Description of studies and RR estimates of the association between syphilis infection and HIV incidence by characteristics and participants characteristics
| No. of studies | RR (95% CI) | Variance explained I2 | P value | |
| Participant characteristics | ||||
| Mean or median age | ||||
| ≤30 years* | 7 | 2.53 (1.50 to 4.26) | 64.3% | 0.010 |
| † | 3 | 3.20 (1.62 to 6.30) | 39.4% | 0.192 |
| >30 years* | 7 | 2.60 (1.76 to 3.84) | 46.1% | 0.084 |
| † | 1 | 3.86 (2.14 to 6.99) | – | – |
| Population | ||||
| MSM* | 12 | 2.60 (1.78 to 3.80) | 58.6% | 0.005 |
| † | 4 | 2.97 (2.05 to 4.31) | 0.0% | 0.520 |
| Other high-risk groups* | 5 | 2.98 (2.15 to 4.14) | 0.0% | 0.861 |
| † | 1 | 6.33 (2.10 to 19.08) | – | – |
| World region* | ||||
| Europe | 3 | 3.26 (2.08 to 5.11) | 0.0% | 0.880 |
| Asia | 11 | 2.80 (2.20 to 3.56) | 0.0% | 0.710 |
| Africa | 3 | 2.77 (1.65 to 4.66) | 16.6% | 0.301 |
| Australia | 2 | 2.08 (1.02 to 4.26) | 0.0% | 0.439 |
| America | 3 | 3.32 (2.32 to 4.73) | 0.0% | 0.920 |
| Syphilis prevalence (baseline) | ||||
| ≤10%* | 12 | 2.63 (1.84 to 3.77) | 59.7% | 0.004 |
| † | 1 | 6.33 (2.10 to 4.57) | ||
| >10%* | 5 | 2.81 (1.97 to 4.00) | 45.5% | 0.022 |
| † | 4 | 2.97 (2.05 to 4.31) | 0.0% | 0.520 |
| Study characteristics | ||||
| Study design | ||||
| Cohort study* | 13 | 2.83 (2.18 to 3.69) | 23.1% | 0.210 |
| † | 5 | 3.21 (2.26 to 4.57) | 0.0% | 0.419 |
| Case-control study* | 4 | 2.20 (1.11 to 4.37) | 74.5% | 0.008 |
| Follow-up rate | ||||
| ≤80%* | 7 | 2.75 (1.71 to 4.41) | 44.2% | 0.096 |
| † | 2 | 3.74 (2.26 to 6.32) | 0.0% | 0.419 |
| >80%* | 4 | 1.87 (0.88 to 3.97) | 54.0% | 0.012 |
| † | 3 | 3.20 (1.62 to 6.30) | 39.4% | 0.192 |
| Syphilis confirm test | ||||
| NTT & TT | 17 | 2.97 (2.43 to 3.62) | 0.0% | 0.818 |
| NTT | 3 | 2.77 (1.93 to 3.96) | 0.0% | 0.534 |
| Length of time between tests | ||||
| ≤6 months* | 10 | 2.74 (2.04 to 3.66) | 27.7% | 0.189 |
| † | 2 | 2.31 (1.36 to 4.57) | 0.0% | 0.487 |
| >6 months* | 4 | 2.61 (1.88 to 3.62) | 53.8% | 0.009 |
| † | 3 | 4.21 (2.61 to 6.77) | 0.0% | 0.715 |
| Definition of unexposed group* | ||||
| Syphilis (−) throughout follow-up | 10 | 3.31 (2.59 to 4.22) | 0.0% | 0.858 |
| Syphilis (−) at baseline | 7 | 1.97 (1.25 to 3.11) | 45.5% | 0.022 |
| Timing sequence | ||||
| Definitely before | 13 | 2.83 (2.28 to 3.51) | 0% | 0.840 |
| Before and indeterminably close | 9 | 3.01 (2.33 to 3.90) | 0% | 0.761 |
*Prevalent syphilis infection.
†Incident syphilis infection.
MSM, men who have sex with men; NTT, non-treponemal test; RR, relative risk; TT, treponemal test.