Earlier quoted context omitted.
I don't understand, aren't future events supposed to be already considered in the stock price? It's not like a vaccine was unpredictable
Yeah, but investors seem to want to hold on to stocks for as long as possible (dividends? I don't know how that works, as far as I know holding stock for dividends is nowhere near as profitable as speculating)
Because of day traders and HFT, it's hard to say how long your average investor holds a stock, but it's fair to say that some hold it for a long time and others "speculate" most of the time. The speculators probably have a greater impact on the movement of a stock, but over time it must move in line with the performance of the company.
The price of a stock is thought to be a "risk adjusted present value" of the underlying company. Even if you could be 100% sure a company would make X dollars per year in perpetuity, the price would not be infinite. Instead each year of earnings is "discounted" by risk, inflation rate, the cost of borrowing money, the value of competing investments, and other factors.
If it was assumed by investors that Zoom would make lots of money for the next 5 years, and then make very little money, and this news meant they would only make lots of money for the next 3 years, then that will have an impact of 2 years worth of discounted earnings immediately.
(of course it's not like Zoom pays or is planning to pay a dividends but there's still an underlying value, for instance they could be purchased by a larger company that does pay dividends.)
[1] https://www.reuters.com/article/us-health-coronavirus-short-...