Why the COVID vaccines need to be stored at such low temperatures

The first two COVID-19 vaccines to get some serious traction are the Pfizer/BioNTech one and the Moderna one. Both vaccines are on the home straight right now, and appear to have an excellent effectiveness rate of around 95%. There are hopes to get them both fast-tracked for use early in the new year.

The vaccines are quite different, but both use varations of a relatively new technique called synthetic messenger RNA (mRNA). Because mRNA is rather delicate, and constantly at risk of damage from other molecules in the environment, it is wrapped within the vaccine in a protective layer made of nanoparticles of fatty lipids, and it needs to be stored at very low temperatures to slow down any chemical reactions and enzyme action that might affect the vaccine's effectiveness and potency.

The Moderna vaccine can be stored long-term at just -20°C (-4°F), a temperature that can relatively easily be achieved in most common freezers. The Pfizer/BioNTech vaccine, however, needs to be stored at around -70°c (-94°F), which requires specialized equipment that is usually available at most urban hospitals but may not be available at smaller rural hospitals

I have not been able to fully ascertain why one needs to be stored at a much colder temperature than the other, but Moderna argues that its lipid nanoparticle protection is superior, and that they just have more experience in the relatively new field of mRNA vaccines. It's also possible that Pfizer/BioNTech may decide, over time and with more testing, that its product can be stored in less extreme temperatures. But, for now, and in the interests of getting the virus out as quickly as possible, they are choosing to err on the side of caution.

Either way, it could well affect the logistics of how the different vaccines are distributed (e.g. the Pfizer product may be reserved for larger population centres with better-equipped hospitals). Some wealthier hospitals are already buying up specialized freezers, despite express advice from the Centers for Disease Control and Prevention against such actions. 

Between the two vaccines, though, and the many others which are set to announce their testing results soon, it is thought that everyone will be accommodated. All we have to do now is to persuade people that they should actually get the vaccine ...