Antibodies Found in Lactating Women Vaccinated Against COVID-19


Investigators from Spain contribute to the dialogue surrounding postvaccine antibodies with a study that looks exclusively at the COVID-19 vaccine’s affect on breastfeeding mothers.

A recent study conducted at the Parc Sanitari Sant Joan de Déu in Spain suggested that breastmilk from mothers who were vaccinated with the Pfizer-BioNTech vaccine for COVID-19 contained specific antibodies against the virus.

The investigators, led by Erika Esteve-Palau, MD, PhD also found that breast milk IgG(S1) levels increased with the second dose of the vaccine and were positively associated with corresponding serum levels.

Prior to the study, questions had been raised regarding the implications of the COVID-19 pandemic on individuals who were breastfeeding. Further questions had been raised on the benefits and risks of the COVID-19 vaccines themselves.

In previous studies, the presence of anti-COVID-19 antibodies had been reported in the breast milk of women who had contracted the virus.

Additionally, several studies had demonstrated the passage of postvaccine antibodies through breast milk in women vaccinated with novel mRNA-based vaccines.

With the newest prospective study, investigators form the Parc Sanitari Sant Joan de Déu set out to characterize the levels of specific COVID-19 antibodies in the breast milk of mRNA-vaccinated women across time, as well as their correlation with serum antibody levels.

The Study

Esteve-Palau and colleagues carried out the study according to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline, which was approved by the ethics committee of the Sant Joan de Déu Research Foundation.

Between February and March 2021, they enrolled 33 lactating women older than 18 years who were vaccinated against COVID-19 with the Pfizer-BioNTech vaccine.

Serum and breast milk samples were taken from each participant at 3 separate points during the study: 2 weeks after the first vaccine dose, 2 weeks after the second vaccine dose, and 4 weeks after the second vaccine dose.

A total of 93 serum samples were collected from the 33 participants.

Additionally, all participants underwent nasopharyngeal COVID-19 rapid antigen testing (Ag-RDT), which determined levels of immunoglobin (Ig) G antibodies against the spike protein (S1 subunit) and against the nucleocapsid (NC) of the virus.

The Results

After confirming that there were no COVID-19 infections prior to the study, the investigators determined the IgG levels from the samples provided.

Median IgG(S1) levels for serum–milk pairs at each time point were 519 (234-937) to 1 (0-2.9) arbitrary units (AU) per mL for time point 1, 18 644 (9923-29 264) to 78 (33.7- 128) AU/mL for time point 2, and 12 478 (6870-20 801) to 50.4 (24.3-104) AU/mL for time point 3.

In keeping with previous studies, Esteve-Palau and colleagues’ suggested that specific antibodies existed in the breast milk of participating women, along with positive serum levels.

The investigators noted the limitations of the study, such as a small sample size. They also urged researchers to study whether breast milk antibody levels decreased or plateaued after vaccination, and if the findings could be reproduced in other mRNA and non-mRNA vaccines.

“Larger prospective studies examining these issues are needed to confirm the safety of SARS- CoV-2 vaccination in individuals who are breastfeeding and further assess the association of vaccination with infants’ health and SARS-CoV-2-specific immunity,” the team said.

The study, “Quantification of Specific Antibodies Against SARS-CoV-2 in Breast Milk of Lactating Women Vaccinated With an mRNA Vaccine,” was published online at JAMA Network Open.

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