Introduction HIV incidence remains high in UK MSM, and cost-effective combination prevention is needed. We estimated numbers of infections averted by targeting testing for highly-infectious primary HIV infection (PHI) in high-risk MSM.

Methods A cohort study of 98 MSM (participation rate 94%; zero loss to follow-up) recently infected with HIV recorded sexual behaviour in the 3 months pre- and post-HIV diagnosis. A stochastic individual-based model calculated numbers of HIV-transmission events expected to occur with and without the effects of (i) immediate ART in those who chose to receive it and (ii) behaviour changes reported post-diagnosis by those who did and did not receive ART. The model incorporates different types of sex-act, condom use, and distinguishes between regular and casual partners.

Results If PHI lasts for 3 months, testing is monthly, and viral load is suppressed by ART after 3 months with an initial rapid decline in the first 2 weeks, then from the 73 patients who took ART the reduction in transmission would be ~75%, from 22–33 to 5–8 events, and in the remaining 25 patients the reduction (due to behaviour change only) would be ~50%, from 8–12 to 5–7. If the duration of PHI is shorter, or testing frequency is lower frequency, and/or viral-load suppression is slower then the number of infections averted is reduced.

Conclusion Diagnosing HIV during PHI can markedly reduce transmission because its high infectivity and short duration make treatment and even short-term behaviour change effective in reducing transmission. Cost-effectiveness would be increased by efficient approaches to identifying PHI (e.g. encouraging HIV testing after episodes of high-risk behaviour and intensive contact tracing from recently-infected individuals, since PHI cases are likely to be clustered). Our quantification of the number of infections averted is an essential component of assessment of the cost-effectiveness of strategies to increase diagnoses of PHI.