Very few studies reporting HIV transmission probabilities through OI or reporting sufficient data to calculate it were found. This may be due to the difficulty in identifying individuals with OI as their sole exposure, the tendency to ascribe any transmission events that occur to any higher risk exposure that is identified, such as AI, and the assumption that the associated risk is very low. Although transmission risk per-act or per-partner through any type of OI activity remains poorly quantified and despite the high estimate from Giesecke et al.
our review suggests a low but non-zero transmission probability.
Where OI exposure with no other HIV risk factors is reported, there are the possibilities of both social desirability bias and recall bias leading to underreporting of higher risk behaviour, which may lead to overestimation of transmission probability estimates. Celum et al.
stated that in their study most men reporting UROI also reported protected AI and that for such studies that rely on self-reported behavioural data, ‘some seroconverters may not have recognized condom failure and others may have over-reported condom use, which could result in over-attribution of HIV transmission to oral sexual exposure’.26
In a MSM cohort study, Keet et al.
reported that of 20 men denying receptive AI (RAI) in the 6–9 months prior to seroconversion in written questionnaires, 11 later reported this practice in face-to-face interviews.2
Conversely, transmission probability estimates from discordant couple studies such as de Vincenzi12
may be low and underestimate infectiousness because index partners are likely to be in the incubation period, after the period of high infectiousness during primary infection.
Different types of OI are likely to have different risks for HIV transmission. The report of a working group of the UK Chief Medical Officers' Expert Advisory Group on AIDS suggested that, ‘it would seem reasonable to assume that ejaculation increases the extent of exposure to HIV and that avoiding it may help reduce the risk of HIV transmission’.27
Again, there are insufficient data to investigate this assumption. Del Romero et al.
provide per-partner estimates by type of act and direction of transmission, but there were no seroconversions in this study.19
Studies reporting risk per-study participant, where number of partners and the serostatus of partners for each participant were often not available, were included in this review because of the limited number of studies reporting any type of OI risk estimate. However, such studies may suffer additional reporting bias, as seroconversions from study participants with no risk factor other than OI are notable because such events are rare and, therefore, their occurrence may increase the likelihood of reports from cohorts mentioning them. For per-act infectiousness, Vittinghoff et al. could only quantify the OI risk per partner infected or of unknown serostatus, which would be an underestimate of risk per infected partner.9
Given the small number of studies, a meta-analysis was not considered appropriate as many zero estimates might have occurred because of the small sample size, the low risk of transmission through OI and the increased influence of misclassification biases. The low risk of transmission means large and expensive studies would be required to provide useful evidence to supply more precise estimates. Such estimates are important for prevention and counselling of individual patients. The relative contribution of OI to HIV transmission, despite its inherent low infectiousness, may be substantial if the frequency of unprotected OI is increasing relative to higher risk sexual practices, which may be protected.1,28,29
Nevertheless, the fact that infected study participants with solely this exposure have remained difficult to identify may suggest that indeed the contribution of OI to HIV incidence remains low. The contribution of OI to HIV incidence needs close monitoring and opportunities for further transmission probability studies should be identified and utilized in order to give greater understanding of this neglected mode of transmission.