Table 2.

Relative risks of factors that increase or decrease per-act HIV transmission risk for sexual exposures.

Cofactor	Relative risk	95% Confidence interval	References
Factors that increase transmission probability
High plasma viral load (log₁₀ copies/ml)	2.89	(2.19, 3.82)	[69]
Genital ulcer disease^a	2.65	(1.35, 5.19)	[69]
Acute versus asymptomatic stage of disease	7.25^b	(3.05, 17.3)	[70]
Late versus asymptomatic stage of disease	5.81^b	(3.00, 11.4)	[70]
Factors that decrease transmission probability
Use of antiretrovirals by HIV-infected partner
Early versus delayed treatment	0.04^c	(0.01, 0.27)	[72]
Received treatment versus no treatment	0.08	(0.00, 0.57)	[73]
Pre-exposure prophylaxis of HIV-uninfected partner
Among heterosexual couples	0.29^d	(0.17, 0.47)	[16,17]
Among MSM	0.56	(0.37, 0.85)	[74]
Among injection drug users	0.52	(0.28, 0.90)	[75]
Condom use	0.20^e	(0.08, 0.47)	[18]
Male circumcision (heterosexual partners)
HIV-uninfected partner is male	0.50^f	(0.34, 0.72)	[76]
HIV-uninfected partner is female	0.80	(0.53, 1.36)	[77]
Male circumcision (MSM)
Insertive partner is HIV-uninfected	0.27^g	(0.17, 0.44)	[78]
Receptive partner is HIV-uninfected	1.20^g	(0.63, 2.29)	[78]

Characteristic of the HIV-uninfected partner; therefore relative risk reflects the increased risk of acquisition of HIV infection from an infected partner

Hazard of transmission that accounts for duration of infectiousness was calculated using these data: hazard of transmission per person-year for early versus asymptomatic stage of disease is 2.76 [95% confidence interval (CI) 1.31–5.09] and for late versus asymptomatic stage is 0.76 (95% CI 0.41–1.28); thus 26 and seven times more infectious, respectively [71].

The reported hazard ratio was used to approximate the relative risk.

For this estimate, we combined the number of events and person-time data from the tenofovir and emtricitabine (TDF-FTC) and placebo arms from two studies of pre-exposure prophylaxis [16,17] and then employed Poisson regression to calculate the estimated relative risk with an exact 95% CI.

This review indicates that consistent use of condoms results in 80% reduction in HIV incidence. Consistent use is defined as using a condom for all acts of penetrative vaginal intercourse. Because the studies used in this review did not report on the ‘correctness’ of use, namely whether condoms were used correctly and perfectly for each and every act of intercourse, effectiveness and not efficacy is estimated. Effectiveness was estimated from two separate incidence estimates: one minus the ratio of incidence among always users to the incidence among never users.

This review combined the survival estimates from three trials [78–80] at 12 months and also at 21 or 24 months in a meta-analysis using the random-effects model. The resultant incidence risk ratio (IRR) was 0.50 at 12 months with a 95% CI of 0.34–0.72, and 0.46 at 21 or 24 months (95% CI 0.34–0.62).

The reported odds ratios were used to approximate the relative risks.