The rapid spread of a new variant of coronavirus has been blamed for the introduction of strict tier four mixing rules for millions of people, harsher restrictions on mixing at Christmas in England, Scotland and Wales, and other countries placing the UK on a travel ban.
So how has it gone from being non-existent to the most common form of the virus in parts of England in a matter of months?
The government's advisers on new infections now say they have "high" confidence that it is more able to transmit than other variants.
All the work is at an early stage, contains huge uncertainties and a long list of unanswered questions.
