The Welsh government have announced on Friday that Cardiff, Swansea and Llanelli will go into local lockdowns from 6pm on Sunday.
The Welsh Health Minister Vaughan Gething said that no-one will be allowed to enter of leave the areas, unless they have a reasonable excuse.
The Health Minister said that people must work from home where they can.
Residents will not be allowed to mix with others that they do not live with and mixing with extended households will also be suspended.
People living in Caerphilly, Rhondda Cynon Taf, Bridgend, Blaenau Gwent, Merthyr Tydfil and Newport are already in lockdown, and are not allowed to leave their areas without a reasonable excuse.
The changes mean that more than 1.5m people will now be under local lockdown, which is just under half the Welsh population.