COVID-19 unlikely to be transmitted via breastfeeding, says small study

SARS-CoV-2 RNA is infrequently detected in the breastmilk of mothers with proven or suspected COVID-19, but just the same, breastfeeding does not represent a risk factor for vertical transmission of the infection, according to data from a small study.

“[E]ven when [SARS-CoV-2 RNA] is detected, it is an unlikely source of infection for the breastfed baby; viral RNA was only transiently present, and we were unable to culture SARS-CoV-2 from any sample,” said a team of researchers led by Paul Krogstad from the University of California, Los Angeles, US.

In the study, Krogstad and colleagues examined 336 breastmilk samples from 110 lactating women, among whom 65 had a positive COVID-19 test, 9 had symptoms but tested negative, and 36 were symptomatic but were not tested. They analysed the samples using reverse transcription-polymerase chain reaction (285 samples) and viral culture (160 samples).

Samples from seven (6 percent) women who either had confirmed infection or reported being symptomatic were positive for SARS-CoV-2 RNA. A second breastmilk sample taken from these seven women between 1 and 97 days later did not contain any traces of the RNA. [Pediatr Res 2022;doi:10.1038/s41390-021-01902-y]

When further examined for the presence of subgenomic RNA (sgRNA), a putative marker of infectivity, samples that contained SARS-CoV-2 viral RNA did not show any infectious SARS-CoV-2 genetic material. The same was true for the second breastmilk samples.

Finally, there was no clinical evidence of infection in the infants who were breastfed by the seven mothers with SARS-CoV-2 RNA in their milk.

Containing numerous antimicrobial factors, breastmilk is an invaluable source of nutrition to infants. In fact, recent studies reported the presence of neutralizing antibodies to SARS-CoV-2 in the breastmilk of lactating women who had been recently infected. [Science 2020;367:1070-1072; Lancet 2016;387:475-490; iScience 2020;23:101735; N Microbes N Infect 2020;37:100752] 

“However, breastfeeding is clearly a route by which either HIV or HTLV (human T cell lymphotropic virus) can be transmitted to infants. In contrast to these pathogens, hepatitis B and C virus infections produce chronic infections in women and, yet, are not contraindications to breastfeeding,” Kroger said.

There are a few small case reports that reported SARS-CoV-2 RNA detection in breastmilk, with one indicating the possibility of vertical transmission, although Kroger noted that milk contamination could not be ruled out as the source. [Eur J Pediatr 2021;180:1009-1103; Matern Child Nutr 2020;16:e13032; BJOG 2020;127:1109-1115]

Kroger pointed out that their study, in comparison to prior studies, is unique in that viral cultures were used to analyse a very large number of breastmilk specimens. “The rationale for this study was to determine how often SARS-CoV-2 viral RNA was present in breastmilk samples and to examine the risk of infection to infants through breastmilk.”

Nevertheless, Kroger cautioned that the collection of breastmilk samples was not directly observed and that he and his team relied on the maternal report of SARS-CoV-2 test results, symptoms, and treatments received. Furthermore, despite the large number of breastmilk samples analysed from women with recent SARS-CoV-2 infection, the size may have been too small to establish factors that would predict the presence of SARS-CoV-2 RNA in breastmilk.

Despite the presence of limitations, the study indicates that breastfeeding by mothers with or who have had COVID-19 is not likely to be a hazard for infants, according to Kroger.