The severity of adverse events in clinical trials refers to how intense the event is, typically classified as mild, moderate, severe, or potentially life-threatening. They are often classified as grades 1-5 or grades 1-4. In vaccine trials, for example, the FDA uses a scale of 1-4.
- Grade 1 is mild, a symptom or discomfort that does not interfere substantially with daily activities, such as feeling feverish or a mild headache.
- Grade 2 is moderate, something manageable but potentially interfering with life, such as nausea or a fever requiring over-the-counter medication to manage.
- Grade 3 is severe, something that substantially interferes with daily activities and prevents you from being able to do normal activities, such as going to work or school. Severe adverse events might include a fever of 102ºF-104ºF, frequent vomiting that requires rehydration or pain that requires pain relievers. Importantly, however, grade 3 does not require someone to go to the hospital.
- Grade 4 is potentially life-threatening or requires a hospital visit, such as a fever over 104ºF or such severe vomiting that you are at risk for dangerous dehydration.
It’s important to note that “severe” and “serious” have different meanings with adverse events. Severity describes the intensity. Serious adverse events are defined by the FDA as “any reaction occurring at any dose that results in any of the following outcomes: death, a life-threatening adverse experience, inpatient hospitalization or prolongation of existing hospitalization, a persistent or significant disability or incapacity, or a congenital anomaly or birth defect.”