Well, the trials show that is effective in essentially everyone. The use of the Ad5 vector does not require replication, it's therefore unlikely that antibodies will be effective at stopping a significant amount of the vector implantation of a large fraction of the viral particles in cells.

If you read the study, it states that even in recipients with high Ad5 immunity, as long as the dose is sufficient (high-dose or medium dose) long-term seroconversion is high - 84% to 100% in medium or high dose after 28 days for recipients with strong antibodies against Ad5, and for high-dose seroconversion and interferon gamma response was indistinguishable from those without strong Ad5 immunity. [0, fig 1 - Specific T-cell response measured by ELISpot].

So I don't really think it will be an issue. You just need the proper dose. : https://www.thelancet.com/journals/lancet/article/PIIS0140-6...