With the onset of the coronavirus disease 2019 (COVID-19) pandemic, hundreds of millions of people became infected worldwide, including thousands of pregnant women near term. The question of vertical transmission to the infant via the placenta or breast milk was very real, leading to a near-total ban on breastfeeding in this group.
A timely new study posted to the preprint server medRxiv* proves that breastfeeding should be encouraged even in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-positive mothers.
Importance of breastfeeding
Breastfeeding has long been the preferred mode of infant nutrition, with its multiple vital contributions to infant growth and development. Its long-term benefits are evident since breastfed infants are less likely to become overweight and develop allergies and bacterial infections.
Exclusive breastfeeding is recommended for six months at least from birth, and even after that, along with other foods, for up to two years.
During the earlier part of the pandemic, in the absence of evidence, most centers chose to separate babies from their mothers until the latter tested positive for the virus. This meant that breastfeeding was also discouraged. While this question has not been conclusively answered, there is little proof of mother-to-fetus transmission, and few newborns have been born infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
While some studies show the virus to be present in breast milk, its ability to cause infection is dubious, and other researchers failed to validate these findings. Many of these used poorly designed protocols to detect the virus.
Moreover, SARS-CoV-2 infection in the mother has been associated with a robust and rapid antibody response in maternal serum and breast milk. Of particular importance is the presence of high amounts of specific neutralizing secretory immunoglobulin A (IgA) antibody in breast milk.
However, the duration and extent of immunity conferred by antibodies in breast milk remain unclear. Cross-reactive antibodies elicited by human endemic coronaviruses are also suspected to be the cause of positive antibody testing, rather than SARS-CoV-2-specific antibodies
The current study was designed to examine how maternal SARS-CoV-2 infection affects the occurrence, titer, and persistence of specific antibodies in breast milk.
Examining 60 mothers who had current or past SARS-CoV-2 infection, the researchers looked for viral genetic material in the form of RNA (ribonucleic acid), specifically its nucleocapsid gene (N1 region) and the envelope gene, in the breast milk.
What were the findings?
Not a single sample showed the presence of viral RNA.
While 85% of breast milk samples were positive for one or more antibodies to the viral receptor-binding domain (RBD), just over half of them contained all three RBD-specific Ig classes. About 17% were non-reactive to the RBD.
The search and measurement of virus-specific immunoglobulins (Igs) – IgA, IgG, and IgM – in breast milk from infected and pre-pandemic women showed vast differences in the antibody responses between individuals and in antibody titer between the antibody subclasses.
Among the various samples, 73% showed IgA and IgM positivity, respectively, with IgG positivity in 64% of samples. These values were confirmed when antibodies to the viral main protease (Mpro) antigen were detected.
While the proportion of IgA-positive samples remained somewhat constant over time, the IgG-positive fraction rose steadily from 48% to 88% over the period from day 41 to day 206. As expected, IgM positivity was most significant at 11-20 days, at 83%, falling after that to 63%.
Total IgA titers were found in the COVID-19 group relative to pre-pandemic controls, and this change could be part of the antiviral response. Over the first 20 post-partum days, RBD-specific IgA titers were found to reduce in all but one woman.
Total IgA fell as the duration of breastfeeding rose and as RBD-specific IgA and IgM titers rose.
Two women showed specific antibody titers at birth even though they had tested positive 96 and 186 days before giving birth. The highest titers were seen with IgG. In one mother who was seropositive 226 days before the sample for this study was collected, the sample did not show RBD-specific antibodies of any subclass.
What are the implications?
Earlier evidence has not shown evidence of viral transmission in breast milk, and only up to 6% of samples in any study have shown viral RNA to be present. The present study confirms this.
However, common sense would dictate that mothers with a history of recent infection should use all measures to prevent transmitting the virus to their infants – whether hand washing, wearing a mask, and avoiding airborne spread.
The presence of antibodies at up to 206 days from the first positive PCR test also corroborates earlier findings showing IgG antibody persistence for up to six months in 90% of infected individuals.
Moreover, the Mpro antigen was used for the first time to detect virus-specific antibodies in human milk, and this bears out earlier findings that different viral antigens react at varying intensities to specific antibodies.
Interestingly, IgA levels in breast milk exceeded those in maternal serum, unlike the other two antibody subclasses, indicating that it may be not just transferred from the blood but produced in the breast tissue. This has earlier been observed with rotavirus infection.
The findings support the presence of antibodies to SARS-CoV-2 in breast milk, though with wide variations between individuals and within the different subclasses. The samples contained not only RBD-targeted antibodies but also those elicited by other viral proteins, such as Mpro.
The presence of IgM in at least some samples may point to a potential protective effect for the infant. These findings thus support the official guidelines on breastfeeding by carefully obtained data.
“Our study endorses the safety of breastfeeding practices and highlights the potential relevance of virus-specific SARS-CoV-2 antibodies providing passive immunity to breastfeeding infants protecting them against COVID-19.”
medRxiv publishes 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.