What is the impact of maternal SARS-CoV-2 booster vaccination on antibodies in blood and breast milk?

By Neha MathurJun 15 2023Reviewed by Lily Ramsey, LLM

In a recent article published in PLOS One Journal, researchers investigated the impact of maternal coronavirus disease 2019 (COVID-19) vaccination on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies in the milk of lactating mothers and mother and infant blood.

Study: Impact of maternal SARS-CoV-2 booster vaccination on blood and breastmilk antibodies. Image Credit: avelIlyukhin/Shutterstock.com

Background

In the United States of America (USA), infants under six months are not eligible to receive COVID-19 vaccines. However, under-one-year-olds are more vulnerable to COVID-19-related hospitalization, respiratory support, and even death compared to one to four-year-old children.

Trans-placental transfer of the anti-receptor binding domain (RBD) and neutralizing immunoglobulin G (IgG) antibodies from an immunized mother to her infant could protect against SARS-CoV-2 infections in infants. COVID-19 vaccination during pregnancy is associated with a 50% reduced risk of infant hospitalization.

Data on the persistence of these antibodies following delivery is limited. Further, the ability of milk antibodies to protect infants against SARS-CoV-2 infection is less understood. Furthermore, data on the impact of maternal boosting on breast milk and infant blood antibody titers is limited.

The researchers hypothesized that infants born to COVID-19-vaccinated women during pregnancy had persistent anti-RBD antibodies in their blood, and while the booster vaccine markedly increased milk anti-RBD antibodies compared to pre-booster samples, the same elevations did not occur in infant blood titers. 

About the study

So in the present study, researchers collected milk and blood samples from a cohort of lactating women and their infants recruited in this study between April 2021 and September 2021. These women, aged ≥18 years and lactating, were provided longitudinal breastmilk samples up to 12 months after their primary COVID-19 vaccination.

Likewise, the team enrolled women who received a primary COVID-19 vaccination series and a booster vaccine and consented to provide at least one blood and milk sample before boosting and 15 to 35 days post-boosting in a substudy collecting longitudinal blood samples in addition to milk.

Some women agreed to provide additional paired breastmilk and blood samples <30 days before and 14 to 35 days after boosting. 

They measured and compared the titers of anti-nucleoprotein (NP) and anti-RBD IgG and IgA in maternal milk and maternal and infant blood before and after the maternal booster vaccination using an indirect enzyme-linked immunosorbent assay (ELISA).

The assay had a sensitivity and specificity of 93.4% and 89.3% and a limit of detection (LoD) of 100. Its cut-off titers for anti-SARS-CoV-2 RBD IgG and IgA were ≥900 and ≥300, respectively.

Likewise, LoD was set to one for the milk assay, and its positive cut-off titer values for milk IgG and secretory IgA were ≥4 and ≥8, assessed using breastmilk samples collected between 2018 and 2019 from a human milk biobank.

Additionally, the team gathered baseline and COVID-19 infection data from all participants using self-reported online surveys during each milk sample collection.

Results

In this analysis, the authors demonstrated that lactating women vaccinated with the primary series of a COVID-19 vaccine and then boosted with mRNA-based vaccine augmented the breastmilk IgG and IgA tigers compared to milk samples taken before boosting. 

These results were encouraging, but the authors were unsure if these increased titers correlated with increased infant protection. The increase in Ig titers was detectable in two weeks post-vaccination; likewise, titers for anti-RBD antibodies peaked between three to four months and began to wane afterward. 

In pregnant women with a history of COVID-19, IgG and IgA titers remained higher for a prolonged duration; however, these differences were insignificant. 

Most infants born to vaccinated women (with primary SARS-CoV-2 vaccination series) had substantial transplacental antibodies five months post-delivery.

This data suggested that optimal antibody placental transfer might occur due to vaccination in the second trimester of pregnancy. This reinforces the current recommendations for all pregnant and lactating females to receive COVID-19 vaccine boosters.

Finally, the researchers showed that despite increased anti-RBD antibodies in breast milk after maternal boosting, there was no corresponding increase in titers of anti-RBD antibodies in infant blood.

It indicated that the gut mucosa poorly absorbed these immunoglobulins transferred via the placenta.

Conclusions

To summarize, the study results demonstrated that mothers who received mRNA-based COVID-19 booster vaccines generated robust and long-lasting IgG and IgA milk antibodies; however, the researchers did not detect these antibodies in infant blood. 

COVID-19 vaccination during pregnancy protects infants aged up to five months from COVID-19 by eliciting significant levels of IgG, a period of vulnerability. This data is clinically relevant and could help clinicians encourage pregnant and lactating women to seek primary and booster vaccines to protect themselves and their children based on solid evidence.

Further studies are warranted to investigate the clinical significance of anti-SARS-CoV-2 antibodies in milk and the optimal time for vaccinating pregnant females to prevent COVID-19 in infants.