The vaccine cold chain is a global network of cold rooms, freezers, refrigerators, cold boxes, and carriers keeping vaccines at the right temperature on the long journey from the manufacturing line to the syringe, to protect them from becoming ineffective.
Majority of cold chain vaccines in broad use are stored between 2 and 8 degrees Celsius, the temperature of a refrigerator, the same from a logistical point of view as moving yogurt or milk around into supermarkets.
Some COVID-19 vaccines can be stored and transported at this chilled temperature, such as the AstraZeneca jab, so do not present a huge challenge.
The Moderna vaccine needs to be kept frozen at minus 20 degrees Celsius, like frozen food, while others need even colder temperatures.
This presents a significantly increased challenge when it comes to distributing the vaccine in hotter climates, and especially when trying to reach rural communities where electricity is unreliable.