I don't think so, or at least, the Covid vaccine trials so far suggest that if it is possible, they either haven't done it or it didn't work, given that some st

I don't think so, or at least, the Covid vaccine trials so far suggest that if it is possible, they either haven't done it or it didn't work, given that some study participants did have pre-existing exposure, and it made the vaccine work less well. Worse still: older people, having had more opportunities for exposure over their lifetimes, were most likely to experience this outcome. Slight caveat, I guess, that where I've seen this discussed, it's been about CanSino's Ad5-vectored vaccine, rather than any of the Ad26 ones, but I don't know of any reason it'd be different. From what I understand, part of the reason that the Oxford group chose a chimp adenovirus was specifically to avoid this issue, as the odds of people having been naturally exposed to chimp adenoviruses is probably pretty low.