Progressive mutation of {HEAD ~ the virus} by {TAIL ~ such nucleoside} analogs renders it non viable 
by
Progressive mutation of { HEAD ~ the virus } by { TAIL ~ such nucleoside } analogs renders it non viable
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH