Decoding Pfizer’s Covid-19 Vaccine Ingredients

People have started receiving doses of the BNT162b2 vaccine in the U.K. This article details the ingredients included in the vaccine.

Image: Wikipedia commons

On December 8 a U.K. woman named Margaret Keenan became the first person to receive a Covid-19 vaccine. At age 90, Keenan marked the beginning of what will be a broad global effort to vaccinate people against Covid-19.

As doses make their way to the United States, this article details the ingredients included in Pfizer’s BNT162b2 vaccine.

The basics

BNT162b2, much like the vaccine being offered by Pfizer-BioNTech, is an mRNA vaccine made up of strands of RNA packed into lipid nanoparticles. These particles, when injected into the human body, will fuse to the cell membrane of human cells and allow the mRNA to gain access to the machinery needed to translate it into viral proteins. Once these proteins are generated, they are packaged and released outside of the cell.

Once the viral proteins are released, our immune system has the opportunity to generate antibodies against them.

Lipid nanoparticle technology

The core of the Pfizer vaccine is the mRNA molecule that encodes for the spike protein (more on this later). In order to get mRNA from injection into the cells of the body, it needs molecular escort. By itself, mRNA cannot freely cross our cell membranes. Lipid nanoparticle technology solves this problem by packaging mRNA — known as a transcript — into a complex vesicle of phospholipid molecules that are designed to fuse with our body’s own cell membranes.

The lipid nanoparticle wall of the BNT162b2 vaccine is made up of four different compounds:

  • ((4-hydroxybutyl)azanediyl)bis(hexane-6,1-diyl)bis(2- hexyldecanoate) — A proprietary phospholipid that makes up the basic structure of the nanoparticle wall. These molecules are modeled after native phospholipids found in living cells.
  • 2-[(polyethylene glycol)-2000]-N,N-ditetradecylacetamide — Another commercially available phospholipid linked to polyethylene glycol.
  • 1,2-distearoyl-sn-glycero-3-phosphocholine — A commercially available glycophospholipid used as a component of the cell wall.
  • Cholesterol — A native molecule ubiquitous in the human body. Cholesterol aids in the fluidity or rigidity of a lipid membrane depending on temperature.

mRNA

The mRNA component of Pfizer’s vaccine provides the human body the blueprint to generate copies of specific coronavirus antigens that elicit an immune response.

Pfizer’s vaccine includes a synthetically generated version of the coronavirus spike protein. This mRNA has been modified to make it more bioavailable by introducing analogs to the RNA sequence. The resulting mRNA is known as nucleoside modified RNA (modRNA).

The DNA template was derived from SARS-CoV-2 viruses isolated in Wuhan, China, at the emergence of the pandemic. The genome from isolated viruses was then sequenced, and the code for specific antigens were made publicly available.

Using an enzyme called RNA polymerase, specific mRNA molecules can be generated using a template plasmid. These sequences also need a few key features to make them recognizable to the human cell, and safe from degradation. These include:

  • 5' cap
  • 5' untranslated region (UTR)
  • Coding sequence (CDS) for SARS-CoV-2 spike protein
  • 3' untranslated region (UTR)
  • Poly-adenosine (Poly-A) tail

Additional ingredients

Beyond the compounds used in making the nanoparticle, there are some ingredients used to control the aqueous environment that the vaccine is delivered in. These ingredients enhance the stability of the nanoparticle, and control against swings in pH. The ingredients in this list function similar to some of our body’s natural buffering systems. These include:

  • Potassium chloride — Simple salt of potassium (K+) and Chloride (Cl-), two ubiquitous elements found in the body.
  • Dipotassium phosphate — A potassium salt of dihydrogen phosphate, a naturally occurring compound. Used as a buffer to protect against swings in pH.
  • Sodium chloride — Laboratory-grade table salt.
  • Disodium phosphate — Similar to dipotassium phosphate, this is a sodium salt of dihydrogen phosphate. Also used as a buffer.
  • Sucrose — Laboratory-grade table sugar. Used to stabilize nanoparticles during transport.

The ingredient list for this vaccine is relatively simple compared to some preparations. Because these vaccines need to be stored at very cold temperatures, there is no need for preservative agents.

Also, because most of the compounds involved in this vaccine are native (or derived from native) compounds to our body, they pose very low risk for adverse reactions. The main risk that could be associated with vaccines of this type are expected reactions like injection site redness and soreness, fatigue, or headache — all of which resolve on their own within a day or two.

Some vaccine recipients have experienced allergic reactions to the BNT162b2 vaccine thus far. Most of these reactions are mild and expected. Some were moderate, but participants recovered well. During clinical trials, only 0.4% of first dose recipients, and less than 0.3% of second dose recipients experienced an immediate allergic reaction.

The BNT162b2 vaccine has been shown to be over 95% effective against Covid-19. RNA vaccines are relatively fast and easy to manufacture. Still, rollout of the vaccine as well as supply chain issues remain significant challenges.

One unique challenge to the BNT162b2 vaccine is the required ultracold storage (-80 degrees Celsius) required to maintain viability of the vaccine. Once thawed and diluted, the vaccine must be used within six hours. U.S. clinics are already preparing training and mock vaccinations for staff to avoid any loss of product.

With patients currently receiving vaccines in the U.K., and doses on their way to the United States in the coming weeks, BNT162b2 will likely play a critical role in curbing the spread of the Covid-19 pandemic.

Find more information on FDA’s approval process of BNT162b2 here.

Medical student, molecular biologist and educator. I write about science and medicine.

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