NHS said about 1mln people had already received invitations
Over 3.5mln doses of the vaccine have been administered so far
Britain is one of the first countries to roll out its vaccination programme
Tens of thousands of tests to be undertaken
The announcement follows similar moves from peers Aldi and Dollar General Corp
Study of healthcare workers shows reinfections are rare
It expects to have enough vaccine doses for all 5.7mln people by the third quarter of 2021