Although babies and young children are at lower risk of getting very sick with Covid-19 compared to older adults, a small proportion of babies will require hospital care.

There has been immense interest among scientists, healthcare workers and new mothers especially in understanding whether potentially protective antibodies against SARS-CoV-2 (the virus that causes Covid-19) can be provided to babies via breast milk.

But what does the evidence tell us? Does a natural Covid infection lead to SARS-CoV-2 antibodies in a woman’s breast milk, and how long do these last? What about after vaccination? If natural Covid-19 infections and vaccination do produce antibodies in a woman’s breast milk, does this mean babies of these mothers will be protected against Covid-19? And could antibody-filled milk be used as a treatment somehow?

There are now several studies looking at breast milk antibody responses in women who have had Covid-19, while research is increasingly exploring breast milk antibody responses in mothers following vaccination with an mRNA vaccine.

After Covid-19 infection, antibodies have been found to persist in breast milk for at least six months, with emerging data suggesting they are still abundant ten months later. Antibodies are found in breast milk even after mild SARS-CoV-2 infections, and in women who have no symptoms at all.

Meanwhile, the breast milk of women who are vaccinated while lactating (who have not had Covid-19) has been found to contain significant levels of SARS-CoV-2 antibodies after vaccination. Determining how long SARS-CoV-2 antibodies in breast milk last after vaccination will take time, but reports suggest they persist for at least six weeks.

It is not surprising that if the mother is infected with or vaccinated against Covid-19 we see SARS-CoV-2 antibodies produced in her breast milk. Pregnant women are often advised to be vaccinated against other diseases, such as whooping cough.

In these cases, we know the resulting antibodies that the mother produces can pass on to the baby through the placenta and in breast milk. This is known as “passive immunity”, meaning the baby gets immune protection while its own immune system matures to the point where it can make antibodies for itself.

It is a continuation of a process that starts while the baby is developing in the womb and receiving maternal antibodies that are passing across the placenta. This is very important to protect the baby against infectious diseases circulating in the community into which it is born.

Unique antibodies

Antibodies are made by specialist antibody-producing immune cells called B cells, which are found in our gastrointestinal tract and other tissues. Antibodies can be found in blood, saliva and other parts of the body.

When a mother’s body is preparing for the birth of a baby, some of these antibody-producing cells travel to the breasts where they produce antibodies locally into the breast milk.

All antibodies can have sugars bound to them. The types and amounts of these sugars vary depending on the part of the body the antibodies are in. We do not yet fully understand the significance of this, but the pattern of sugars associated with antibodies in breast milk probably supports them in promoting the baby’s wellbeing. For example, these factors might help the antibodies avoid being digested too quickly in the baby’s gastrointestinal tract.

After natural infection or vaccination, breast milk contains both types of key antibodies – immunoglobulin A and immunoglobulin G. These SARS-CoV-2 antibodies in breast milk have been found to neutralise the virus in laboratory models. This confirms they are likely to protect a baby from infection.

Interestingly, breast milk collected before the pandemic has also been shown to contain antibodies that respond to SARS-CoV-2. This suggests some women have developed antibodies to other human coronaviruses that might protect newborns against Covid-19 – though we do not know for sure.

Breast milk

While a Covid-19 infection and vaccination confer protective antibodies, there is no danger the virus itself can be transmitted through breast milk from mother to baby.

In a study that tested breast milk from women while they were Covid-positive, the researchers could not detect viral RNA (the genetic material of SARS-CoV-2) in the samples. Meanwhile, where unpasteurised expressed breast milk was fed to babies separated from their mothers who had Covid-19, none of these babies showed evidence of infection.

Similarly, the mRNA from Covid-19 vaccines is not detectable in the breast milk of women vaccinated while breastfeeding.

Using milk therapeutically

The presence of SARS-CoV-2 antibodies in the breast milk of women who have had Covid-19 or been vaccinated is incredibly important, as these antibodies will help to protect babies from infection.

This knowledge also paves the way for questions as to whether we could use breast milk to treat or prevent Covid-19.

Some of the health benefits of breast milk are already harnessed in various ways. Through human breast milk banks, for example, donated breast milk is used to save the lives of premature and sick babies.

The ability of SARS-CoV-2 breast milk antibodies to neutralise the virus is retained after high-pressure pasteurisation, which is a good sign.

If we were to consider using SARS-CoV-2 antibodies in breast milk to treat Covid-19, such an approach would be similar to that of convalescent plasma therapy. This is where antibodies from the blood of people who have had Covid-19 are administered to hospital patients with the virus to limit disease severity – although these trials have not been very successful.

We are still a long way off any kind of treatment like this. But the ongoing research to understand SARS-CoV-2 antibodies in breast milk is a good start.

Catherine Thornton is a Professor of Human Immunology and April Rees is a PhD Researcher in Immunology at the Swansea University.

This article first appeared on The Conversation.