My personal theory is that lethal mutations or the ones that cause severe symptoms get detected and isolated where possible, while the mutations that cause only

My personal theory is that lethal mutations or the ones that cause severe symptoms get detected and isolated where possible, while the mutations that cause only mild or no symptoms can continue spreading silently without anyone noticing. If both create the same immunity, the non lethal mutations will win. If this theory holds, testing only if a person is showing symptoms would be the right thing to do. Filter the dangerous mutations, let the other ones spread.