Online violin and viola lessons can be extremely effective.  It is just like being in the room with me.  Some of my students that switched to online lessons in the wake of covid-19 are absolutely flourishing during this time.  The is because the single most important factor in a student's progress is the effort and focus the student puts forth into learning an instrument, regardless of what medium they learn on.  The quality of the instructor is also more important than the medium they use to teach.

There are a few important factors to make online lessons a success.  A good internet connection is a must  The device used to connect must have a good quality camera (or use a quality webcam).  The camera should be set up to show the student's hands and arms clearly when they play.  If meeting over Zoom, "suppress background noise" must be disabled.

My students that have prepared for auditions online have so far passed with a 100% success rate.  Those that started from scratch with me online have made noticeable strides in their progress.