Ethics statement: Ethical approval was granted by the Kilimanjaro Christian Medical Centre (KCMC) University Research Ethics Committee (698 and 829); National Institute of Medical Research (NIMR), Tanzania (NIMR/HQ/R.8c/Vol. I/1140 and NIMR/HQ/R.8a/Vol. IX/2079) University of Otago Human Ethics Committee (H17/052), and University of Glasgow College of Medical, Veterinary and Life Sciences Ethics Committee (200140149).