• Scores on the practice of safe sex with steady partners at a six-months follow-up
  

• Scores on response efficacy, intentions and perceived behavioral control regarding the practice of safe sex with steady partners directly after the administration of the intervention
  

Relationships are a high-risk setting for HIV-infection. This trial aimed at testing the efficacy of an online theory-based tailored intervention for preparing single MSM to practice safe sex with future steady partners—labeling it the ‘cognitive vaccine approach’.

The target was the promotion of negotiated safety (NS): steady partners testing for HIV and reaching agreements to either be monogamous or to only have safe sex outside the relationship in order to have safe unprotected anal intercourse with each other. The intervention content was based on the information, motivation, behavioral-skills model and the intervention was tailored according to knowledge, motivation, and skill-related deficiencies of each participant. Condom use was promoted as the default alternative for NS. Using an online randomized controlled trial we examined the effects of a tailored versus non-tailored version of the intervention. The cognitive effect (i.e. response efficacy, intentions, and perceived behavioral control (PBC) was measured directly after the intervention and, after 6-months, the behavioral effect (i.e. NS and condom use) via e-mail follow-up.