The mRNA moves out of the nucleus to our cells’ cytoplasm. Here, molecules called ribosomes translate the RNA’s code into proteins. In short, the ribosome is like a protein-making factory, and the mRNA created from our DNA is the blueprint for the proteins it makes.
RNA vaccines take advantage of the fact that our ribosome factory doesn’t care where a blueprint comes from. So if we can smuggle a new blueprint for the virus spike protein into this factory, the ribosome will assemble the protein without question. Once it’s manufactured, the spike protein sticks to the surface of our cells and triggers a response from our immune system.
Smuggling the blueprint into our cells isn’t straightforward. If we simply inject the RNA on its own, enzymes in our bodies would break it down before it could enter our cells. For this reason, it’s encapsulated in lipid nanoparticles: tiny fat droplets around one billionth of a metre in diameter. These nanoparticles shield the RNA, stopping it from breaking down, and help it get taken up by our cells.
Amongst the COVID-19 vaccines, there are two types of RNA vaccine. These are messenger RNA (mRNA) vaccines, like those produced by Moderna and Pfizer/BioNTech, and self-amplifying RNA (saRNA) vaccines, like that developed by Imperial College London.
The structures of mRNA and saRNA used in the vaccines are very similar but have one key difference. Both contain the region of the RNA which codes for the virus spike protein. Both also contain a cap, which stops the RNA breaking down and helps start protein synthesis in our cells, and a tail which helps stabilise the RNA. Unlike mRNA, saRNA also contains the code for a virus enzyme. This enzyme helps create multiple copies of the virus RNA once it’s in our cells, leading to quicker protein production.
As saRNA produces more copies of itself once it’s in a cell, it means that we can give vaccines containing it in smaller doses than mRNA vaccines. This means that the cost per dose is lower and that the same volume of vaccine produces more doses.
RNA stability is an important consideration for the way in which we store and transport these vaccines. Some need low-temperature storage to remain stable. The Pfizer/BioNTech vaccine requires a transportation temperature of –70 ˚C and can be stored for up to five days in a fridge after delivery. The Moderna vaccine requires a transportation temperature of –20 ˚C, and after thawing can be stored at refrigerator temperature for 30 days. Temperature matters: chemical reactions happen more quickly at higher temperatures, so low temperatures ensure the RNA remains intact.
Though these vaccines will be the first licensed RNA vaccines, they are not the first to be developed. They’ve been under development for several years for other viruses, including influenza, HIV and Zika. They’re also not the first RNA-based medication to gain approval. That title goes to Onpattro, a medication approved in the US and EU in 2018, which treats nerve damage.
RNA vaccines have several benefits over other vaccine types. The most obvious is the pace at which we can make them. COVID-19 vaccines are setting new records for the speed with which a vaccine has gone from development to approval. Synthetic RNA is straightforward to make in a lab, so it doesn’t take long to design and produce these vaccines. Take Moderna: they finalised the RNA sequence for their vaccine just two days after Chinese scientists shared the genetic sequence of SARS-CoV-2, and they made the first clinical batch of the vaccine just 25 days after this.
RNA vaccines also have safety benefits. The synthetic RNA can’t cause illness – though it’s a blueprint for virus spike protein production in our bodies’ cells, production of this protein alone can’t trigger an infection. The RNA itself gets broken down by normal processes in our cells, so it doesn’t hang around for long anyway.
There’s good evidence from the trials that these RNA vaccines are effective in preventing COVID-19. Moderna’s vaccine has shown 100% efficacy against severe disease, and general efficacy of 94.1% – a higher figure than perhaps expected. The Pfizer/BioNTech vaccine has reported similarly impressive results of 95% efficacy. This compares pretty well with vaccine efficacies for other diseases. For example, the average influenza vaccine effectiveness since 2010 is 42%.
Like vaccinations for many diseases, the RNA vaccines for COVID-19 need two doses. The body’s immune response, when confronted with the coronavirus spike protein, is to produce antibodies and memory cells. This response helps the body respond quickly if it spots the virus. Multiple doses increase the quantities of memory cells created, meaning a faster and more effective response if we encounter the virus.