I am unable to create a summary of the requested article because the search results provided do not include the article titled "Commensal-driven serotonin production modulates in vivo delivery of synthetic and viral vectors" from the journal Science, Volume 391, Issue 6791, March 2026. The search results include related studies on serotonin and the gut microbiome, but not the specific article you listed. In order to create an accurate and factual summary as per your requirements, I need to have the content of this particular article available.