[1] UCL, Inst Sport Exercise & Hlth, London W1T 7HA, England
[2] Royal Brompton Hosp, Dept Resp Med, London, England
[3] Univ Pretoria, Fac Hlth Sci, Sport Exercise Med & Lifestyle Inst, Pretoria, South Africa
[4] Int Olympic Comm Res Ctr South Africa, Pretoria, South Africa
[5] Univ Canberra, Res Inst Sport & Exercise, Canberra, NSW, Australia
[6] Imperial Coll London, Med Res Council, Ctr Global Infect Dis Anal, Dept Infect Dis Epidemiol,Sch Publ Hlth, London, England