Analyst downgrades typically lead to a negative short-term reaction in the stock price as they often reflect a less optimistic outlook on the company's future performance. The downgrade from Buy to Neutral suggests that Mizuho sees limited upside potential for Rivian's stock, which could lead to decreased investor confidence and a potential decrease in stock price in the short term. The setting of a $12 price target, presumably lower than the current trading price, further emphasizes the analyst's reduced expectations for the stock.