up: 42 months of life	Breastfeeding was not associated with reduced risk of hospitalization for diarrhea. Breastfeeding was associated with reduced risk of hospitalization for respiratory infections between ages 30 and 42 months (OR of exclusive breastfeeding 0.76).
Raheem, R.A. et al., 2017 [45]	Respiratory infections and diarrhea	Australia	Cohort	458	Length of follow-up: 6 months of life	The risk of acquiring respiratory infections is significantly reduced when the infants were predominantly breastfed for 3 months (OR 0.56) and 6 months (OR 0.45). The risk of getting diarrhea is significantly reduced even when the babies were partially breastfed for 6 months (OR 0.31).
Tarrant, M. et al., 2010 [46]	Respiratory and gastrointestinal infections	Honk Kong	Cohort	8327	Length of follow-up: 8 years of life	Breastfeeding for ≥3 months was associated with a lower risk of hospital admission in the first 6 months of life for respiratory infections (hazard ratio 0.64), gastrointestinal infections (0.51), and any infection (0.61).
Frank, N.M. et al., 2019 [47]	Respiratory and gastrointestinal infections	USA	Prospective longitudinal study	6861	Length of follow-up: 4 years of life	At 3–6 months of age, breastfeeding was found to be inversely associated with the odds of respiratory infections with fever (OR 0.82), otitis media (OR 0.76), and infective gastroenteritis (OR 0.55). Between 6 and 18 months of age, breastfeeding continued to be inversely associated with the odds of ear infection and infective gastroenteritis, and additionally with the odds of conjunctivitis, and laryngitis and tracheitis.
Nakamura, K. et al., 2020 [48]	Gastrointestinal infections	Japan	Cohort	31,578	Length of follow-up: 18 months of life	Exclusively breastfed late preterm infants did not show an increased risk of hospitalization for gastrointestinal infection.
Morales, E. et al., 2012 [49]	Various infections	Spain	Cohort	580	Length of follow-up: 14 months of life	In comparison with never breastfeeding, predominant breastfeeding for 4–6 months was associated with lower risk of wheezing (OR 0.53), low respiratory tract infections (OR 0.51) and atopic eczema (OR 0.58) between months 7 and 14 of life. Predominant breastfeeding for 4–6 months was associated with lower risk of gastroenteritis during the first 6 months of life (OR 0.34)
Quigley, M.A. et al., 2016 [50]	Various infections	UK	Cohort	15,809	Length of follow-up: 9 months of life	Exclusive breastfeeding for <4 months was associated with an increased risk of chest infection (risk ratios 1.24–1.28) and diarrhea (risk ratios 1.42–1.66). There was also an excess risk of the chest infection (risk ratios 1.19) and diarrhea (risk ratios 1.66) among infants exclusive breastfed for 4–6 months who stopped breastfeeding by 6 months.
Størdal, K. et al., 2017 [51]	Various infections	Norway	Cohort	70,511	Length of follow-up: 18 months of life	Higher risk of hospitalization was observed in breastfed children ≤ 6 months compared to ≥12 months (RR 1.22).
Davisse-Paturet, C. et al., 2020 [52]	Various infections	France	Cohort	1603	Length of follow-up: 8 years of life	Compared with never breastfed infants, ever-breastfed infants were at a lower risk of diarrhea events in early infancy as well as infrequent events of bronchitis/bronchiolitis throughout infancy. Only predominant breastfeeding duration was related to frequent events of bronchitis/bronchiolitis and infrequent events of otitis.
Davisse-Paturet, C. et al., 2019 [53]	Various infections	France	Cohort	9703	Length of follow-up: 2 years of life	Any breastfeeding for <3 months was associated with higher risks of hospitalizations from gastrointestinal infections or fever. Predominant breastfeeding for <1 month was associated with higher risk of a single hospital admission. Ever breastfeeding was associated with lower risk of antibiotic use.
Christensen, N. et al., 2020 [54]	Various infections	Denmark	Cohort	815	Length of follow-up: 3 years of life	Adjusted incidence rate ratio (IRR) for hospitalization due to any infection decreased with a longer duration of any breastfeeding (IRR 0.96; 0.88 for exclusively breastfed infants). The strongest associations between the duration of any breastfeeding and hospitalizations due to infection were found within the first year of life and for lower respiratory tract infections.
Ardiç, C. et al., 2018 [55]	Various infections	Turkey	Cohort	411	Length of follow-up: 5 years of life	Infants breastfed longer than 12 months had less acute otitis media and acute gastroenteritis when compared with the infants breastfed less than 12 months.
Li, R. et al., 2014 [56]	Various infections	USA	Prospective longitudinal	1281	Length of follow-up: 6 years of life	Children breastfed for ≥9 months had lower odds of past-year ear (OR 0.69), throat (OR 0.68), and sinus (OR 0.47) infections compared with those breastfed >0 to <3 months.
Mulatu, T. et al., 2021 [57]	Various infections	Ethiopia	Nationally representative survey	1034	Age of infants: <6 months	Compared to infants who were non-exclusively breastfed, the odds of having an illness with fever in the last 2 weeks among infants who were exclusively breastfed decreased by 66% (OR 0.34). Exclusively breastfed infants had lower odds of having an illness with cough (OR 0.38) and having diarrhea (OR 0.33) compared to non-exclusively breastfed infants.
Ladomenou, F. et al., 2010 [58]	Various infections	Greece	Prospective study	926	Length of follow-up: 12 months of life	Infants exclusively breastfed for 6 months presented with fewer infectious episodes than their partially breastfed or non-breastfed peers (OR 0.58 for respiratory infections and 0.37 for acute otitis media). Prolonged exclusive breastfeeding was associated with fewer infectious and fewer admissions to hospital for infection in the first year of life.

OR: odds ratio; RR: relative risk; IRR: incidence rate ratio; IR: incidence ratio; RCT: randomized controlled trial.