The breastmilk secretory IgA (sIgA) capsular antibody was associated with lower risks for late-onset disease from group B streptococcal (GBS) invasive disease, according to study data published in Clinical Infectious Diseases.

Using a matched case-control study in infants <3months of age in Johannesburg, South Africa, investigators collected breastmilk samples from case and control participants, matched for gestational age, maternal age, and HIV-status at time of enrollment. Capsular serotype Ia, Ib, III, and V sIgA antibody concentrations were also measured.

Samples were available for 31 cases of late-onset GBS infection: 8 were a result of serotype Ia, 23 due to serotype III, 10 due to serotype Ia, and 11 due to serotype III. Investigators also obtained samples from women who had recto-vaginal colonization as matched controls (10 serotype Ia and 11 serotype III), as well as 84 serotype Ia samples and 105 serotype III samples from non-colonized matched controls. Using a Bayesian model to estimate the probability of disease, investigators found an approximate 90% reduction in the risk for developing serotype Ia and III late-onset disease with sIgA concentrations ≥0.14 μg/mL and ≥2.52 μg/mL, respectively.

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Investigators noted that this was a small sample size but Bayesian analysis can still be appropriately undertaken using small samples. However, Bayesian analysis estimates are highly sensitive to the specification of the prior distribution and certain assumptions need to be made. Also, the study was primarily designed to determine serum capsular serocorrelates of protection against invasive GBS disease and controls were matched for variables that have been shown to influence serum antibody concentrations in the mother or infant.

According to investigators, these data suggest that a maternal GBS vaccine able to induce serotype-specific sIgA responses might offer supplementary protection against invasive disease than that conferred through transplacental serum IgG alone.

Reference

Dangor Z, Khan M, Kwatra G, et al. The association between breastmilk group B Streptococcal capsular antibody levels and late-onset disease in young infants [published online May 6 2019]. Clin Infect Dis. doi:10.1093/cid/ciz360