U00-U49: Provisional assignment of new diseases of uncertain etiology or emergency use

U21: Emergency use of U21

The U21 category in the ICD-10 WHO 2016 classification refers to the designation for emergency use of codes that are not classified in other parts of the classified system. This category is typically utilized during situations of public health urgency, such as during an outbreak or a pandemic. Conditions under this category may vary significantly in causes and symptoms. These conditions are often highly infectious and potentially lethal, requiring rapid identification and subsequent coding for data collection and tracking. They may impact various bodily systems and manifest in a wide range of symptoms, often influencing respiratory or immunological systems heavily. The disease progression can be swift or slow, contingent on the specific circumstances of the emergent public health event.