A mass action model developed by the World Health Organization (WHO) estimates that the re-use of contaminated syringes for medical care accounted for 2.5% of HIV infections in sub-Saharan Africa in 2000. The WHO's model applies the population prevalence of HIV infection rather than the clinical prevalence to calculate patients' frequency of exposure to contaminated injections. This approach underestimates iatrogenic exposure risks when progression to advanced HIV disease is widespread. This sensitivity analysis applies the clinical prevalence of HIV to the model and re-evaluates the transmission efficiency of HIV in injections. These adjustments show that no less than 12-17%, and up to 34-47%, of new HIV infections in sub-Saharan Africa may be attributed to medical injections. The present estimates undermine persistent claims that injection safety improvements would have only a minor impact on HIV incidence in Africa.