always take with a big grain of salt the "reasons" offered for why the stock market moves on any one day.
https://www.cnbc.com/2020/05/17/stock-futures-up-after-powell-says-economy-could-recover-some-this-year.html
Stocks rallied on Monday, rebounding from losses last week, as news from a Moderna trial stoked optimism about a potential coronavirus vaccine.
The Dow Jones Industrial Average traded 800 points higher, or 3.4%. The S&P 500 gained 3% while the Nasdaq Composite advanced 2.5%.
Moderna shares rallied 26% after the company reported “positive” phase one results for a potential coronavirus vaccine. The company said that after two doses all 45 trial participants had developed coronavirus antibodies.