I think this is referring to the classification of the patient according to the presence of what are called biomarkers and other biological data that reveal his risk group/symptomatic severity.

This is done using a technique of "stratification", which is a kind of data analysis in the field of epidemiology.

Here's a definition of "estratificación": "La estratificación, también conocida como segmentación de datos, se define como la acción de ordenar datos, personas y objetos en distintos grupos o estratos. Cuando los datos provenientes de varias fuentes o categorías se presentan de manera desordenada o mezclada, interpretarlos se hace mucho más complejo."

Also see: "Son numerosos los estudios que tratan de encontrar nuevos biomarcadores útiles para la estratificación del riesgo de los pacientes infectados por SARS-CoV-2."

Here's a definition of the target-text translation in English: "Stratification is defined as the act of sorting data, people, and objects into distinct groups or layers. It is a technique used in combination with other data analysis tools. When data from a variety of sources or categories have been lumped together, the meaning of the data can be difficult to see"

Also this: "Stratification analyses have been widely utilized in molecular association meta-analyses to estimate the interaction between genetic and environmental factors or to control for the confounding variables linked to a disease."