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mRNA vaccines for COVID-19 – how do they work?

A newly authorized vaccine technology is being rolled out around the world to protect against COVID-19. Although Messenger RNA (mRNA) vaccine technology has been available for quite some time, these recent authorizations have prompted many questions about how they’re made and how they work.

While we are familiar with DNA, which encodes all our genes, we may be less familiar with mRNA. If you think of DNA as the blueprint of the cell and proteins as the materials that make up the structure, mRNA are the instructions that come from the interpretation of the blueprint to make all the structural components. Each gene in DNA is transcribed into an mRNA message in the nucleus of the cell. Then the mRNA message travels to the cytoplasm to be translated into protein by a ribosome. Afterwards, the mRNA message is degraded.

DNA ‎‎→ mRNA ‎‎→ Protein


Applying existing knowledge to SARS-CoV-2

Years of research have gone into understanding how to make stable mRNA and encapsulate it into lipid nanoparticles for delivery into cells. While research in this area continues to evolve, this knowledge was available at the start of the COVID-19 pandemic. So, to make an mRNA vaccine for COVID-19, all that was needed was the genetic sequence of the target antigen on the pathogen. For SARS-CoV-2, this was the spike gene. Shortly after the genetic sequence of SARS-CoV-2 was determined in January 2020, companies specializing in mRNA technology including BioNTech and Moderna started making mRNA vaccines for COVID-19.

The only active ingredient in both mRNA vaccines is the mRNA itself. Importantly, mRNA cannot alter cellular DNA -as it never enters the nucleus where the DNA resides. The mRNA vaccines also contain lipids, which protect the mRNA and help it get into cells, as well as salts, sugars, and buffers for stabilization. While the specific lipids and stabilizers differ between the vaccines, both mRNA vaccines have been found to be safe, with overall anaphylactic reactions in fewer than five cases per million doses.

mRNA vaccine is injected into the muscle of the upper arm, creating an immune response. The details are described in the paragraphs below.

Image courtesy of DynaMed. Copyright© 2021 EBSCO Information Services.


mRNA vaccines and the immune response

Both mRNA vaccines are injected intramuscularly into the deltoid muscle. Because of the hydrophobic nature of the lipid nanoparticles, they are attracted to cell membranes. Once the lipids are associated with cells in the muscle, the mRNA is released into the cytoplasm of the cell. In the cytoplasm, the vaccine mRNA gets translated by a ribosome into protein – in this case, the SARS-CoV-2 spike protein. Muscle cells around the injection site make lots of spike protein, which can be detected by immune cells in the tissue.

The injection site, a puncture wound and a bolus of injected fluid, cause cellular stress that alerts the immune system of a possible issue. The mRNA itself is also thought to stimulate an inflammatory reaction that kicks off the immune response. This inflammatory and stress response attracts immune cells into the muscle, which can lead to pain, swelling, and redness of the arm at the injection site.

Antigen presenting cells (APCs) are one type of immune cell that gets attracted to the site. The mRNA vaccine can enter APCs and make spike protein (the same way it does in the muscle cells) and stimulate B cells and CD8 T cells. APCs also play another pivotal role in initiating the immune response by picking up extracellular proteins, dead cells, and debris at the injection site. They break down the proteins, including the spike protein, and present peptide fragments to CD4 T cells. Furthermore, APCs take antigens from the injection site to the local draining lymph nodes where they have a greater chance of encountering B cells, CD4 T cells, and CD8 T cells with receptors that can recognize the spike protein.

When antigen-specific lymphocytes are activated, they undergo proliferation and differentiation from naïve cells to effector cells. Effector B cells make antibodies of different varieties that perform distinct tasks. An important function of antibodies generated after vaccination is neutralization (binding up the virus and blocking it from infecting cells). Effector CD4 T cells have a wide range of functions, from helping B cells make the most targeted antibodies to quelling the immune response. Effector CD8 T cells, often known as cytotoxic T cells, survey the body and eliminate infected cells. All this immune activity can cause systemic symptoms such as fever, fatigue, and muscle aches. This normal response, known as reactogenicity, is more common after the second dose of mRNA vaccine.


mRNA vaccine safety and efficacy

The most important consequence of vaccination is that some effector cells persist and retain memory of the SARS-CoV-2 spike protein (these cells are called memory B cells and memory T cells). If a vaccinated person is infected with SARS-CoV-2, circulating antibodies can bind up the virus, and the memory B cells and memory T cells initiate a rapid and strong response to clear the virus. For the mRNA vaccines, this response can prevent symptoms in about 95 percent of people and prevent severe COVID-19 and death in 100 percent.

mRNA vaccines may be a new technology, but they have been subjected to the same evaluation in clinical trials as other vaccines have previously. The more rapid authorization and distribution of a COVID-19 vaccine within one year was possible for a few reasons. First, many years of research on common cold coronaviruses, as well as SARS and MERS viruses were instrumental in understanding that the spike protein would be a key target for vaccination. Second, the need was so immediate that many companies initiated clinical trials at the same time they were doing preclinical studies, at great financial risk. Finally, because COVID-19 is so widespread, determining vaccine efficacy did not take as long as it had for other vaccines that prevent less-common infections.

Now that hundreds of millions of people have been vaccinated worldwide, it appears that mRNA vaccines against COVID-19 are both safe and effective and will be a key weapon in the arsenal to end this pandemic.


For more information, see the topic COVID-19 (Novel Coronavirus) in DynaMed

Original article published on EBSCO Health Notes. Written by:

  • Vito Iacoviello, MD, Deputy Editor for Infectious Disease, Allergy, and Immunology at DynaMed; and
  • Heather D. Marshall, PhD, Public Health Content Manager at DynaMed.

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About the author(s)

DynaMed is a clinician-focused tool designed to facilitate efficient and evidence-based patient care. Rigorous, daily review of medical literature by physician and specialist staff ensures timely and objective analysis, synthesis and guidance. DynaMed includes drug content from Micromedex, Canadian and international guidelines, and clinical images. CMA members have access to DynaMed, a point-of-care tool, included with their membership ― a tool valued at US$399 a year.