I actually came up with one more explanation, which is added to the main body of the post but reproduced here for your convenience:
A fourth explanation is that Merck’s 50% effectiveness result on Oct. 1 updated the market on two points:
A treatment in this class was going to be more profitable and easier to make than the market originally thought. This increases the price of both Merck and Pfizer.
Merck seemed more likely than before to beat Pfizer. This increases the price of Merck and decreases the price of Pfizer.
If the first factor cancels out the second factor for Pfizer, we’d obtain the result of no motion in the stock price.
I actually came up with one more explanation, which is added to the main body of the post but reproduced here for your convenience: