There is no standard definition. The clinical trial would have a set of predefined outcomes ("endpoints") that are used for the efficacy analysis. The endpoints

There is no standard definition. The clinical trial would have a set of predefined outcomes ("endpoints") that are used for the efficacy analysis. The endpoints will not necessarily be the same ones for each vaccine, even for the same disease. You'll need to read the actual trial protocol for a specific vaccine to find out just what they were measuring, and how they defined e.g. "serious" vs. "non-serious" cases.