(a), mean fitness of the viral population as a function of mean SPVL when transmission and time to AIDS are fitted as step functions (solid line; shaded area shows the 95% C.I.) or generalised Hill functions (dashed line). (b), evolutionary predictions for the temporal dynamics of mean SPVL given by the ODE model (thin solid and dashed lines), and the stochastic IBM (dotted lines), under three scenarios for the impact of within-host evolution (biased mutation) on SPVL in blue (1, α = −0.47 log10 copies/mL), red (2, α = −0.093 log10 copies/mL) and green (3, α = +0.057 log10 copies/mL). The thick line is the data, showing the linear regression of SPVL on date of seroconversion, with 95% bootstrap confidence intervals shown as a shaded area. (c), distribution of SPVL in the population over time; grey points show the data, and the line is the unadjusted regression of SPVL over time. (d) coefficient of regression of SPVL over time in the adjusted linear regression, with confidence intervals, in various subsets of the data (Material and methods). All data; SPVL strict definition; SPVL measured with Abbott assay and Roche 1.5 assay; SPVL measured at Walter Reed (WR), John Hopkins (JH) and RHSP laboratories; SPVL in males and females; subtype A, subtype D, and other/unknown subtype viruses.