I wouldn't use a tvs/varistorsTVS or varistors to protect the 5V from an overvoltage. Instead, I would use a zenerZener clamping circuit or an IC that provides an overvoltage protection. TVS/varistors are more suited for fast transients. Unlike overvoltages, the voltage would remain for longer period and requires a 'resolve' either through clamping or cutoff when a fault is detected.