Study examines safety of COVID-19 vaccination during lactation

As more and more women have become pregnant and delivered over the long-drawn-out pandemic of coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), it has become important to extend the benefits of vaccination to them as well. This is despite the lack of definitive efficacy and safety data in this cohort, largely because this group has not been included in early vaccine trials.

A new preprint, recently released on the medRxiv* server, reports from real life in a cohort of mother-child dyads in Singapore. The mothers were lactating after having received COVID-19 vaccination.

No serious side effects of COVID-19 vaccination

The mothers all received two doses of the Pfizer/BioNTech BNT162b2 vaccine, which is based on a messenger ribonucleic acid (mRNA) platform that encodes the wildtype SARS-CoV-2 spike antigen, to be expressed within the host cells and trigger an immune response. The two doses were 21 days apart.

The study was carried out at 28 days from the second day, using a questionnaire to elicit side effects. These were then evaluated for their possible relationship to vaccination.

The researchers found that about a quarter of the women reported any side effects following vaccination. No serious adverse events were described. This corroborates the findings of earlier studies.


The study was intended to explore potential beneficial and adverse outcomes following vaccination. Vaccination is intended to protect the mother against symptomatic SARS-CoV-2 infection, and also the infant via the transfer of antibodies in breast milk.

Earlier studies showed that a few women experienced a decline in the production of milk and/or mastitis following vaccination. These are common events in early lactation, however.

The current study aimed to investigate whether these effects were more frequent in this group to enable better guidance to be provided to women considering vaccination while lactating.

The researchers specifically looked for axillary lymph node enlargement, which drains most of the breast tissue. Their theory was that the robust immunogenicity of the vaccine might increase the incidence of this side effect. They also asked about mastitis and breast engorgement, besides other systemic or local adverse events.

What were the findings?

Among the 88 dyads who were analyzed over the one month during which the study was carried out, asthma, allergic rhinitis, alpha thalassemia minor and thyroid cancer were present in a small number. Over 80% were practicing exclusive breastfeeding, while the others also used infant formula.

Over three-quarters began breastfeeding within 72 hours of vaccination. The mean age of the breastfed infants was 10 months.

Two-thirds of the women said they had pain, redness or swelling at the injection site, but none had serious allergic reactions, and none required hospital admission.

Approximately 6% of the women said they had enlarged axillary or cervical glands, while 3% had mastitis. One woman said she had breast engorgement, but this resolved within a day and did not compromise milk production.

Milk supply remained intact throughout the study period, though one woman said she noticed a faint change in color, with a bluish-green tint, only after the first dose of the vaccine.

No infant was reported to have any adverse events, including fever, cough, vomiting or diarrhea, or behavioral changes after the mother was vaccinated.

What are the implications?

This is the first cohort study directed at assessing the adverse events in mothers and children following mRNA vaccination in a cohort of breastfeeding women. It shows that there is little risk of concerning adverse events, lymph node enlargement.

Despite the presence of this symptom in 6% (reported as 0.3% in Pfizer trial data), these women did not report inflamed or engorged breasts. The higher incidence may be because it was specifically enquired about in this study, unlike in the trial, which could have led to mild lymph node enlargement going unnoticed.

Mastitis and breast engorgement occurred at expected rates in the vaccinated group. The infants also appeared to be completely unaffected by the vaccine, and the milk supply remained unchanged.

Our findings suggest that the BNT162b2 vaccination is well tolerated in lactating women and not associated with short term adverse effects in their breastfed infants,” write the researchers.

This supports current recommendations that vaccinated women who are lactating continue to breastfeed without interruption, in view of the vaccine's safety.

*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.

Journal reference:
Dr. Liji Thomas

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.


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