Regarding Richard Muller and Steven Quay’s op-ed “Science Closes In on Covid’s Origins” (Oct. 6): In SARS-1 the coronavirus underwent a few mutations in the horseshoe bat to accommodate to the civet cat; in the civet there were several more mutations to make it human-adoptable; once it became “humanized” it underwent still more mutations to make it virulent. How do we know that? Every one of those mutations has been identified. In SARS-2 not a single intermediary mutation can be found despite what has likely been an exhaustive effort to find such a scapegoat.

The...

Photo: Martin Kozlowski

Regarding Richard Muller and Steven Quay’s op-ed “Science Closes In on Covid’s Origins” (Oct. 6): In SARS-1 the coronavirus underwent a few mutations in the horseshoe bat to accommodate to the civet cat; in the civet there were several more mutations to make it human-adoptable; once it became “humanized” it underwent still more mutations to make it virulent. How do we know that? Every one of those mutations has been identified. In SARS-2 not a single intermediary mutation can be found despite what has likely been an exhaustive effort to find such a scapegoat.

The coronavirus is characterized by a crown of spikes (RNA proteins). For the virus to become activated, the spikes have to be cleaved or split. The enzyme that causes the cleavage derives from the host cell. The virus needs the host to cooperate, but the host does a better job if the cleavage site is made up of a certain arrangement of amino acids: proline-arginine-arginine-alanine. That array in SARS-2 is unique, not seen in other coronaviruses, suggesting it may have been “man-made” or retroengineered. This variant also has quite a low affinity for horseshoe bat cells. One can be pretty sure that this virus has never seen the inside of a bat. That only leaves a single viable source of origin.

Clinical Prof. Stanley Alexander, M.D.

USC Keck Medical School

Arcadia, Calif.