You don't need to prove or disprove a negative. You simply must allow for it to be a possibility unless evidence comes forth which proves that the doubts are unfounded. For instance, if patient zero was found and they had no contact with the lab, if a wild source of COVID-19 with the same genetic structure was found, or if it was proven than none of the samples in the lab could be the source of the infection.

The problem here is that I opened up the possibility by asking the question, and hateful, partial, non-scientific minds decided to dogpile me for threatening their beliefs and faith in authority.