Human immunodeficiency virus fitness in vivo: Calculations based on a single zidovudine resistance mutation at codon 215 of reverse transcriptase

We monitored a subject newly infected with a zidovudine-resistant human immunodeficiency virus type 1 strain and found that in the absence of drug, the viral population with the resistance conferring tyrosine (TAG) codon 215 of reverse transcriptase was gradually replaced. By using standard formulas to model the effects of selection at a single locus in an asexual haploid population, the relative fitness gain of the viral population with a single mutation at codon 215 creating a serine (TCC) was calculated. We concluded that a viral population with a serine at reverse transcriptase codon 215 conferring zidovudine sensitivity was between 0.4 and 2.3% more fit.