Almost since the beginning of the COVID-19 pandemic, researchers have raised worries that the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) could be transmitted to infants born to infected mothers via breastmilk. As a result, these mothers face conflicting advice on breastfeeding, with many organizations advising them to continue breastfeeding as long as they observe precautions to prevent respiratory droplet spread. A recent study published in the journal JAMA in August 2020 suggests that SARS-CoV-2 RNA does not represent replication-competent virus and that breast milk may not be a source of infection for infants.
Viral RNA in Some Breastmilk Samples
Currently, 24 case reports have been published on the viral content of breastmilk samples from women who were infected with SARS-CoV-2. Of these, 10 samples, from 4 women, were found to have viral RNA. However, the studies failed to confirm if the presence of the virus was due to maternal infection and not because of retrograde flow from the oral cavity of an infected agent. Another possibility is that of environmental contamination.
Secondly, the presence of viral RNA as detected by reverse transcriptase-polymerase chain reaction (RT-PCR) is not the same as the presence of infectious viral particles. Until this point, infectious virions have not been isolated from breastmilk. Moreover, the undisputed infection of a single infant through breastmilk is yet to be proved.
Testing Breastmilk from Infected Women
The current study aims to explore the potential for transmission of SARS-CoV-2 through breastmilk from an infected woman. This would answer the question that many such women have: is it safe to breastfeed while still infected?
The study includes women living in the US, who have a history of symptomatic COVID-19, or have a history of exposure to a confirmed patient, or have been tested positive for the virus, and who were currently breastfeeding. Beginning from March 27 up to May 6, 2020, the study enrolled 18 women who met these criteria.
Of the 18 women, most were Caucasian, with a mean age of 34 years, with children aged anywhere from newborn to 19 months. Each woman supplied 1-12 samples, for a total of 64 samples. Some of these samples were from stored milk in milk banks dating from pre-pandemic days, or before the woman tested positive for the virus. All of the women had symptomatic disease except for one.
The researchers carried out a specially developed RT-PCR assay for detecting the virus in breastmilk and looked for the presence of infectious virus particles as well.
Looking for Virus in Pasteurized Breastmilk
The researchers also simulated holder pasteurization conditions that are typically provided in human milk banks. They added SARS-CoV-2 (200 × median tissue culture infectious dose 50% [TCID50]) to breastmilk samples collected from two non-infected control donors before the pandemic began. The samples were then heated to 62.5 °C for 30 minutes and then cooled to 4 °C. These samples were also added to tissue culture along with the other samples. Specimens of the same virus-spiked milk samples were also cultured in tissue culture without pasteurization.
Only one breastmilk sample was found to have SARS-CoV-2 RNA, collected on the day the woman first developed symptoms. This patient had negative samples both two days before and 12 and 41 days after symptoms began. The infant was not tested for infection. No sample contained infectious virus capable of replication.
The pasteurized samples also failed to show the presence of the infectious virus or viral RNA, though the unpasteurized mixed samples did contain the virus. The viral culture of the sample, which returned a positive viral RNA result on RT PCR was negative, suggesting that the milk did not contain an infectious form of the virus.
This study implies that breast milk may not be a route of vertical transmission. Pasteurization of breastmilk samples also resulted in failure to detect either viral RNA or culturable virus.
This shows that breastfeeding can be continued even in the presence of maternal infection, and milk from milk banks can be safely used without the risk of transmitting the virus to the woman. The study was small, and the sample was not random. Thus a more extensive study needs to be carried out with more stringent procedures to validate this preliminary result.