Breast Milk of Women Infected with SARS-CoV-2 Unlikely to Transmit Virus

An evaluation of breast milk from 18 women infected with SARS-CoV-2 found no evidence that the virus that causes the coronavirus 2019 (COVID-19) disease is transmitted through breast milk, according to a research letter, published in JAMA Network.

Investigators at the University of California, San Diego, and the University of California, Los Angeles, invited breastfeeding mothers who tested positive for the novel coronavirus to participate in the study beginning in March. It involved gathering clinical data via telephone interviews, and breast milk samples were mailed to the study center.

“These findings were reassuring for infected mothers who choose to start or continue breastfeeding,” lead author Christina Chambers, PhD, MPH, co-director of the Center for Better Beginnings and professor in the Division of Dysmorphology and Teratology at the University of California, San Diego, told Contagion^®. “We found no evidence that virus capable of infecting the infant could be transmitted via mom’s milk.”

A total of 64 breast milk samples were included in the study, with each mother providing between 1 and 12 samples from time points before and after diagnosis. Detectable SARS-CoV-2 RNA, which does not equate to infectivity, was found in 1 of the 64 samples. It was taken from a mother on the day of symptom onset, and the viral culture from that sample was negative. Other samples from that mother taken 2 days before and 12 and 41 days after tested negative for viral RNA.

“We were not sure how often we would see viral RNA (not infectious virus) in the milk, and we found this in one sample out of 64 tested,” Chambers said. “We would like clinicians to know that although we want to generate more data, these findings support safety of breastmilk from infected moms.”

The study included 2 control samples spiked with replication-competent SARS-CoV-2 virus that were treated with Holder pasteurization commonly used in human milk banks. They were heated to 62.5° C for 30 minutes and then cooled to 4° C, after which no replication-competent virus or viral RNA was detectable.

“Our next steps are to test the many more samples mothers have sent in, and to start looking at when infected mothers who have recovered start producing antibodies to the virus that can be detected in milk,” Chambers said.

The potential risks of SARS-CoV-2 for infants and pregnant women have been a topic of ongoing research. Investigators in New York found risk of perinatal transmission of the virus to be very unlikely in a recent observational cohort study that evaluated babies born to mothers who tested positive for the virus and found that none of the infants were positive for the virus.

While the evidence is reassuring, women who are pregnant or planning to become pregnant should remain cautious. Another study found a statistically greater risk of severe COVID-19 illness among pregnant women, who were associated with greater risk of hospitalization, ICU admission, and even mechanical ventilator care than for women who were not pregnant.

An earlier study conducted in Wuhan, China, found that a group of 33 babies born to mothers with coronavirus included 3 babies with coronavirus themselves.