First, I don't know how many of us are already immune. We may already be past the point where it can spread or we might soon be there. A blood test to identify people who have been exposed and developed immunity is needed to know when herd immunity is sufficient. That said, we don't know how long immunity will last.
Lockdown was necessary in New York, Seattle and New Orleans. Not to save lives but to keep the medical system from being overloaded with very sick people. I don't know where you are coming from when you say "lockdowns don't work". They do work to cut the number of sick people in hospitals. The purpose of lockdowns is to allow capacity to treat Covid patients and other sick people who would die if the medical system is swamped.
Lockdowns do cut the rates of new cases from developing in hot spots. Once rates in all parts of the country are low, if as predicted, 95% are still susceptible, then we'll need to put controls in place. These would be wide scale testing, monitor for new cases, isolate them and well as all who came into contact with the infection. Also, trace back to find asymptomatic carriers or other new sources of the infection. This is close to what S. Korea is doing.
A vaccine is the end game. Developing the vaccine is an event driven schedule and not one that can be predicted. From what I've read, it's going to be years before a vaccine is ready for deployment.
