November has brought optimistic news about the vaccines being produced by biotech companies Moderna and Pfizer / BioNTech. Both use a new technology, that of messenger RNA (mRNA). Being labile molecules, they face the same challenge: they must be stored at very cold temperatures.
Let us remember that the more than 180 vaccine candidates against SARS-CoV-2 are based on different synthesis platforms (Figure 1). Some chose to use inactivated complete SARS-CoV-2 viruses that cannot multiply. Others decided to use the vector technique with non-replicative viruses: the virus is genetically modified to reduce its virulence and prevent it from replicating. These types of vaccines, which follow the classical model, are the most common and are used to develop doses against influenza, measles or Ebola.
For their part, Moderna and Pfizer / BioNTech have tested and developed a new type of vaccine that uses a small fragment of the virus’s messenger RNA (mRNA) to convert a patient’s cells into factories that produce a coronavirus protein, specifically the one that forms the spikes with which the virus penetrates our cells.
The viral mRNA sequence injected by the vaccine is not enough to cause the damage that a complete virus would do, but it is enough for our immune system to learn to recognize it and be able to unleash an immune response to fight future infections. By detecting the sequence of the spiculated protein, our immune system produces antibodies and stimulates T cells, the lymphocytes produced in the thymus whose main purpose is to identify and destroy infected cells.
Building the vaccine
Think of the vaccine as an ounce of chocolate that melts easily at room temperature. Just as there are ways to prevent chocolate from melting, biotechnologists at both drug companies devised mechanisms to protect their trials. Given the reactive promiscuity of some biomolecules, protecting them is not easy.
Organic reactions blindly obey (if they don’t, things will go very wrong) the genes. What genes do is give instructions to make proteins. Most of the useful things in the body are. Some, hormones, transmit chemical messages. Others, antibodies, attack pathogens. Other proteins, enzymes, are catalysts that accelerate an organic reaction by lowering the activation energy or changing the reaction mechanism.
The problem for biotechnologists working with RNA is that it is a molecule under constant threat of being destroyed by ribonucleases (RNases), the enzymes that catalyze the hydrolysis of RNA by breaking its ribonucleotide chain into smaller components. In other words, they break down the genetic puzzle of the virus by taking its pieces apart.
Ribonucleases are a primary line of defense against infectious agents that use RNA as genetic information and that is why they are extremely common, so common that they can be on a researcher’s breath, on their skin, or on anything they have touched, whatever. the consequence is that any RNA has a very short life in an unprotected environment.
The protection mechanism used in biotechnology is the ribonuclease inhibitor (IR), which binds to certain ribonucleases with higher affinity than any other protein-protein interaction, preventing them from “attacking” the RNA. IR is used in most laboratories studying RNA to protect their samples from degradation by environmental RNases under laboratory conditions.
Therefore, biotechnologists first made some modifications to the molecular structure of mRNA to make it more stable. Basically, what they did was modify the nucleotides of the mRNA (the “puzzle pieces” of the viral genome) using more stable modified versions. That is something like changing some ingredients of the chocolate so that it does not melt too much without losing its essences.
Commercial chocolate, in whatever form it is presented, is immersed in a recipe that includes stabilizers and preservatives, among other additives whose use is unthinkable in a vaccine. Therefore, to avoid any damage, researchers not only have to make chemical changes to the mRNA and wrap it in a protective layer, they must also store it at low temperatures until the chemical reactions slow to a near stop.
Pure chemistry or, rather, pure chemical kinetics. Chemical reactions have different kinetics, that is, they proceed at different speeds. For example, the oxidation of iron under atmospheric conditions is a slow reaction that can take years to complete, while the combustion of natural gas is a reaction that happens in fractions of seconds.
Everything happens more slowly as the temperature drops. Therefore, the chemical reactions triggered by the enzymes that break down RNA are slowed down. It is the same as freezing food to avoid spoiling.
Why are such low temperatures needed?
In addition to their immune trigger mechanism, the two vaccines pending approval have something in common. A membrane of lipid nanoparticles surrounds the mRNA, covering it with a kind of envelope that we can compare with a Lacasito, a chocolate lentil inside a caramel envelope that ensures that the chocolate does not melt. In the different lipids used by Moderna and Pfizer / BioNTech reside the temperature differences that the conservation of their respective vaccines requires.
In the long term, Moderna’s vaccine can be stored at -20 ⁰C, while Pfizer / BioNTech’s vaccine needs to be frozen at -70 ⁰C , colder than an Antarctic winter. Both vaccines require people to receive two injections, three weeks apart. That means doubling the capacity requirements, which is an additional complication: Health centers, hospitals and other potential vaccination sites will need to store a large quantity of vaccines in their facilities without breaking the cold chain.
This is where the Achilles heel of the Pfizer / BioNTech vaccine resides, because to achieve its preservation temperature you will have to use deep freezers, which need electrical energy, are expensive and quite bulky, or dry ice, some cylinders of frozen CO₂ that can be put in a box to transport the vaccine and can last a fortnight if the dry ice is changed every five days. At this point Moderna has an advantage, because its vaccine lasts six months at the temperature reached in domestic freezers and even a month in the main cabin of a refrigerator.
However, from Covax, the alliance formed by more than 170 countries to facilitate equitable global access to vaccines, they are optimistic about the challenge of storing vaccines that require deep-freezing: the current Ebola vaccine requires storage between -70 ° C and -80 ° C and yet mass vaccinations have been carried out during recent African epidemics.
For the rest, the first trials of a “hot” vaccine, that is, thermostable, are already underway. Science has not had the last word. There is still a long way to go.
Manuel Peinado Lorca, University Professor. Department of Life Sciences and Researcher at the Franklin Institute for North American Studies, University of Alcalá; José Miguel Sanz Anquela, Associate Professor in Health Sciences. Department of Medicine and Medical Specialties, University of Alcalá and Luis Monje, Biologist. Professor of Scientific Photography, University of Alcalá
This article was originally published on The Conversation. Read the original.
