Passive Immunity: Does COVID-19 Disease or Vaccination Provide any Protection for your Fetus?

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If you have been reading articles here on The Pulse, and throughout the Pregistry site, then you know that the consequences of infection with SARS-CoV2 (the virus that causes COVID-19) can have a worse outcome, if you are pregnant than if you are not. In particular, pregnancy makes you more likely to suffer a severe case of COVID-19 if you are infected, more likely to be admitted to the ICU, and more likely to be put on invasive ventilation, meaning on a mechanical ventilator with a tube down the throat. Furthermore, having COVID-19 makes a woman three times more likely to give birth to a preterm infant compared with a woman who does not have COVID-19, plus it increases the likelihood that the newborn will be admitted to the neonatal unit. But you also may have heard that aspects of immunity from a mother can transfer to the fetus, or newborn, protecting the baby from infectious disease. Known as passive immunity, such protection can result from the transfer of antibodies, and possibly certain immune cells called T lymphocytes, that can recognize the disease-causing agent and neutralize, or partially-neutralize it. Antibodies, which are proteins, come in several different flavors, some of which are able to pass from a mother’s blood, through the placenta, to the fetal blood, while others are able to penetrate into breast milk. Let’s unpack these phenomena in terms of what we know specific to SARS-CoV2 and the vaccines that are being given to protect against it.

 Of the different products of the immune system, which you can liken to the different branches of the Defense Department, such as the Army, the Air Force, Navy, etc., antibodies are the force that we are most certain can reach the fetus and/or newborn from the mother. Above, we mentioned T lymphocytes, of which there are various subtypes, but there are also B lymphocytes. These are cells that make antibodies that initially they display on the outer surface of their cell membranes. Antibodies can bind things that the immune system sees as foreign, which are known as antigens. The antibodies can do this binding while sticking out from the surface of a B lymphocyte, but B lymphocytes also can become activated in a way that makes them reproduce into modified cells called plasma cells, which not only make antibodies, but also release them so that they travel freely in blood or other fluids.

 Antibodies are very specific to particular antigens —actually to parts of antigens, called epitopes— but they also come in a few different flavors. These flavors of antibodies include IgA, the flavor of antibody that is most abundant in body secretions, such as breast milk. It turns out that there is some evidence mounting that, if you have been infected previously with SARS-CoV2, the immune system that your body mounted may include IgA antibodies that penetrate into your breast milk, which may —possibly— give a nursing infant a little bit of protection against the virus, although this is by all means not certain, given that the infection is spread mostly by respiratory droplets.

 Following an infection, you also produce antibodies in your blood, especially of the flavors IgM, which comes in clusters of five antibodies together, and IgG, which comes as individual antibodies. Initially after an infection, most of the antibody response is IgM, which does not penetrate through the placenta, but, over time, your immune system produces less IgM against the virus, and more IgG. When it comes to COVID-19, there is growing evidence that IgG from a previously infected pregnant woman passes into the fetal blood, which, theoretically, should provide some protection.

 So what about the vaccines? Unlike the immune response to the virus itself, which includes antibodies against a handful of virus proteins, the vaccines are designed to cause an immune response against just the spike protein of the virus. So far, there isn’t much evidence of any anti-SARS-CoV2 IgA in breastmilk, meaning that a vaccinated, nursing mother would not be giving the nursing infant any protection against COVID-19. But there is growing evidence that, if you have been vaccinated, and if several weeks have gone by so that your immune system is now producing good levels of IgG against the virus, such IgG will indeed reach your fetus through the placenta. Stay tuned as we learn more about this exciting possibility.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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