The challenges of keeping a Covid-19 vaccine cold
As the UK became the first country in the world to approve Pfizer/BioNTech’s novel coronavirus vaccine, the hard work to overcome significant logistical hurdles began. Roger Mainwaring-Burton of Cambridge Consultants looks behind the glare of public attention to unravel the challenges of vaccine cooling.
Following one of the most consequential scientific endeavours any of us can remember, three Covid vaccines emerged with promising preliminary results. But all bring with them vastly different storage and distribution challenges. The Pfizer/BioNTech jab, first to be announced and approved, has to be stored at -70°C (-94°F), the second by Moderna requires -20°C (-4°F) and the third by Oxford/AstraZeneca needs 4°C (39°F). There are relatively few methods of moving heat away from something to keep it cold, and which method is most appropriate varies, depending on the desired temperature, the surrounding environment, power availability and length of journey.
So what cooling systems are available for this crucial supply ‘cold chain’, and which are best suited to each vaccine? The three most common ways of moving heat are phase-change, mechanical and electrical (although mechanical is technically also phase-change). Phase-change includes ice (0°C), freezer blocks (0° – 8°C) and dry ice (-78.5°C). They can move a huge amount of energy, but they need recharging and can produce waste. The mechanical method is just like a conventional domestic refrigerator. A vapour compression (VC) system compresses a highly volatile refrigerant which flows around a circuit, moving the heat from a colder to a hotter place. Electrical uses thermoelectric coolers (TECs), where applying a voltage to a specific combination of metals causes heat to flow from one side to the other.
Oxford/AstraZeneca's vaccine can be maintained with conventional refrigeration. This means that the same systems which keep your fridge at home cold will work. Freezer packs or ice cubes would also suffice, as would TECs. The biggest challenge with this vaccine will be monitoring the storage conditions to ensure that the temperature is constantly maintained. Although the temperatures are similar to those used to keep food, the vaccines will demand more careful monitoring – since failures are harder to detect and the consequences are far more life threatening.
Ice is not cold enough for Moderna's vaccine which will require more power to keep it cold. The best solution here is likely to be the same as that used in a domestic freezer, a vapour compression system. However, these systems need a source of power and can suffer damage if they aren't kept very stable.
This is why you are asked to wait a day before turning on a newly installed fridge.
Inefficient and expensive
Now let’s return to the Pfizer/BioNTech vaccine – which demands such low temperatures that conventional cooling methods can't keep up. Specialised VC systems can reach -80°C but are inefficient and expensive due to the extreme pressures involved. And although dry ice can cool sufficiently, it must be replaced regularly and is not safe in enclosed spaces, such as aeroplanes. The danger is that people could by suffocated by the CO2 it releases.
The equation for heat transfer through a wall – which every thermodynamic engineer is familiar with – is P=hAΔT. What this means is that the power through a surface is proportional to the resistance of the wall (h), the area of the wall (A) and the temperature difference between the two sides of the wall. This means that the colder the vaccine needs to be, the more power will be needed to keep it there.
While some of these vaccines are more thermodynamically challenging than others, they all require solutions which can cope with the unprecedented scale of this new distribution challenge. And while methods exist for distribution of these kinds of products, they are not established to move billions of vaccines across the globe.
Making treatment available for the world will require a combination of different vaccines, different cooling technologies and different logistical techniques. It's unlikely that this challenge will be solved by old methods. Instead we need to understand the unique requirements of this situation. That means repurposing existing technology and creating new methods to maximise the rate at which the vulnerable can be treated.