Breastfeeding women vaccinated against COVID-19 had elevated levels of antibodies against SARS-CoV-2 in their breast milk as soon as 2 weeks after their first vaccination dose, according to a prospective cohort study published in JAMA.

Investigators vaccinated exclusively or partially breastfeeding women between December 23, 2020, and January 15, 2021, with 2 doses of the Pfizer-BioNTech vaccine, 21 days apart. Breast milk samples were collected before vaccination, then once weekly starting 2 weeks after the first vaccine dose for 6 weeks. Immunoglobulin G (IgG) levels of more than 0.8 U/mL and Immunoglobulin A (IgA) with an extinction ratio of samples over calibrator of more than 0.8 U/mL were considered positive. Investigators also collected information on maternal and infant demographics and vaccine-related adverse events. The study was conducted in Israel, where a national COVID-19 vaccination program began on December 20, 2020, and prioritized breastfeeding women belonging to vaccine target groups.

In total, investigators collected 504 breast milk samples from the 84 women who completed the study. The mean maternal age was 34 years (SD, 4) and mean infant age at the time of the first maternal vaccine was 10.32 months (SD, 7.3).

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At 2 weeks after the first vaccine dose, 61.8% of breast milk samples tested positive for SARS-CoV-2 IgA antibodies, and mean IgA levels were 2.05 ratio (P <.001). At week 4 (1 week after the second vaccine dose), 86.1% of breast milk samples tested positive for SARS-CoV-2 IgA antibodies. Mean IgA levels remained elevated through 6 weeks, by which point 65.7% of breast milk samples tested positive.

Anti-SARS-CoV-2 IgG antibodies remained low for 3 weeks after the first vaccine dose, then at week 4, IgG levels increased to 20.5 U/mL (P =.004), and 91.7% of breast milk samples tested positive for IgG antibodies. At weeks 5 and 6, 97% of breast milk samples tested positive for IgG antibodies.

Mothers and infants did not experience any serious adverse events throughout the study period, with local pain at the injection site being the most common complaint. There were 4 infants who developed a fever during the study period; all had symptoms of upper respiratory tract infection that resolved spontaneously except for 1 infant, who was hospitalized for neonatal fever due to his age and was treated with antibiotics.

Though investigators in this study did not perform functional assays, previous studies reported strong neutralizing effects of antibodies to COVID-19 in breast milk. This implies that infants of mothers with anti-SARS-CoV-2 antibodies in their breast milk are potentially protected against COVID-19.


Perl SH, Uzan-Yulzari A, Klainer H, et al. SARS-CoV-2-specific antibodies in breast milk after COVID-19 vaccination of breastfeeding women. JAMA. Published online April 12, 2021. doi:10.1001/jama.2021.5782