COVID-19 virus stable in frozen human milk

Cold storage does not inactivate the COVID-19 virus in human milk, a new study by researchers from UNSW and Australian Red Cross Lifeblood Milk has confirmed.

The researchers tested if storing SARS-CoV-2 in human milk at 4°C or -30°C would inactivate the virus. “We found that cold storage did not significantly impact infectious viral load over a 48-hour period,” said Greg Walker, lead author and PhD candidate in Professor Bill Rawlinson’s group at UNSW Medicine.

“While there is no evidence that the virus can be transmitted through breast milk, there is always a theoretical risk,” said Greg Walker, lead author and PhD candidate in Professor Bill Rawlinson’s group at UNSW Medicine.

Freezing the milk resulted in a slight reduction in the virus present, but we still recovered viable virus after 48 hours of storage, said Walker.

The fact that SARS-CoV-2 was stable in refrigerated or frozen human milk could help inform guidelines around safe expressing and storing of milk from COVID-19 infected mothers, according to the researchers.

“For example, we now know that it is particularly important for mothers with COVID-19 to ensure their expressed breast milk does not become contaminated with SARS-CoV-2,” Dr Klein said. “But it’s also important to note that breastfeeding is still safe for mothers with COVID-19 — there is no evidence to suggest that SARS-CoV-2 can be transmitted through breastmilk.”

The study has also confirmed what researchers already suspected to be the case — pasteurisation inactivates COVID-19 virus in human milk.

There are five human milk banks in Australia. As the COVID-19 pandemic evolves, these milk banks continue to provide donated breast milk to preterm babies who lack access to their mother’s own milk. Donors are screened for diseases, and milk is tested and pasteurised to ensure that it is safe for medically fragile babies.

“While there is no evidence that the virus can be transmitted through breastmilk, there is always a theoretical risk,” Walker said.

“We’ve seen in previous pandemics that pasteurised donor human milk (PDHM) supplies may be interrupted because of safety considerations, so that’s why we wanted to show that PDHM remains safe,” Walker said.

For this study, the team worked in the Kirby Institute’s PC3 lab to experimentally infect small amounts of frozen and freshly expressed breast milk from healthy Lifeblood Milk donors. “We then heated the milk samples — now infected with SARS-CoV-2 — to 63˚C for 30 minutes to simulate the pasteurisation process that occurs in milk banks, and found that after this process, they did not contain any infectious, live virus,” Walker said.

“The amount of virus we use in the lab is a lot higher than what would be found in breast milk from women who have COVID-19 — so we can be really confident in these findings,” he said.

Dr Laura Klein, Research Fellow and Lifeblood Milk senior study author, explains that the purpose of the research was to provide evidence behind what people already expected. “Pasteurisation is well known to inactivate many viruses, including the coronaviruses that cause SARS and MERS.

“These findings are also consistent with a recent study that reported SARS-CoV-2 is inactivated by heat treatment in some contexts,” she said.

Image credit: ©stock.adobe.com/au/Andrey Kiselev