The Secretory IgA Response in Human Milk Against the SARS-CoV-2 Spike Is Highly Durable and Neutralizing for At Least 1 Year of Lactation Postinfection

Abstract

Background: Although in the early pandemic period COVID-19 pathology among young children and infants was typically less severe compared with that observed among adults, this has not remained entirely consistent as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants have emerged. There is an enormous body of evidence demonstrating the benefits of human milk antibodies (Abs) in protecting infants against a wide range of enteric and respiratory infections. It is highly plausible that the same holds true for protection against SARS-CoV-2 as this virus infects cells of the gastrointestinal and respiratory mucosae. Understanding the durability of a human milk Ab response over time after infection is critical. Objective: Previously, we examined the Abs present in milk of those recently infected with SARS-CoV-2 and concluded that the response was secretory immunoglobulin A (sIgA) dominant and that these titers were highly correlated with neutralization potency. The present study aimed to monitor the durability of the SARS-CoV-2 IgA and secretory Ab (sAb) response in milk from COVID-19-recovered lactating individuals over 12 months in the absence of vaccination or reinfection. Results: This analysis revealed a robust and durable spike-specific milk sIgA response, and at 9-12 months after infection, 88% of the samples exhibited titers above the positive cutoff for IgA and 94% were above the cutoff for sAb. Fifty percent of participants exhibited less than twofold reduction of spike-specific IgA through 12 months. A strong, significant positive correlation between IgA and sAb against spike persisted throughout the study period. Nucleocapsid-specific Abs were also assessed, which revealed significant background or cross-reactivity of milk IgA against this immunogen, as well as limited/inconsistent durability compared with Spike titers. Conclusion: These data suggest that lactating individuals are likely to continue producing spike-specific Abs in their milk for 1 year or more, which may provide critical passive immunity to infants against SARS-CoV-2 throughout the lactation period.

Keywords: COVID-19; SARS-CoV-2; antibodies; human milk; lactation; secretory IgA.

FIG. 1.

Longitudinal human milk IgA and sAb reactivity against SARS-CoV-2 spike demonstrates significant durability over time. SARS-CoV-2 Ab levels were measured using an optimized Luminex assay, as described in the Materials and Methods section. (A, B) Titration curves of IgA and sAbs 3–8 weeks postinfection; (A) IgA and (B) sAbs. (C, D) Individual endpoint titers over time; (C) IgA and (D) sAbs. Dotted lines: positive cutoff value. (E, F) Grouped endpoint titers with mean values and ANOVA for change over time; (E) IgA and (F) sAbs. Dotted lines: positive cutoff value. (G, H) Relative endpoint titers over time compared with 3–8 weeks postinfection; (G) IgA and (H) sAbs. Dotted line indicates relative titer of 1, meaning no change. PI, postinfection; sAbs, secretory antibodies; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

FIG. 2.

Longitudinal human milk IgA and sAbs against SARS-CoV-2 nucleocapsid are maintained less consistently compared with spike Abs and subject to significant background reactivity. (A–C) Titration curves of IgA and sAbs 3–8 weeks postinfection; (A) IgA, (B) sAbs, and (C) the high background IgA of COVID-19-naïve milk, which hinders determination of a positive cutoff value. MFI at 1/18 dilution (screening dilution). (D, E) Individual endpoint titers over time; (D) IgA and (E) sAbs. (F, G) Grouped endpoint titers with mean values and ANOVA for change over time; (F) IgA and (G) sAbs. Relative endpoint titers over time compared with 3–8 weeks postinfection. (H) IgA and (I) sAbs. Dotted line indicates relative titer of 1, meaning no change. MFI, median fluorescence intensity; PI, postinfection.

FIG. 3.

Spike-specific human milk IgA and sAbs remain highly correlated throughout the study period, while nucleocapsid Abs exhibit moderate correlation and spike versus nucleocapsid Ab reactivities are unrelated. (A) Spike IgA versus sAbs 3–8 weeks postinfection. (B) Nucleocapsid IgA versus sAbs 3–8 weeks postinfection. (C) Correlation between spike IgA and sAbs at the end of the study period. (D) Spike versus nucleocapsid IgA 3–8 weeks postinfection. (E) Spike versus nucleocapsid sAbs 3–8 weeks postinfection. Endpoint titers were compared using Pearson correlation analysis.

FIG. 4.

SARS-CoV-2-neutralizing Ab in human milk is maintained for at least 9 months after infection. A proxy neutralization assay that measures Abs specific for the spike RBD, which block the RBD-ACE2 interaction, was performed using a premade kit according to instructions (GenScript). Dotted line: background neutralization level of COVID-19-naïve milk. Paired t-test was used for the analysis. PI, postinfection; RBD, receptor-binding domain.