Thanks for the comment and for letting me know about the link. I was having problems with it yesterday too. Hopefully it’s fixed now. Here’s a link to the abstract in the Journal of Clinical Infectious Diseases and a separate link to the full pdf. I’ll admit, these estimates aren’t perfect and can vary based on testing availability. I’ve mostly seen factors in the range of 4 to 10. I’d be interested to know where you saw the factor of 2. Also, please note that I’m suggesting some places are nearing ‘herd protection’ not ‘herd immunity’ which are similar concepts but not the same thing. Hopefully, that makes sense. If not, please let me know what questions you have. Take care!