This is a really great question. Technically speaking, any single mutation defines a new strain. But that's not how we tend to talk about it either to the gener

This is a really great question. Technically speaking, any single mutation defines a new strain. But that's not how we tend to talk about it either to the general public or amongst clinicians. Rather we use a more informal notion of a significant functional change. We do this because there is, somewhat surprisingly, no real term that reflects the category of "new strain with significant or interesting functional effects."