Serotype 1 isolates used in this study.

*Lineage is as described in Brueggemann et al.[26].∧Non-Indigenous isolate. Other isolates were all of Indigenous origin.+Non-invasive isolates were both non-invasive in humans and mice: intermediately virulent strains were virulent in humans, but significantly less virulent in mice than strains 1861 and 4496. Highly virulent strains were significantly more virulent than the other four strains.