Not true
We don't allow deadly virus's to spread because that would kill a load of people. Herd immunity as you think of it is done with a NON DEADLY vaccine. The difference being one kills, the exact thing you're trying to avoid.
@"Lockdown and wait for a vaccine is a terrible plan"
That's not the plan, the plan is lockdown, get the number of cases low enough to manage by contact tracing and get on with your life like every other country that's already tackled it.
i.e. the same thing we did with MERS, SARS1, H1N1, FOXNEWS:
Contain the sickness, isolate it, let it die, and watch out for any signs of the disease coming back to contain it.
Also you don't want it to widely spread because it would have more opportunity to mutate, speaking of which....
https://www.sciencedirect.com/science/article/pii/S1567134820301829?via%3Dihub