I see 4.9% of 555 newly diagnosed HIV infections in France between 2003-10, were the CXCR4-tropic variant. And of these, 1.5% were involved in a transmission clusters (one case).

I think it means the CXCR4-tropic variant usually arises/evolves after infection, it's less often involved in transmission?

I think they hypothesize, if I understand the paper correctly, that the CXCR4-tropic variant is less infectious than CCR5 variants.

Frange, P., Meyer, L., Ghosn, J., Deveau, C., Goujard, C., Duvivier, C., Tubiana, R., Rouzioux, C., Chaix, M.L. and ANRS CO6 PRIMO Cohort Study Group, 2013. Prevalence of CXCR 4‐tropic viruses in clustered transmission chains at the time of primary HIV‐1 infection. Clinical Microbiology and Infection, 19(5), pp.E252-E255.