Further confirmation Holder pasteurization inactivates SARS-CoV-2 in breast milk

Researchers in Australia have shown that a standard pasteurization method commonly used in human milk banks is effective at inactivating severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Although freezing or refrigerating milk was not enough to reduce the infectivity of the milk, a process called Holder pasteurization successfully inactivated the virus.

Holder pasteurization, which involves heating milk at 62.5ºC for 30 minutes, is the most widely used pasteurization method in milk banks globally.

The team, from the University of New South Wales, University of Melbourne, and the University of Queensland, say the findings confirm that current milk bank processes are sufficient to mitigate the potential risk of SARS-CoV-2 transmission to infants.

The findings should help guide international recommendations for the storage of milk expressed by mothers with coronavirus disease 2019 (COVID-19), they add.

A pre-print version of the paper is available in the server medRxiv*, while the article undergoes peer review.

Study: SARS-CoV-2 in human milk is inactivated by Holder pasteurization but not cold storage. Image Credit: sommart sombutwanitkul / Shutterstock
Study: SARS-CoV-2 in human milk is inactivated by Holder pasteurization but not cold storage. Image Credit: sommart sombutwanitkul / Shutterstock

Risk mitigation strategies

As the COVID-19 pandemic continues to sweep the globe, human milk banks are busy ensuring donor milk is available to infants who do not have access to their mother’s milk. The risk of SARS-CoV-2 transmission via infected milk is not yet well understood, and milk banks currently employ various strategies to mitigate any potential risk.

The milk banks use several approaches to ensure the safety of donor milk, including strict selection criteria, microbial screening, cold storage, and pasteurization.

In the case of SARS-CoV-2, donors are checked for COVID-19 symptoms, and infected individuals or those who have been in close contact with the virus are excluded. The most widely used pasteurization method is Holder pasteurization, which has previously been shown to inactivate SARs-CoV-1 and Middle East respiratory syndrome coronavirus (MERS-CoV).

Concerns surrounding transmission via breast milk

Although several studies had reported that SARS-CoV-2 is not detected in breast milk, some more recent reports have described its detection in cases of symptomatic COVID-19.

While there is not yet any evidence that SARS-CoV-2 can be transmitted to infants via breast milk, theoretically, milk could become infected through maternal exhalation or contact with the skin.

Based on the existing data, international guidelines currently advise that mothers continue to breastfeed, even if they are infected with COVID-19 since the health benefits to infants significantly outweigh the risk of SARS-CoV-2 transmission.

Although the potential risk of SARS-CoV-2 being transmitted to infants via breast milk is low, it is still a significant concern among milk banks worldwide.

Testing pasteurization and cold storage

Now, William Rawlinson and colleagues have tested how effective pasteurization is at inactivating SARS-CoV-2 in breast milk, as well as how stable the virus is in refrigerated or frozen milk.

“This is the first study to assess the stability of SARS-CoV-2 in human milk,” writes the team.

The team acquired milk that had been frozen for 14 weeks and fresh milk that had been expressed within the previous 12 hours. The milk had been donated to Australian Red Cross Lifeblood Milk by healthy individuals.

Previously frozen milk and Minimal Essential Medium (MEM) were infected with SARS-CoV-2 and then either not pasteurized or subjected to Holder pasteurization (heating at 62.5ºC for 30 minutes) or under-pasteurization (heating at 56ºC for 30 minutes).

Freshly expressed milk and MEM were also infected with SARS-CoV-2 and put in cold storage (either frozen or refrigerated). Infectious titers were measured at the time of inoculation and after two days of storage at 4°C or -30°C.

Pasteurization, but not cold storage, inactivated the virus

The team reports that following heating at 63°C or 56°C for 30 minutes, no live virus was detected in the milk samples or MEM.

After 48 hours of cold storage, there was no reduction in infectious titers of milk samples stored at 4°C and a slight reduction in samples stored at -30°C. Among the MEM samples, there was a slight reduction in infectious titers after storage at both 4°C and -30°C

Although storing samples at -30°C did slightly reduce infectious titers in milk samples and MEM, and the live virus was still detectable after 48 hours, warn Rawlinson and colleagues.

“Our findings demonstrate that SARS-CoV-2 can be effectively inactivated by Holder pasteurization, and confirm that existing milk bank processes will effectively mitigate the risk of transmission of SARS-COV-2 to vulnerable infants through pasteurized donor human milk,” writes the team.

The findings support a recent Germany study that also concluded Holder pasteurization to be effective at inactivating SARS-CoV-2 in breast milk. Authors Stephanie Pfaender and colleagues concluded, “viral infectivity is completely eliminated by this treatment.”

“Whilst it is yet to be determined whether SARS-CoV-2 detected in breast milk is infectious, these findings may assist in the development of guidelines around expressing and storing milk from COVID-19 infected mothers,” conclude Rawlinson and team.

*Important Notice

bioRxiv and medRxix publish preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal references:
Sally Robertson

Written by

Sally Robertson

Sally has a Bachelor's Degree in Biomedical Sciences (B.Sc.). She is a specialist in reviewing and summarising the latest findings across all areas of medicine covered in major, high-impact, world-leading international medical journals, international press conferences and bulletins from governmental agencies and regulatory bodies. At News-Medical, Sally generates daily news features, life science articles and interview coverage.

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