How I'd put it:
Most of the reduction in COVID so far has been down to lockdown, but vaccines are now really kicking in too, even though we're only half way through the vaccination programme.
By keeping up the vaccination until everyone is treated we can continue to open up slowly without any new surge in cases and deaths.
This is a new disease and new vaccines, so there is uncertainty in exactly how fast it is safe to reopen, hence we need to be cautious and keep an eye on the spread of the virus as we open up.