California shut down very hard and fast. When they reopened, nearly 100% of the population was still susceptible to the virus, so it spread like wildfire. Especially since the reopening was not exactly slow. Compare this to the Northeast and much of Western Europe where the virus spread too quickly until lockdowns reduced the spread. Now, keeping the transmission rate low in those areas has become comparatively easy,…
That may be true from a pure logical/mathematical point of view, but that doesn't mean it makes a practical difference.
Herd immunity is something that happens when a very large percentage of the population is immune. For mumps (which has a very high R0), you need about 92% immunity level for it to be effective.
For COVID-19, it is currently estimated to be around 60+%. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7151357/)
We are nowhere near these kind of immunity levels right now, and it's impossible to get to that level without vaccine unless you're willing to accept that a huge amount of people will die or get seriously ill.
Any suggestion that herd immunity is relevant in today's environment is plain wrong.