Updated: Nov 12, 2021
Pfizer/BioNtech and Moderna were on the center stage in November when they announced the development of a 95%-efficacy vaccine against COVID-19. This vaccine has already been approved in more than 40 countries, including the 27 members of the EU, and the vaccination campaign started late December in France. These vaccines, called RNA vaccines, rely on very recent scientific discoveries and will be used world-wide for the first time. But what are they? Which advantages and drawbacks do they imply? Can RNA integrate and thus modify our cells' genome? How could they be developed so quickly? Here is a small overview of what we know.
What is RNA?
RiboNucleic Acids (RNA) are molecules found in each and every living organism, and some viruses. There are several types of such molecules, but those present in the vaccines are called messenger RNA (mRNA) and are used by the cells to produce proteins. Among various roles, cells produce proteins that are necessary for the whole organism. The information needed for the making of these proteins is contained in the DNA composing our genome, stored and protected inside the cell nucleus, as a recipe is in a book. A fragment of DNA is, therefore, the recipe for a specific protein. This fragment is firstly copied as a messenger RNA, which then gets out of the nucleus to enter the cytoplasm. This is where it is converted into a protein thanks to a specific cellular machinery.
Figure 1: Protein synthesis from DNA. 1) The RNA is synthesized in the nucleus from gene sequences contained in the DNA. 2) The RNA leaves the nucleus to enter the cytoplasm. 3) The RNA is translated into a protein by the cellular machinery. 4) Proteins are used by the cell for its own maintenance or are secreted in the extracellular matrix to serve a role in the organism. Made with BioRender.
How does a vaccine work?
The aim of a vaccine is to ‘’train’’ a person’s immune system to recognize a pathogen such as coronavirus. It will make it faster and better to respond and fight when the actual virus is encountered. For that, several different viral components can be used, such as live attenuated viruses (measles, yellow fever), inactivated ones (poliomyelitis, flu) or viral protein fragments (Hepatitis B) which is then administered to the population. The viral components stimulate the person’s immune system without threatening the organism. The immune system recognizes the vaccine as a foreign body and triggers a defence strategy to eliminate it. Vaccination activates two main mechanisms: one ‘’blocks’’ the virus entrance in the cells using neutralizing antibodies produced by B cells, while the other detects and obliterates infected cells through activation of T cells. This immune response usually takes 2 to 5 days to develop, but up to 3 weeks are needed to reach an efficient response level. The human body then keeps the signature of the virus through memory B and T cells roaming throughout the entire organism. If a new infection occurs, the immune system will be able to react much faster, thus preventing the disease.
What about the RNA vaccine?
RNA vaccines are based on this very same model described above, but without involving the direct injection of a viral component. A synthetic messenger RNA containing the coding sequence (the recipe) for a fragment of a virus is first created in a lab. Then, non-coding sequences are added that help stabilize the molecule and make it easier for the cell to synthesize the protein 1.
This mRNA is then injected during vaccination, either directly, or associated with a transporting molecule that eases the RNA entry in the organism’s cells. For instance,the transporter can be a liposome, a ‘’fat bubble’’ with a structure similar to other molecules already present in the human body.
As previously described, the mRNA will enter the cells at the injection site (usually in the arms muscle cells) and allow the production of the viral proteins in the cell’s cytoplasm. These proteins are then exposed at the surface of the cells, where they will be used as a bait for immune system cells, which will recognize and eliminate the infected cells.
Figure 2: Action mechanism of the RNA vaccine. The mRNA contains a sequence (purple) encoding a part of the virus and non-coding sequences making it more stable and easier to translate into a protein (blue). It is encapsulated in a lipidic bubble. 1. After vaccine injection, the mRNA enters the cell. 2. The lipidic bubble breaks up and the RNA is released. 3. The mRNA is translated into a protein that can be secreted outside the cell to activate the immune system. 4. The protein can also be degraded by the cell components. 5. The protein is displayed at the cell surface and serves as a bait identifying the cell as infected. 6. B and T cells are activated. 7. B and T cells obliterate the cell. Made with BioRender.
The mRNA used in the vaccines developed by Moderna 2 and Pfizer/BioNTech 3 encodes part of the ‘’Spike S’’ surface protein of SARS-CoV2. This key molecule allows the virus to adhere and infect the organism’s cells. This protein provides such interest for vaccines, because it is located on the surface of the virus. Our immune system is better at targeting surface proteins, especially for antibody-mediated responses that are triggered by vaccination. Thus, the S protein is the ideal candidate from a vaccination point of view.
A specific feature of the Moderna vaccine is the addition of a viral protein sequence encoding a replicase. It allows the mRNA containing it to replicate itself, making it a self-replicating RNA. Thus, the immune response increases from the small dose of initially injected virus.
