Adenoviral infections are common in kids since they have typically not encountered them before. Not common in adults as we have residual resistance from our own encounters as children. This type of cross resistance is also common with coronaviruses which include several of the common cold viruses.

A key aspect of the cited hypothesis is host taking the Gilead pre-exposure HIV med, which may have caused the replication of the Adenoviral vector.

Moral of the story, careful when you fuck with nature because nature knows how to fuck too.