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The CDC 7/20/20, in a series of Scenarios, indicate the best estimate is 40%. Thus we might increase the number of confirmed cases by 40%. For LA county increasing the 21.5K by 1.4 times yields 30.2K or 30,200/1,000,000 = 3%. This is still very low. For Delaware adding 40% yields ~ 4.2%.
Professor Gomes of the University of Strathclyde in Glasgow produced a model suggesting herd immunity may be as low as 10-20%, see https://www.medrxiv.org/content/10.1101/2020.04.27.20081893v3.
Dr. Manoj Jain on infectious disease physician at the Rollins School of Public Health says "By definition, herd immunity means we can let our guard down, that we can go life as usual. That is not the case in our declining numbers. Our numbers are declining in part because we are masking and distancing. This lowers our transmission number, which in turn lowers the percent of the population required to achieve 'herd immunity'... If we stop that, then the population percent required or herd immunity will go up."