Nineteen of the 25 estimates were derived from prospective studies (one for IDU and 18 for needlestick injury), in which study participants were typically followed up for about 9 to 12 months following one contact/exposure for needlestick or many injection events for IDUs. Studies tended to be prospective in that health care workers or laboratory workers were encouraged to report exposures, after which they were followed up, rather than enrolment in a study before exposure. Two studies of needlestick transmission were retrospective with health care workers sometimes reporting more than one contact/exposure [20,21]. One was of unspecified design , while Gisselquist  used data from a case–control study  to derive an estimate. The remaining two estimates, one for IDU  and one for contaminated injections , were also indirectly derived from mathematical models (one for each type of exposure).
Transmission probabilities are difficult to measure for most modes of transmission, partly because of difficulty in confirming the infected source, and also because they depend on other risk factors, such as HIV viral load  and exposure to antiretroviral therapy . The lack of data for both IDU and contaminated medical injection exposure means that transmission probabilities can only be indirectly derived from studies on HIV transmission per accidental percutaneous injury. The risk of infection from percutaneous accidents depends on factors that influence exposure to a greater volume of blood (e.g. visible blood on the device, procedures involving needle placement directly into a vein or artery, etc), viral load (e.g. clinical status of the source patient, post-exposure prophylaxis), which affect the ‘effective’ viral dose received [57,60]. The type of injection equipment affects infectivity because syringes with detachable needles retain – and probably transfer – substantially more blood than integral cannula syringes with a permanently attached needle [61–63]. Laboratory studies have suggested that a greater volume of blood is transferred by deeper injuries and by hollow-bore needles (especially those with larger gauges) .
It is important to provide reliable estimates of transmission probabilities for iatrogenic modes of transmission, in order to predict their relative impact on the HIV/AIDS pandemic. Our estimates are based on the most comprehensive review of the literature and best studies, and will be critical when assessing the contribution of these routes of infection to HIV incidence and designing appropriate prevention strategies.
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