Pregnant and lactating women have been excluded from all clinical trials with the new messenger ribonucleic acid (mRNA) vaccines against the novel severe acute respiratory syndrome coronavirus 2 (SARS- CoV-2). The presence of antibodies to SARS-CoV-2 in breastmilk and its immunological protection is well-known.
A new study published in BMC Pregnancy and Childbirth studies the presence of antibodies in breastmilk and the immune responses of lactating women when compared to non-lactating women after vaccination using the BNT162b2 Pfizer-BioNTech vaccine.
Study: COVID-19 mRNA vaccine and antibody response in lactating women: a prospective cohort study. Image Credit: Pixel-Shot / Shutterstock.com
Vaccination and human milk
Neonates have an immature immune system. In the first months of their life, they receive immunological protection through breastfeeding.
There are several characteristics of lactating mothers that may affect the presence of antibodies in human milk. Some of these can include previous infections, age, genetic factors, and immunity.
Human milk contains secretory immunoglobulin A (IgA) (SIgA), IgM, and IgG antibodies. SIgA represents 90% of the antibodies present in breastmilk and is essential in defending mucous membranes.
The American College of Obstetricians and Gynecologists have declared that lactating women should be offered the coronavirus disease 2019 (COVID-19) vaccines. Further, the Academy of Breastfeeding Medicine does not recommend lactating women stop breastfeeding after receiving the COVID-19 vaccine.
There is no data on the potential risks or benefits to babies breastfed by women vaccinated against COVID-19. However, vaccination against influenza during pregnancy causes specific antibody production through lactation. The same principle may be extrapolated to mRNA vaccines.
A previous study on 31 lactating women suggests that COVID-19 mRNA vaccines result in humoral immune responses in lactating women which are comparable to those observed in non-pregnant populations. This study also confirmed the presence of anti-SARS-CoV-2 IgG antibodies are present in breastmilk.
In the current study, the researchers aimed to determine the blood antibody profile of lactating women as compared to non-lactating women after vaccination with the Pfizer vaccine. Furthermore, the researchers were interested in determining whether antibodies can transfer to the infant through breastmilk.
Measuring antibody response after vaccination
In the current prospective cohort study, 24 healthcare workers were included, of which 14 were lactating and 10 were non-lactating women. All eligible participants underwent vaccination with the BNT162b2 Pfizer vaccine.
Clinical data related to demographics, date and type of delivery, breastfeeding details, the timing of COVID-19 vaccine doses, and post-vaccination symptoms were registered through a written questionnaire.
A blood sample was collected from all participants one to three weeks after they had received the first and second dose of vaccination. Simultaneously, breastmilk was collected from lactating women on the same days.
Levels of anti-SARS-CoV-2 antibodies including spike IgG, IgA, and IgM were quantified in post-vaccination blood and breastmilk. Antibody quantification was performed by enzyme-linked immunosorbent assay (ELISA) to the SARS-CoV-2 trimeric spike protein.
Post-vaccination antibodies in serum and breastmilk
Both lactating and non-lactating women exhibited no differences in terms of age, smoking habits, comorbidities, and post-vaccination side effects. Furthermore, none of these participants reported a prior SARS-CoV-2 infection.
All women developed immunity after vaccination and showed positive serum IgM, IgA, and IgG antibodies after the second dose. IgG levels were dominant in both lactating and non-lactating women. However, the non-lactating women had higher levels of IgG antibodies after the first dose. In lactating women, IgM and IgG antibodies increased after the 2nd dose.
The breastmilk of lactating women showed modest levels of antibodies, particularly IgG at 42.9%. Notably, high levels of IgG were associated with a longer duration of breastfeeding, although this association was moderate.
Limitations of the study
No antibody testing was performed before vaccination. Therefore, it is possible that the participants were previously infected by SARS-CoV-2.
However, since the participants were health workers, they were routinely tested and none of them reported prior infection. Notably, the sample size included in this study was small; therefore, the findings cannot be generalized.
More than 90% of the population show antibodies against at least three human coronaviruses. The possibility of cross-reactivity during antibody testing cannot be denied.
Implications of the study
There is not much known about COVID-19 vaccine immunization and breastfeeding. Furthermore, there is also little evidence indicating the transfer of antibodies to breastmilk after the COVID-19 vaccination.
This is the first study to relate the duration of breastfeeding with higher levels of antibodies in breastmilk. While the presence of antibodies in breastmilk is reported, immunization through breastfeeding has yet to be verified. Thus, additional clinical studies are required in lactating women to address scientific-based recommendations.