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Replication Capacity
Replication Capacity (RC), a component of "viral fitness," is a measure of how well a virus is able to replicate. Although drug resistance mutations enable viruses to replicate in the presence of drug, they may do so at some fitness cost to the virus overall. For instance, mutations that enable the virus's replication machinery to evade drug activity may also impair the normal function of this machinery. Thus, by acquiring drug resistance, the virus may be forced to replicate less efficiently than a typical (wild-type) drug sensitive virus.
Clinical studies investigating RC have already identified correlations between RC information and virologic and immunologic response to therapy. These studies have identified the following:
- Change in RC is significantly correlated with drug resistance, however RC is a unique viral characteristic that is not simply a surrogate for drug resistance
- A virus strain with a low RC is associated with slower viral load increase and immune destruction compared to a similar strain with a higher RC
- In treatment-naïve patients, RC correlates with viral load and CD4 count — patients with lower RC have lower viral loads and higher CD4 counts
- At the time of treatment failure, RC is associated with CD4 count — patients with lower RC have a higher CD4 count
- In patients experiencing prolonged virologic treatment failure with sustained high CD4 count, RC is consistently low
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