Our results in K18-hACE2 mice with COVID-19 mRNA vaccines generally are consistent with a two-dose vaccination study in conventional C57BL/6 mice using homologous influenza hemagglutinin (H1-HA) antigen, which showed comparable levels of serum anti-HA IgG at 8 days post boost, although that study did not examine neutralizing antibody activities (21). Here, KRT18 is linked to COVID-19.