I have been looking for published, or even pre-print, data that evaluates the serological response to a boost of the Janssen COVID-19 Vaccine against the adenoviral vector Ad26 rather than the intended SARS-CoV-2 spike protein target.
Vector-targeting immune responses are a generic and known issue with adenovirus vectors. Janssen has made the claim that Ad26 is superior to Ad5 in this regard, but while increased response to the target epitopes is obtained in boost (though slight in my opinion), a frank discussion or display of differences in anti-vector response is not covered in any of the references:
In contrast to Ad5-based vaccines, a similarly clear impact of natural- or vector-induced pre-existing immunity to Ad26 on vaccine immunogenicity has not been observed to date in clinical studies. Results from clinical trials assessing repeated administration of Ad26-based vaccination approaches showed that a second or subsequent dose of study vaccine was able to boost humoral and cellular immune responses to HIV antigens (Ad26.EnvA, study IPCAVD001; and Ad26.Mos.HIV, APPROACH study; even in the presence of high Ad26 neutralizing antibody titers induced by the first dose. Similarly, naturally occurring Ad26 neutralizing antibody titers or Ad26-targeting T cell responses at baseline were not associated with decreased immune responses against the vaccine antigen. (DOI)
Not destroying your target immune response with a boost is not the same thing as a meaningful boost to response or a lack of boost to vector.
I was hoping to find a report on changes (or lack there of) reaction to any Ad26 proteins via Western, ELISA, or any acceptable protein interaction assay for individuals after both prime and boost with Jansen vaccine. Generic reaction to the vector, even related ones, would be apricated if a clear division in prime and boost responses were present.
Most studies that use the vector (an example with RSV that I know had the problem but was unreported) seem to avoid discussing it altogether.