Why is "could" being used here? If I use "can" or "be able to" instead of "could", will the meaning of this sentence change?
"Could" is used here to state a conditional. "Could" poses the option of the Adeno-associated virus possibly being able to work, although it has not been fully tested yet. If you replaced "could" with "can" here, it would imply that they know this option will work. However, in this case, they do not know for sure that it will work and so he uses "could" instead to express that there is only a possibility that it will succeed.
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