Pfizer Covid vaccine temperature: Why does it have to be stored at -70 degrees Celsius?

PFIZER vaccine temperature: Covid vaccines appear to be getting ever closer, with the UK government now ruling the Pfizer/BioNTech coronavirus vaccine safe enough for mass immunisation. Why does the Pfizer Covid vaccine have to be stored a such a cold temperature, of around -70 degrees Celsius?
Source: Daily Express - Health - Category: Consumer Health News Source Type: news