As the current pandemic of coronavirus disease 2019 (COVID-19) continues, the pace of research on a vaccine to the virus responsible, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been intense, leading to the accelerated release of several vaccines candidates.

A new study, released on the medRxiv* preprint server, shows the correlation between serum levels of antibodies to SARS-CoV-2 and levels in breast milk following the vaccination of lactating women with a vaccine based on messenger ribonucleic acid (mRNA) technology. With such vaccines, the viral spike antigen is encoded in the mRNA, to be expressed in the host cells.

The expressed viral spike is expected to stimulate host immunity in the form of antibodies specifically directed against the virus.

Vaccines and lactation

The vaccine rollout in many countries has been extended to pregnant and lactating women, despite the fact that they were not included in any of the clinical trials. This has been a concern for many women, since vaccination may have potential adverse effects on the breastfeeding unit.

The current study aimed to answer a specific question: in lactating women vaccinated with the Pfizer-BioNTech COVID-19 vaccine (BNT162b2), how do breast milk antibodies to the virus correlate with serum levels across time?

The fact that many women in the healthcare profession are lactating mothers has led to the recommendation that the mRNA Pfizer/BioNTech and Moderna vaccines be extended to them, since this platform is considered to be a low-risk one during lactation and since the risk of getting COVID-19 is less than the potential health hazards of vaccination.

Earlier, several studies have shown that milk from infected mothers contains anti-SARS-CoV-2 antibodies, and this is also known to be the case with mothers who have received the vaccines. The protective effect of these antibodies on the breastfed infant is still unknown.

Study details

The current results are from the first 18 lactating mothers in the study, each of whom had three paired serum-breast milk samples given at three time points: after the first dose of the vaccine, at two weeks after the second dose, and at four weeks after the second dose.

Simultaneously, all were tested by nasopharyngeal swab for rapid antigen determination of the presence of SARS-CoV-2.

Serum immunoglobulin G (IgG) levels directed against the spike S1 protein and the nucleocapsid was also measured.

What were the results?

All the 18 participants were free of confirmed SARS-CoV-2 infection before they were vaccinated and during the study. The mean age was 37 years, and that of the infant being breastfed, 19 months.

The median IgG anti-S1 level in serum was about 400 AU/mL after the first dose and shot up to 11,500 AU/mL at two weeks after the second dose. At four weeks after the second dose, it fell to 8,000 AU/mL.

In breast milk, the median IgG levels against the S1 spike subunit of the virus were 2 AU/mL, 52 AU/mL, and 42 AU/mL, respectively.

No serious adverse effects were reported in any of the lactating women and none in the infants.

What are the implications?

The researchers show that lactating mothers who are vaccinated with the Pfizer vaccine have significant specific antibodies in their breast milk. The levels of these antibodies show a marked rise after the boost dose, in proportion to the serum levels at these time points.

This promising finding argues in favor of a potential protective effect of breastfeeding mothers’ vaccination in their infants.”

Longitudinal follow-up will be required to reveal if the breast milk antibody levels go down further, or remain at this level, and whether this pattern will be seen with other vaccines, both mRNA and non-mRNA. The safety of this vaccine in lactation and in the breastfed infant remains to be seen, as well as its protective effect against COVID-19 in particular.

The current study is still continuing, and further analyses are expected as more participants are tested.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.