I don't know which measures are you going to do, but if it is for measuring purposes, you must take off the varistor and replace it by a TVS.
(I don't like varistors because they die fast).
Install in the primary an X2 rated capacitor in paralel with the input. You can place the varistor in the input in paralel with the X2 capacitor.
As the final protection, install a GDT, between an output and GND. Select a proper GDT.
With that you will have a true isolation without loosing security and the wire insulation will last forever.