It seems we are only beginning to learn about this thing. A key question is whether casual exposure builds “herd immunity”. There seems to be no definitive answer yet. Just opinions bandied about. It probably varies from person to person but even a general case answer would be helpful.
Forget WHO. With the whole world working on this I would have expected more definitive scientific answers by now.
Weighing in from Southern California with herd immunity anecdotal evidence. Almost 9 thousand Chinese nationals arrive/ depart or did arrive /depart every single day in California up to the day of lockdown and yet, not a big outbreak at all here and a totally unjustified shelter in place deal.
I know 3 people locally that have likely had this bug as far back as October 2019. 2 had it in January 2020. I was traveling with them 2 weeks prior and never got sick.