The increased infection of brains by BA.5 and XBB over BA.1 in K18-hACE2 mice may be associated with the enhanced fusion activity of the later omicron variants (Tamura et al., 2023; Tang et al., 2023), which is usually associated with an enhanced ability to utilize TMPRSS2 and/or increased binding affinity for ACE2 (Aggarwal et al., 2022; Tamura et al., 2023). Here, TMPRSS2 is linked to infection.