We don't for absolute certainty, but there's no reason to think there would be. Consider: a) A virus that is known to cause severe disease in a significant pro

We don't for absolute certainty, but there's no reason to think there would be. Consider: a) A virus that is known to cause severe disease in a significant proportion of people, including months-long serious health effects, hospitalisation and death. b) A virus chosen because it doesn't causing any ill-effects in humans, modified to take on some non-harmful aspects of virus a) and found to not cause any significant harm in tens of thousands of people over several months. Which is more likely to cause long-term side effects?