Yes, there are certain strains that wouldn't work as vectors. HAd26 (used in J&J) has not shown reduced immunogenicity based on studies conducted. It was used i

Yes, there are certain strains that wouldn't work as vectors. HAd26 (used in J&J) has not shown reduced immunogenicity based on studies conducted. It was used in other vaccines previously, and that hasn't impacted its performance here. While it's not a problem now, the situation might change in the future, so you were right to ask the question. These issues of pre-existing immunity are taken seriously inside vaccine companies too. (Note that while I work in biotech, I'm not claiming to be an expert on this topic, so don't take these as definitive claims of truth. People shouldn't be taking HN comments as a source of any kind of medical advice anyway.)