That really depends on your definition of ‘to work’. The credible informed estimates I’ve seen talk about 80% uptake being good enough. Sure, you will definitely miss some folks, and you will definitely need supplemental old-school contact tracers to really close the net if you want to get to elimination (rather than mere suppression) but 80% means you very quickly have the skeleton - and most of the flesh - of a contact network, and can prevent things spiralling out of control. And, of course, the pieces of the network missed because their case 0s lie in the 20% are themselves subject to the 80% meaning the impact of any squirters gets shutdown quickly too.
To be sure, 100% would absolutely be better, but it is absolutely not a pre-req for an effective implementation.
