Regardless where you put your colony to, if you keep up a transportation system to that colony and back, you end up enabling spread of the disease. Coronavirus only spread so quickly because of airplane travel. Had we cancelled all airplane, train and ship traffic to and from China early enough, the virus wouldn't have spread as quickly. If in the future we have an intergalactic society with FTL travel, and one colony discovers an ancient virus that kills everyone, the virus will spread with speeds faster than light, because that's our underlying travel method.

To meet threats like coronavirus, you don't need a different planet. Any remote island would do, like easter island, as long as you shut down traffic soon enough.