Whats means the term (prototype vírus strain)is a first virus isolated and without mutations?
Different lines or isolates of the same virus (e.g., from different geographical location or patients)
the first example of something, such as a machine or other industrial product, from which all later forms are developed
I think prototype of virus strain is first isolated virus of that strain.
Principles of Molecular Virology fourth edition; Alan J. Cann; page 64. https://en.wikipedia.org/wiki/Strain_(biology) https://dictionary.cambridge.org/dictionary/english/prototype