Moreover, it is possible that the TMPRSS2 route of cell entry, along with the high membrane fusion capacity, is required for the rapid induction of anosmia in SARS-CoV-2 variant Delta, and this route may be the only one able to cause rapid anosmia in COVID-19 due to its enhanced infectivity, while the Omicron variant uses the less efficient endosomal route, thus appearing in the failure to induce frequent anosmia [84]. The gene discussed is TMPRSS2; the disease is Kallmann syndrome.