Is it sometimes necessary to repeat this ThetaHealing process for viruses, as with other healing work, to ensure their complete removal?

This is true, and you have experienced this also. And so this is yet another reason for caution, to not be boldly claiming cures when you cannot guarantee this because of such pitfalls. If you are honest with people and explain the possibility but not the certainty, you will be setting the stage for improvement. And this shift in perspective, and attitude, and marshaling of beliefs it will happen, can be just the ticket, so to speak, to achieve a successful healing. This will always be a boon to the person because such disorders take a heavy toll emotionally and physically and can worsen over time if not truly healed in some way, which rarely happens.