No Evidence of Infectious SARS-CoV-2 in Human Milk: Analysis of a Cohort of 110 Lactating Women

BACKGROUND: SARS-CoV-2 infections of infants and toddlers are usually mild but can result in life-threatening disease. SARS-CoV-2 RNA been detected in the breast milk of lactating women, but the potential role of breastfeeding in transmission to infants has remained uncertain. METHODS: Breast milk specimens were examined for the presence of the virus by RT-PCR and/or culture. Specimens that contained viral RNA (vRNA) were examined for the presence of subgenomic coronavirus RNA (sgRNA), a putative marker of infectivity. Culture methods were used to determine the thermal stability of SARS-CoV-2 in human milk. RESULTS: Breast milk samples from 110 women (65 confirmed with a SARS-CoV-2 diagnostic test, 36 with symptoms but without tests, and 9 with symptoms but a negative SARS-CoV-2 diagnostic test) were tested by RT-PCR (285 samples) and/or viral culture (160 samples). Although vRNA of SARS-CoV-2 was detected in the milk of 7 of 110 (6%) women with either a confirmed infection or symptomatic illness, and in 6 of 65 (9%) of women with a positive SARS-CoV-2 diagnostic test, virus was not detected in any culture. None of the 7 milk specimens with detectable vRNA contained sgRNA. Notably, when artificially added to human milk in control experiments, infectious SARS-CoV-2 could be cultured despite several freeze-thaw cycles, as occurs in the storage and usage of human milk. CONCLUSIONS: SARS-CoV-2 RNA can be found infrequently in the breastmilk of women with recent infection, but we found no evidence that breastmilk contains infectious virus or that breastfeeding represents a risk factor for transmission of infection to infants.