In late March, popular consensus was that Japan was managing its coronavirus epidemic well, and was unlikely to be seriously affected. Patrick McKenzie, a software entrepreneur in Japan, thought that that was wrong, and on March 23rd through 25th he and three other locals produced a white paper describing their reasoning, which they shared with Japanese news organizations.
They were right; Japan is presently in a state of emergency. According to Patrick, their work helped accelerate the Japanese response.
(You may recall reading on Marginal Revolution that "The coronavirus situation in Japan is probably much worse than you think"; this is the group Tyler Cowen was talking to).
This essay describes how he reached his conclusion and how he acted on it. It's rather long and elliptically written, taking great care not to attribute blame to any of the institutions involved. Read it anyway: it's an account of concrete, successful action against Covid-19, and seems to contain a few LW easter eggs for alert readers.