Why are these vaccines developed only now?
The first studies on RNA vaccination were completed during the 90s. Researchers demonstrated that the intramuscular injection of mRNA allowed the production of encoded proteins and triggered an immune response in mice 4. However, the development of this technique was slowed down because of the instability and quick degradation of mRNAs. Furthermore, the injection of the mRNA alone had a poor efficiency because the molecules do not enter the cells easily, which is a crucial step for protein production. Recent advances such as mRNA sequence optimization and addition of transporters such as lipidic bubbles that ease the entry allowed the development of the first applications based on this technique, with a first clinical trial in cancer therapy in 2002 5.
However, we had to wait until 2012 for the first preclinical trials on pathogens to be made. BioNTech and Moderna currently have numerous ongoing trials against other diseases such as Zika virus, flu, prostate cancer or melanoma 1. For instance, a vaccine based on this method has been available since 2018 to prevent swine flu in pigs 6.
All those studies have allowed fast development of these two vaccines against COVID-19, which also benefited from a great investment from researchers, huge funding from public institutions, and fast-tracked procedures for clinical trials and marketing authorization.
Can RNA integrate our genome?
Messenger RNA vaccines are safe, because the RNA cannot enter in the cell’s nucleus where the genome is encapsulated. For the mRNA to enter the nucleus, it would indeed need first to be copied into DNA, or retro-transcripted. This mechanism never occurs spontaneously in human cells. There is consequently absolutely no risk for it to permanently integrate our genome, to induce modification or even to be transmitted to our children.
What are the advantages?
mRNA vaccines only encode a small part of the virus, which is not sufficient to produce a complete virus. Thus, there is no risk of infection through vaccination. Moreover, messenger RNA never stays in the organism for more than a few days, because it is unstable and quickly degraded by the cell.
Another advantage of RNA vaccines is their cheap and fast production, which makes it easy for a mass production scale. Those are the main reasons why they were the first solution available on the market, a good thing in the context of a global pandemic.
Yet another advantage is that RNA vaccines are able to trigger a strong innate immune response 6. Indeed, an additional immune response called PAMPs-PRR exists. This recognizes foreign RNAs and results in their degradation. When triggered, this system induces the production of inflammatory molecules (inflammatory cytokines) such as interferon I (IFN-I), which attract immune cells, namely CD4+ T and B cells. Thus, the strong innate immune response allows to get rid of adjuvants, a type of molecule widely used in other types of vaccines to strengthen the induced immune response.
And the drawbacks?
On the down side, the main disadvantage of RNA vaccines is the lack of studies on the long-term impact of this recent technology. A possible risk of this method could be an excessive activation of the innate immune system. Consequently, this technique could put at risk people affected by auto-immune diseases. A clear limitation of some of these vaccines is their stability. They need to be stored at extremely low temperatures (-70°C) to remain usable. It is thus impossible to buy big quantities in advance, or to store them in your fridge before getting the injection. This makes it necessary for vaccination centers to have a great and special storage capacity and be able to inject them quickly after thawing.
Vaccines based on other methods are currently being developed, such as the DNA vaccine from AstraZeneca and Oxford University, or the Sanofi/GSK one. France already ordered some of these and is counting on a strategy using various candidates, subject to clearance by authorities. Despite those limitations, among other candidates, mRNA vaccines remain promising for now, easy and fast to implement. For these reasons, they seem quite fitted to vaccine development in the context of pandemic.
1. Pardi, N. et al. (2018) mRNA vaccines-a new era in vaccinology. Nat. Rev. Drug Discov. https://doi.org/10.1038/nrd.2017.243
2. Corbett, K. S. et al. (2020) SARS-CoV-2 mRNA vaccine design enabled by prototype pathogen preparedness. Nature. https://doi.org/10.1038/s41586-020-2622-0
3. Sahin, U. et al. (2020) COVID-19 vaccine BNT162b1 elicits human antibody and TH1 T cell responses. Nature. https://doi.org/10.1038/s41586-020-2814-7
4. Wolff, J. A. et al. (1990) Direct gene transfer into mouse muscle in vivo. Science. https://doi.org/10.1126/science.1690918
5. Kyte, J. A. et al. (2006) Phase I/II trial of melanoma therapy with dendritic cells transfected with autologous tumor-mRNA. Cancer Gene Ther. https://doi.org/10.1038/sj.cgt.7700961
6. Pardi, N. et al. (2018) Nucleoside-modified mRNA immunization elicits influenza virus hemagglutinin stalk-specific antibodies. Nat. Commun. https://doi.org/10.1038/s41467-018-05482-0