What we don't know is what we don't know. For me, one of the biggest black holes is that we have no clear biodistribution or in-situ expression data for these gene delivery vectors, meaning we don't know where these go and where they're being expressed.
As an immunologist, it's still unclear to me how these expressed proteins are presented to the immune system. Typically our immune system relies on a danger signal, or what we call pathogen-associated molecular patterns or PAMPs. This helps us to differentiate cell from non-cell and what's dangerous from not dangerous, to launch an appropriate type of immune response. Antiviral immunity is different from antifungal immunity, which is different from antiparasitic. These strategies are different.
In respect, for example, to the lead mRNA vaccine candidates we have, I don't see where that instructive information is contained for the immune system to know what exactly it's supposed to be fighting. It's like eliciting an answer without knowing what the question is. The long-term consequences of this immune ambivalence I think are yet to be determined.
Lastly, in terms of the DNA vector vaccines, we have not evaluated and we should—so I would add that to my wish list—the antibody and the immune response to the adenovirus vector itself, the thing carrying the message. Presumably we'd be generating a pretty strong response to the vector, which theoretically means that each subsequent booster shot would elicit a lower immune response to the message, because the messenger is being wiped before it delivers it.
We have not asked for any of this more detailed nuance. We could be giving booster shots eventually, by the third time, and they're just blanks for our immune system. We're not launching sufficient immune response to the spike protein that is being encoded in the message there.