Why haven't the markets reacted positively to this news?
But I think the market is reacting not to the virus, which by itself would have a limited economic impact (and if you think that it affects mostly the elderly, from a pure, cold economic point of view, would rather reduce the long term burden on healthcare, so would be a net economic benefit).
I think the market instead reacts to the actions taken to contain the virus, which will likely severely supress the economic activity. Until there is any sign of these actions being reversed (and everything is pointing to the opposite, with France and the EU hinting at more restrictions yesterday), it is rational for the market to be bearish.
Then as always there are other considerations, stocks were overly expensive, economic cycles, over leverage, etc.
The markets has not reacted positively to this news because something possibly helping makes no difference. Actually making a difference to the situation makes a difference. Until the point where there clearly is something that makes a difference the outlook is really bleak.
The paper says that people are being successfully treated with this medication. It sounds like a slam dunk and a definite end to all the uncertainty we're experiencing. It's somewhat akin to magically having a vaccine available today, no? A situation in which we'd expect markets to react positively. Maybe, this information has not disseminated widely enough in the media, and the markets are acting as a trailing, not leading, indicator.