Characteristics of repeat syphilis infections and interinfection intervals in a retrospective clinical cohort: implications for syphilis retesting and prevention.

Background: Syphilis infections are rising in many countries. Syphilis reinfections can occur among patients with ongoing risk. We aimed to describe the characteristics of syphilis reinfections and the interval time between syphilis reinfections.

Methods: This was a retrospective study of patients with two or more syphilis infections between 2011 and 2022 diagnosed at Melbourne Sexual Health Centre, Australia.

Results: 474 patients, 98% men who have sex with men (MSM), had 1327 syphilis infections over 2062 person-years; 26.9% (n=357) primary syphilis, 20.9% (n=278) secondary and 48.3% (n=641) early latent syphilis. Individuals experienced up to nine syphilis infections. The proportion of people living with HIV (PLWH) increased with each subsequent syphilis infection from 40.1% (190/474) of first infections to 78.4% (29/37) of fifth infections. For first infections, the proportions were 36.5% for early latent, 31.6% for primary and 22.2% for secondary syphilis. In contrast, for fifth infections, proportions increased to 78.4% for early latent and decreased to 16.2% for primary and 5.4% for secondary syphilis. The median interinfection interval, between the first and second syphilis infection, for the entire cohort, was 656 days (IQR 325-1262 days). Pre-exposure prophylaxis (PrEP) users had a shorter median interinfection interval of 341 days (IQR 206-762 days), compared with the entire cohort (p<0.0001), HIV-negative patients not using PrEP (p<0.0001) and PLWH (p<0.0001).

Conclusion: Among MSM, syphilis reinfections were common, especially among PLWH and HIV-negative PrEP users, with the latter having the shortest interval between infections. These groups should be retested for syphilis with frequent serological screening, PCR testing of syphilis lesions and should be targeted for syphilis prevention.