Efficiency of human immunodeficiency virus type 1 postentry infection processes: Evidence against disproportionate numbers of defective virions

The vast majority of human immunodeficiency virus type I particles are claimed to be noninfectious, but there is disagreement as to whether they are defective or simply lack the opportunity to initiate an infection. We have examined the efficiencies of reverse transcription and integration and find that approximately 1 of every 8 virions that initiate reverse transcription form proviruses, a quantity significantly different from the commonly reported ratio of I in 1,000. In addition, results from two different infectivity assays demonstrate that the titers are not equivalent to the number of infectious particles. The apparent predominance of noninfectious particles is due to infrequent occurrences of successful virus-cell interactions.