“Antibodies found in breast milk of these women showed strong neutralizing effects, suggesting a potential protective effect against infection in the infant,” Sivan Haia Perl, MD, of the Pulmonary Institute at the Shamir Medical Center in Zerifin, Israel, and colleagues wrote in JAMA.
The researchers prospectively analyzed 504 samples of breast milk from 84 women recruited from hospitals all over Israel. The women had a mean age of 34 years, and all had received two doses of the Pfizer-BioNTech vaccine. Their infants had a mean age of 10.32 months.
IgA antibodies were found in 61.8% of the breast milk samples 2 weeks after the first dose and were significantly elevated, the researchers reported. After 4 weeks — or 1 week after receipt of the second dose — 86.1% of samples were positive. Antibody levels remained elevated through follow-up, and after 6 weeks, 65.7% of the samples still tested positive, they said.
Anti-SARS-CoV-2-specific IgG antibodies remained low until week 4, when 91.7% of the samples tested positive. Almost all of the samples — 97% — were positive after weeks 5 and 6.
According to the researchers, no mother or infant experienced any serious adverse events during the study period. However, 47 women (55.9%) reported vaccine-related adverse events after their first dose, and 52 (61.9%) of women reported adverse events after their second dose. The most common event reported was local pain (40.5% of women), followed by fatigue (33.3%) and fever (11.9%).
Among the infants, four developed fever within the first 3 weeks of the study period, and all experienced symptoms of upper respiratory tract infection, which included cough and congestion.
The researchers said the study had two limitations, including that no serum antibody testing or real-time reverse-transcriptase PCR testing was performed.