Human milk: Pasteurization inactivates COVID-19 – Cold storage doesn’t inactivate the virus

A team of medical researchers has found that in human milk, pasteurization inactivates the virus that causes COVID-19, confirming milk bank processes have been safe throughout the pandemic, and will remain safe going forward, too.

The study – published this month in the Journal of Pediatrics and Child Health – was a partnership between UNSW and a multidisciplinary team from Australian Red Cross Lifeblood 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 pasteurized to ensure that it is safe for medically fragile babies.

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

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

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 pasteurization process that occurs in milk banks, and found that after this process, they did not contain any infectious, live virus,” Mr Walker says.

“Our findings demonstrate that the SARS-CoV-2 virus can be effectively inactivated by pasteurization.”

The researchers say their experiments simulated a theoretical worst-case scenario.

“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,” Mr Walker says.

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.

“Pasteurization is well known to inactivate many viruses, including the coronaviruses that cause SARS and MERS,” she says.

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

Kirby Institute researcher and study co-author, Associate Professor Stuart Turville, says this work was a first.

“We’ve been working in real time to grow and make tools against this new pathogen, which has been an exponential learning curve for everyone involved. This work and many others that are continuing in the PC3 lab tell us how we can be safe at the front line working with this virus in the real world.”

Cold storage doesn’t inactivate the virus

The researchers also tested if storing SARS-CoV-2 in human milk at 4°C or -30°C would inactivate the virus – the first time a study has assessed the stability of experimentally infected SARS-CoV-2 in human milk under common storage conditions.

“We found that cold storage did not significantly impact infectious viral load over a 48-hour period,” Mr Walker says.

“While freezing the milk resulted in a slight reduction in the virus present, we still recovered viable virus after 48 hours of storage.”

The researchers say 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.

“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 says.

“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.”

Donated breast milk is recommended by the World Health Organization when mother’s own milk is not available to reduce the risks of some health challenges premature babies can face.

Lifeblood Milk has provided donor milk to over 1500 babies born premature in 11 NICUs across New South Wales, South Australia, and Queensland since launching in 2018.

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Pasteurization of human milk by the Holder method (62.5°C for 30 min) inactivates SARS-CoV-2.

Thus, in the event that donated human milk contains SARS-CoV-2 by transmission through the mammary gland or by contamination, this method of pasteurization renders milk safe for consumption and handling by care providers.

Mother’s milk is the optimal source of nutrition for infants and contains a myriad of bioactive and immunomodulatory factors, including cytokines, lactoferrin, oligosaccharides and secretory immunoglobulins, which help orchestrate immune system development and provide first-line defence against respiratory tract and gastrointestinal tract infection.1–5

For vulnerable infants, such as very low-birth-weight (born < 1500 g) infants, use of mother’s milk is associated with a shorter hospital stay and reduces their risk of sepsis and necrotizing enterocolitis, a severe bowel emergency.6–9 It is the standard of care in Canada to provide very low-birth-weight infants in hospital with pasteurized donor human milk until their mother’s supply is established.10

Past global epidemics, such as HIV/AIDS, have had devastating effects on donor human milk banking because of perceived risks.

In the 1980s, with the knowledge that HIV could be transmitted into human milk, 22 of the 23 Canadian donor human milk banks closed.11

Several viruses, in addition to HIV, can be transmitted through human milk, including hepatitis, cytomegalovirus and human T-cell lymphotropic virus type 1.12

Some viruses may be secreted into milk by paracellular passage as tight junctions open in response to maternal illness and inflammation.3

Other routes of transmission include contamination from respiratory droplets, skin, breast pumps and milk containers.

Milk banks affiliated with the Human Milk Banking Association of North America (HMBANA) and the European Milk Bank Association (EMBA) pasteurize milk using the Holder method (62.5°C for 30 min) before dispensing for use; the Holder method is effective in inactivating the aforementioned viruses.13,14

Very little is known of the prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in human milk, or its infectivity; however, the virus has been detected in human milk by reverse transcription polymerase chain reaction (RT-PCR) testing.15–18

Mothers donating milk are verbally screened for symptoms associated with coronavirus disease 2019 (COVID-19) at HMBANA-affiliated milk banks, but direct assessment for SARS-CoV-2 by nasopharyngeal swabs and RT-PCR testing is not performed.

Although there is no direct evidence showing that Holder pasteurization inactivates SARS-CoV-2 in human milk, this virus is known to be heat sensitive.19

The aim of this research was to confirm that Holder pasteurization would be sufficient to inactivate SARS-CoV-2 in donated human milk samples.

Interpretation

Very few milk samples from women positive for COVID-19 have been tested for SARS-CoV-2. Of the few available cases reported in the literature, there are now at least 3 reports of the presence of SARS-CoV-2 nucleic acid in human milk, although none of these have measured the viability of the virus in these samples.

The World Health Organization recommends that human donor milk be fed to low-birth-weight infants when there is an insufficient volume of mother’s milk.27

Human milk banking is growing rapidly internationally, with more than 650 milk banks globally that rely on the Holder method to ensure the safety of donor milk.28

Although this technique is assumed to result in inactivation of SARS-CoV-2, it is important to confirm this in a human milk matrix, for the safety of milk bank staff, caregivers and recipients of human donor milk.

In this study, pasteurization of human milk spiked with SARS-CoV-2 using the Holder method (62.5°C for 30 min) resulted in complete viral inactivation, as measured by TCID50/mL.

The high viral titre used to spike samples in these experiments enabled us to confirm a 106 reduction. The impact of pasteurization on coronaviruses in a human milk matrix has not previously been reported in the literature.29

The results are in keeping with evidence of coronavirus inactivation in other matrices, including culture media and plasma using a variety of pasteurization protocols.

The virus causing severe acute respiratory syndrome, SARS-CoV, has been shown to be completely inactivated with temperatures as low as 56°C for 20 minutes, as well as at higher temperatures, such as 70°C for 5 minutes.30–33

The virus causing Middle East respiratory syndrome was shown to be inactivated at 56°C for 60 minutes.34,35 A recent report by Chin and colleagues showed SARS-CoV-2 in virus transport media to be completely inactivated at 56°C for 30 minutes or 70°C for 5 minutes.19
In the present investigation, we did not see complete inactivation of SARS-CoV-2 in media that did not contain human milk (positive control) after pasteurization at 62.5°C for 30 minutes, which differs from the report of Chin and colleagues.

This finding suggested to us that the biological matrix in which the virus resides needs to be considered when assessing effective inactivation conditions.

Interestingly, we observed some reduction in the cytopathic effects of SARS-CoV-2 in milk samples that were not heat treated but held at room temperature for 30 minutes.

This is very likely a result of the multitude of immune components found in human milk — including secretory 1gA antibodies, lactoferrin, lactadherin, mucins from milk fat globules and oligosaccharides — that have significant antiviral activity.1,2

Notably, Hamilton Spence and colleagues reported the same finding for human milk samples inoculated with Ebola virus and held at room temperature for 30 minutes.36

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More information: Gregory J Walker et al. SARS‐CoV ‐2 in human milk is inactivated by Holder pasteurization but not cold storage, Journal of Pediatrics and Child Health (2020). DOI: 10.1111/jpc.15065