Pregnancy Vaccine Tutorial: The Pfizer-BioNTech and Moderna Vaccines for COVID-19

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Things are moving quickly in the realm of COVID-19 vaccines and there are now two vaccines that could be available to you in the weeks or months to come, depending on factors influencing your risk, such as your line of work. All of this can be particularly frustrating if you are pregnant, or maybe even holding off on getting pregnant to avoid possibly being last in line to receive a COVID-19 vaccine. In several countries, including the United Kingdom and the United States, the Pfizer-BioNTech vaccine is now being administered beyond the scope of Phase 3 testing to the extended population, beginning with people at high risk for COVID-19, such as health care workers. Because of their high visibility and the need to set an example for the public, certain government officials, such as the vice president and the surgeon general of the United States, also have been administered the first of two doses of the Pfizer-BioNTech vaccine. Meanwhile, at the time that I’m writing this, the Moderna vaccine effectively is on-deck to be administered in the United States; during the third week of December, it was endorsed by a review panel in the US Food and Drug Administration (FDA), implying that soon (perhaps already by the time that you are reading this) Moderna will receive the emergency use authorization (EUA) that the Pfizer-BioNTech vaccine received the evening of December 11.

The Pfizer-BioNTech and Moderna vaccines use the same technology. Both consist of a strip of messenger RNA (mRNA) contained within what’s called a lipid nanoparticle, a little sphere built of the same kind of molecules —called phospholipids— that comprise the membranes of your body cells, the cell membrane, although the cells also have numerous proteins embedded between the phospholipid molecules. The composition of the lipid nanoparticles allows the particles to merge with the cell membranes of your cells, causing the mRNA payload to get deposited into the cytoplasm of the cell, region of the cell outside the cell nucleus. The mRNA encodes a protein, called the spike protein that SARS-CoV2 (the virus that causes COVID-19) has sticking out from its protein coat that causes the corona (crown) appearance and also enables the virus to hijack a protein present on many different types of body cells, called the ACE-2 protein, and utilize the ACE-2 to invade body cells. But the same spike protein also stimulates the immune system, so it was selected as a target for the vaccines being developed to teach the immune system to recognize and respond to SARS-CoV2 much faster than occurs when one is infected with the virus itself. The mRNA gets into muscle cells, because you are given the shot into the deltoid muscle of your arm, but some of the mRNA also gets into various types of other cells. Inside the cells, biological machines called ribosomes read the instructions in the vaccine mRNA and translate those instructions into numerous copies of a unit of spike protein, and each spike protein is made by combining three of these spike protein units. In all cells that have a nucleus (which is most types of body cells) where the vaccine mRNA arrives, the spike protein ends up on the outside surface of the cell, connected to a particular other protein that is always there. Muscle cells, have this other protein, which means that muscle cells where you get injected will display the spike protein on their surface, connected with that other protein. Probably more importantly, especially for the first shot, is that a tiny bit of the vaccine gets into what scientists call antigen displaying cells, of which there are three kinds: B lymphocytes, dendritic cells, and macrophages. These cells make not only that other protein to which the spike protein is attached and gets displayed on the surface of the cell, but also make another surface protein. The spike protein gets delivered to the surface of these antigen-displaying cells also attached to this other special protein. When this happens, other cells called T-lymphocytes are introduced to the spike protein and this starts the process of your immune system learning to defend against SARS-CoV2. While most of the vaccine probably goes to muscle cells, which don’t that second, very special protein, the spike protein attaching to the more universally made protein (the first kind that, as noted above, all body cells with nuclei make) does stimulate the transformation of what are called B lymphocytes into more specialized cells called plasma cells, which make antibodies. When this is triggered by a protein that the immune system knows already, it’s very significant, which means that the bulk of the vaccine getting into muscle cells may be doing something very big after you receive the second dose of the vaccine. It really is very beautiful how all of this molecular biology and immunology fits together. It’s like the musicians and instruments of an orchestra that have been rehearsing together for many months.

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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