Wall Street Journal:

Hong Kong, with over 7.5 million residents, is one of the most densely populated places on earth, but recorded only six deaths from Covid-19 despite having no lockdown and recelving nearly three million travelers a day from abroad, around half of them from mainland China, where the virus originated.

The key secret of Hong Kong’s success, Prof. Yuen said, is that the mask compliance rate during morning rush hour is 97%. The 3% who don’t comply are mainly Americans and Europeans, he said.

“The only thing you can do is universal masking, that’s what stopped it,” Prof. Yuen said.