Expert reaction to antibodies found in babies born to COVID-19 mothers

A study, published in JAMA, reports on antibodies found in babies born to COVID-19 mothers.

Professor Sarah Rowland-Jones, Florey Professor of Infection and Immunity, U. Sheffield/Professor of Immunology, U. Oxford

“In the last trimester of pregnancy, particularly in the last few weeks, there is an active process going on in the placenta which leads to the capture of the mother’s antibodies (to whatever pathogens she has previously encountered) from the circulation. These are then transferred across the placenta to her developing baby and will provide the infant with some protection against infections in the early months of life.

“Usually only the IgG class of antibody crosses the placenta in this way, and certain subclasses of IgG are transferred more efficiently than others. If the placenta is healthy (and not damaged, for example by infections such as malaria, HIV or cytomegalovirus) the end result is that the infant has roughly the same levels of antibodies against bacteria and viruses as his/her mother.

“The JAMA study looks at a small number of women who acquired confirmed Covid-19 in pregnancy in Wuhan. None of the six women had severe disease, and all their infants appeared to be healthy after delivery with no evidence of SARS CoV2 infection. All six infants had IgG antibodies against the SARS CoV2, which are likely to have been given to them by their mothers through the process described above, and may well have provided some protection against coronavirus infection for these children. This is to be expected, but it is more surprising that two infants had IgM antibodies against the virus as well as IgG. IgM is the kind of antibody that the body makes first in response to an infection, but this is later replaced by IgG antibodies which last longer and are usually more effective against the virus.

“IgM antibodies form complexes that are larger than IgG antibodies and are not usually transferred across the placenta. The authors speculate that either the babies were able to generate their own antibodies to the infection (even though there is no evidence that any of them were actually infected) or that the placenta might have been damaged by the Covid-19 infection leading to transfer of larger IgM molecules – the placenta was not however examined in these women so it is impossible to know if this was the case.”

Prof Richard Tedder, Visiting Professor in Medical Virology, Imperial College London, said:

“This study is comforting in one sense, there was no evidence of viral presence in the six children. Viral PCR did not reveal the presence of viral nucleic acid at any site. The implications are that at the time of birth the infants were not infected.

“All six children were found to have IgG antibody in their plasma which in five cases was commensurate with the level in the mothers plasma, clearly indicating likely transplacental passing of IgG antibody from mother to infant. One infant whose mother had very low levels of IgG appears to have a twofold higher level in the infant serum than in the mother, this probably reflects variability in the assay performance. Two infants are shown to have easily detectable IgM in their plasma, however at a lower level than present in the maternal sample. These would be compatible with a possible serological response arising in the infant since IgM does not usually cross the placenta.

“Whilst this could indicate an infection in utero, the fact that the two infants had no detectable virus at the time of birth renders transplacental infection in utero unlikely.

“The human placenta has evolved, along with other higher primates, to allow the passage of IgG antibody into the infant. It is generally accepted that this feature of the primate placenta confers protection to the unborn child which becomes manifest in the first stages of life by the protection against a range of common microbial infections in the first few months of life. To confirm the existence of intrauterine infection, undetectable by PCR, continued follow-up of the children should show that the IgG levels persist. If this is done and the infants antibody levels wane with time after delivery then infection would be deemed not to have occurred.”

Dr Simon Clarke, Associate Professor in Cellular Microbiology, University of Reading, said:

“This report tells us that there is transfer of antibodies against the coronavirus causing COVID19 in the womb, during pregnancy. It does not tell us that the presence of antibodies in the babies has protected them against infection; it should be noted that they were separated from their mothers immediately after birth. Even if the antibodies were protective, and there is no evidence of that, any immunity would dwindle because B-cells which produce the antibodies, are not transferred.”