The acquisition announcement typically leads to a positive short-term impact on the stock price of the company being acquired, as the offer price is usually at a premium to the current trading price. In this case, Harpoon Therapeutics is likely to see an increase in its stock price in the short term due to the acquisition by Merck.

The impact on Merck's stock (MRK) will depend on how investors view the acquisition's strategic fit, the price paid, and potential for future growth. Acquisitions can be seen as positive if they are expected to lead to synergies and growth, or negative if they are considered overpriced or not strategically sound. Without specific details on market reception, the short-term impact on MRK is uncertain.