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ACEP Now: Vol 40 – No 02 – February 2021

COVID-19 mRNA vaccine induces an immune response through the activation of T cells with antigenes obtained out of the viral spike protein (red)., which is encoded by the mRNA contained in the vaccine. APC=antigen-presenting cell. Credit: JUAN GAERTNER / SCIENCE SOURCE

Two COVID-19 vaccine candidates, one from Pfizer-BioNTech and the other from Moderna, have received emergency use authorization (EUA) from the Food and Drug Administration. For the first time outside of clinical trial environment, vaccinations began in the United States in mid-December. 

Ingredients in the Pfizer and Moderna vaccines are listed in Tables 1 and 2. Both vaccines use gene-based technology, relying on synthetic messenger RNA (mRNA).¹ In each, a synthetic strand of mRNA created for these vaccines was designed to emulate the SARS-CoV-2 mRNA strand that specifically codes for the production of the coronavirus-specific spike protein. Once inside of the cell, our own ribosomes translate the synthetic mRNA into the spike protein. That protein is then recognized as foreign, and our immune systems create antibodies. 

This spike protein was chosen because it binds and fuses with human cells, allowing entry.1 It resides on the viral capsid (envelope) of the coronavirus, a lipid bilayer that encloses the viral nuclear material.² Thus, this viral surface spike protein is visible to the human immune system and is highly antigenic during virus invasion. 

Unlike conventional vaccines, the chosen mRNA segment does not come from “live” viruses grown in eggs or cell culture. Rather, it is a hybrid mRNA synthesized in the lab.^1,3 Therefore, such vaccines can be developed rapidly in large quantities, with the potential for rapid production and delivery to large-scale recipient populations. This technology is highly attractive for the ability to produce and provide vaccine doses rapidly, both to address current needs and to address subsequent pandemics that may appear and spread rapidly in the future.

Vaccines based on mRNA represent a new approach, one that had not previously been used clinically for infectious diseases. There have been some small, early-phase trials researching mRNA vaccines for diseases such as rabies, influenza, and Zika. However, the majority of mRNA research until recently had been directed at cancer immunotherapy. This is the first time mRNA vaccines have been proposed for wide-scale clinical release and utilization. Of note, two other vaccine candidates are currently either in trials or widespread use: the University of Oxford/AstraZeneca vaccine (now in use in the United Kingdom) and the Johnson & Johnson/Janssen Pharmaceuticals vaccine (trials are under way). These use more traditional vaccine technologies based on adenoviruses.  

Table 1: Ingredients of Pfizer-BioNTech mRNA Vaccine

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Table 2: Ingredients of Moderna Vaccine

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