How important is the acute phase in HIV epidemiology

At present, the best hope for eliminating HIV transmission and bringing the epidemic of HIV to an end lies in the use of anti-retroviral therapy for prevention, a strategy referred to variously as Test and Treat (T&T), Treatment as Prevention (TasP) or Treatment centred Prevention (TcP). One of the key objections to the use of T&T to stop transmission concerns the role of the acute phase in HIV transmission. The acute phase of infection lasts for one to three months after HIV-seroconversion during which time the risk of transmission may be ten to twenty times higher, per sexual encounter, than it is during the chronic phase which lasts for the next ten years. Regular testing for HIV is more likely to miss people who are in the acute phase than in the chronic phase and it is essential to determine the extent to which this might compromise the impact of T&T on HIV-transmission. Here we show that 1) provided the initial epidemic doubling time is about 1.0 to 1.5 years, as observed in South Africa, random testing with an average test interval of one year will still bring the epidemic close to elimination even if the acute phase lasts for 3 months during which time transmission is 26 times higher than in the chronic phase; 2) testing people regularly at yearly intervals is significantly more effective then testing them randomly; 3) testing people regularly at six monthly intervals and starting them on ART immediately, will almost certainly guarantee elimination. In general it seems unlikely that elevated transmission during the acute phase is likely to change predictions of the impact of treatment on transmission significantly. Other factors, in particular age structure, the structure of sexual networks and variation in set-point viral load are likely to be more important and should be given priority in further analyses.