I think the "expected value" was priced in, but regardless of the result, this trial would move the stock price towards that direction.

Biotech is one of the very very very few areas of the stockmarket where specialist knowledge generates alpha.

Trolling through random biotechs, learning about their tech, and investing in the most promising applications of that tech in the most impactful areas is something that happens somewhat on publicly traded markets. With tech, most of that happens pre-IPO.

Even if you were to optimally price in all available information you still have uncertainty of what will actually happen. i.e. A 1 dollar coin flip has an expected value of 50c before it’s thrown and it will collapse either to zero or to one after the uncertainty is resolved.

I don't believe the efficient market hypothesis. However even if you do, new information has arrived and that will affect the price.

Hard to price in the political factors now involved in Moderna's success or lack of success.

It was priced in, the share price prior to the news being released was taking into account a probability measurement of the trial succeeding or failing. You can’t price in both options of a binary outcome simultaneously, so part of the equity price was functionally equivalent to a call option on the outcome of the drug trial that only resolved after the trial results are announced.

It’s similar to the situation where a company puts out a public offer to buy another company at say, $50 a share on a day that the acquisition target is trading at $40/share.

If the stock of the target company is trading at $46/share, the market is pricing in the probability of the acquisition not happening. If the market knew the acquisition would happen with 100% certainty, the price of the target company shares would be equal to the buyout share price offer. You can assume the risk of the acquisition not happening by purchasing shares at $46, and if it does end up going through, you earn $4 a share from assuming that risk.