Rapid dissociation of HIV-1 from cultured cells severely limits infectivity assays, causes the inactivation ascribed to entry inhibitors, and masks the inherently high level of infectivity of virions.

By using immunofluorescence microscopy to observe and analyze freshly made HIV-1 virions adsorbed onto cells, we found that they are inherently highly infectious, rather than predominantly defective as previously suggested. Surprisingly, polycations enhance titers 20- to 30-fold by stabilizing adsorption and preventing a previously undescribed process of rapid dissociation, strongly implying that infectivity assays for many viruses are limited not only by inefficient virus diffusion onto cells but also by a postattachment race between entry and dissociation. This kinetic competition underlies inhibitory effects of CCR5 antagonists and explains why adaptive HIV-1 mutations overcome many cell entry limitations by accelerating entry.