Increased pathogenicity and transmission of SARS-CoV-2 Omicron XBB.1.9 sublineage, including HK.3 and EG.5.1

With the SARS-CoV-2 Omicron XBB.1.9 sublineage circulating worldwide, two XBB.1.9 variants, EG.5.1 and HK.3 spread rapidly and became dominant from middle 2023. However, the spike features, pathogenicity, and transmissibility of HK.3 are largely unknown. Here, we performed multiscale investigations to reveal the virological features of XBB.1.9 subvariants, especially the newly emerging HK.3. HK.3 revealed high replication efficiency in vitro. The HK.3 spike exhibited enhanced processing, although its infectivity, fusogenicity, and hACE2 binding affinity were comparable to those of the EG.5 and XBB.1 spikes. All XBB.1.9.1, EG.5.1 and HK.3 strains demonstrated efficient transmission in hamsters, although XBB.1.9.1 exhibited stronger fitness in the upper airways. HK.3 and EG.5.1 exhibited greater pathogenicity than XBB.1.9.1 and BA.2 in H11-K18-hACE2 hamsters. Our studies provide insight into the newly emerging pathogens HK.3 and EG.5.1. ImportanceIn animal models, the ongoing attenuated pathogenicity and poor transmission of Omicron subvariants seems to reach a consensus. However, our results revealed that Omicron XBB.1.9 subvariants, including one of the key variants of interest, EG.5 with its another key subvariant HK.3, universally exhibited both increased pathogenicity and highly transmission. This study reemphasized the importance of surveillance in characteristics of epidemic Omicron subvariants.