Koalas first: lessons from a wildlife Chlamydia vaccine.

Koalas are the first wildlife species with a conditionally approved Chlamydia pecorum vaccine, offering a natural-host system for a pathogen whose human counterpart, Chlamydia trachomatis, lacks a licensed vaccine. Human vaccine efforts are slowed by uncertain immune correlates, asymptomatic infection, licensure-relevant disease end points, strict regulatory requirements, and limited commercial incentives, with only one candidate completing Phase 1. Koalas share these constraints, but visible ocular and urogenital disease enables clinical assessment under natural infection. Koala vaccine design integrated dominant circulating C. pecorum genotypes with a single-dose Tri-Adjuvant, evaluated for safety, immunogenicity, and clinical outcomes. Conditional approval under Australia's veterinary minor-use pathway relied on real-world evidence rather than controlled challenge studies. This opinion article highlights how natural-host evidence frameworks can inform intracellular pathogen vaccinology.