There are a number of direct risks related to the Adenovirus itself. Multiple adenovirus vaccine programs have been scrapped over the years because of these iss

There are a number of direct risks related to the Adenovirus itself. Multiple adenovirus vaccine programs have been scrapped over the years because of these issues: https://cen.acs.org/pharmaceuticals/vaccines/Adenoviral-vectors-new-COVID-19/98/i19 https://cen.acs.org/pharmaceuticals/vaccines/Adenoviral-vect... For example, famously, one Ad-based HIV vaccine was found to increase the chances of getting HIV. People still theorize that it might be due to the vector itself: https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)32156-5/fulltext https://www.thelancet.com/journals/lancet/article/PIIS0140-6... (that said, these platforms are certainly better validated than the mRNA-based vaccines. There's an approved rabies virus vaccine based on adenovirus.)