CROI 2025 Abstract eBook

Abstract eBook

Poster Abstracts

acts (UA), defined as a condomless anal sex act with a casual partner not fully covered with PrEP (ie, proxy for HIV acquisition risk). Data were obtained through daily digital diary entries. We calculated the IR ratio (IRR) of UAs between online-mediated and in-clinic monitoring arms, and between 6- and 3-monthly monitoring arms. Noninferiority was concluded if the upper limit of the 97.5%CI of the IRR was <1.8. Results: Between September 2021 and August 2022 we included 469 participants, who were randomized as follows: arm 1, n =117; arm 2, n =114; arm 3, n =119; and arm 4, n =119. During 605 person years (py) of follow-up, 816 UAs were reported; overall IR of UA was 1.3/py [95%CI:1.2-1.4]. The IR of UA was lower in the online-mediated arms than in the in-clinic arms (IRR=0.83, 95%CI:0.72-0.95). The IR of UA in the 6-month arms was similar to that in the 3-month arms (IRR=0.96, 95%CI:0.84-1.11). The results showed noninferiority with the upper limit of the 97.5%CI of both IRRs <1.8 (Figure). Conclusions: Online-mediated and 6-monthly PrEP monitoring are non inferior to in-clinic and to 3-monthly monitoring, respectively, regarding PrEP adherence. These findings suggest that, in terms of HIV acquisition risk, online mediated and 6-monthly PrEP monitoring can be safely implemented as part of routine PrEP care for MSM.

We recommend that studies of infectious disease prevention consider the impact of sources of transmission beyond the reach of the intervention when designing and evaluating interventions to inform public health programs. 1208 Cost-Effectiveness of Vaccination Strategies to Control Future Mpox Outbreaks in England Xu-Sheng Zhang 1 , Siwaporn Niyomsri 2 , Sema Mandal 1 , Hamish Mohammed 1 , Miranda Mindlin 1 , Gayatri Amirthalingam 1 , Andre Charlett 1 , Claire Dewsnap 3 , Bennet Dugbazah 1 , David Phillips 4 , Mary E. Ramsay 1 , Josephine G. Walker 2 , Peter Vickerman 2 1 Public Health England, London, UK, 2 University of Bristol, Bristol, UK, 3 Sheffield Teaching Hospitals, Sheffield, UK, 4 Croydon University Hospital, Thornton Health, UK Background: In 2022, a global outbreak of mpox occurred among gay and bisexual men who have sex with men (GBMSM). In England, the outbreak was controlled through reductions in sexual risk behaviour and vaccination of high risk GBMSM. However, mpox continues to circulate, including an expanding outbreak in Africa, and so there is a risk of future outbreaks in the UK and other noon-endemic countries. We evaluated the most cost-effective vaccination strategy to minimise the risk of future mpox outbreaks among GBMSM in England. Methods: A mathematical model of mpox transmission among GBMSM was developed to estimate the costs per quality-adjusted-life-year (QALY) gained for different vaccination strategies starting in 2024 (20-year time-horizon; 3.5% discount rate; willingness-to-pay threshold £20,000/QALY). The model was calibrated using English surveillance data from the 2022 outbreak and two community surveys. Reactive vaccination (only during outbreaks) and pre-emptive vaccination (continuous routine) strategies targeting GBMSM at high-risk for mpox were compared to no vaccination. Baseline projections incorporated importation of new mpox cases, and a vaccine effectiveness of 78%/89% for 5/10 years with 1/2 doses at £160/dose. Based on the 2022 outbreak, costs were estimated for case management, vaccination and public health responses (PHR) during futures outbreaks. Results: All vaccination strategies reduced future outbreaks, gained QALYs and reduced costs compared to no vaccination. Continuous pre-emptive vaccination (daily rate 41 doses) was most cost-effective, saving £38.4 million and gaining 546.3 QALYs over 20-years. Threshold analyses suggested vaccination of GBMSM at high-risk for mpox is cost-effective if the vaccine costs less than £636 per dose. Pre-emptive vaccination remains the optimal strategy across numerous sensitivity analyses, including for importation of more severe and transmissible clades, but the optimal vaccination rate can vary. Reactive vaccination only becomes more cost-effective when public health response costs are not included. Conclusions: Pre-emptive vaccination of high-risk GBMSM is a crucial cost saving strategy to prevent future mpox outbreaks. Similar vaccination strategies should be considered for GBMSM in other countries to minimise the risk of future outbreaks. 1209 Estimating Population Immunity and Impact of COVID-19 Vaccination in Washington and Oregon Mia Moore 1 , Larissa Anderson 1 , Chloe Bracis 2 , David Swan 1 , Ian Painter 3 , Erik Everson 4 , Holly Janes 1 , Joshua T. Schiffer 1 , Laura Matrajt 1 , Dobromir Dimitrov 1 1 Fred Hutchinson Cancer Center, Seattle, WA, USA, 2 Lixoft, Inc, Antony, France, 3 Washington State Department of Health, Olympia, WA, USA, 4 Multnomah County Health Department and the State of Oregon Public Health Division, Portland, OR, USA Background: The highly efficacious mRNA-based COVID-19 vaccines had good uptake in Washington State and Oregon with at least 75% of the population in each state receiving at least one dose. Estimating the impact of COVID-19 vaccination campaigns has been challenging given 1) the concurrent buildup of population-level immunity from infection, primary vaccination, and booster doses, 2) the spread of variants during 2021 and 2022 which evaded population immunity, and 3) the changing backdrop of non-pharmaceutical interventions as the epidemic waned. Model-based counterfactuals, which simulate the epidemic without any vaccination, can account for these factors. Methods: To create vaccine-free counterfactual scenarios, we used a compartmental, age-structured epidemic model, with time-varying rates of transmission and vaccination. Our model incorporated 1) protections against SARS-CoV-2 infection and COVID-19 hospitalization due to vaccination and prior infections and 2) waning of those protections due to loss of immune memory and novel SARS-CoV-2 variants. We formulated and parameterized our model using a prior meta-analysis on the durability of immunity. We then

Poster Abstracts

1207 Spillover Infections Drive Transmission in HIV Prevention Trials: BCPP/Ya Tsie Study Lerato Magosi 1 , Eric Tchetgen Tchetgen 2 , Vlad Novitsky 1 , Molly Pretorius Holme 1 , Janet Moore 3 , Pam Bachanas 3 , Christophe Fraser 4 , Sikhulile Moyo 5 , Kathleen Hurwitz 6 , Tendani Gaolathe 5 , Joseph Makhema 5 , Shahin Lockman 7 , Max Essex 1 , Victor De Gruttola 1 , Marc Lipsitch 1 , for the Botswana Combination Prevention Project (BCPP / Ya Tsie Trial) 1 Harvard University, Cambridge, MA, USA, 2 University of Pennsylvania, Philadelphia, PA, USA, 3 Centers for Disease Control and Prevention, Atlanta, GA, USA, 4 Oxford University, Oxford, UK, 5 Botswana Harvard AIDS Institute Partnership, Gabarone, Botswana, 6 Target RWE, Durham, NC, USA, 7 Harvard TH Chan School of Public Health, Boston, MA, USA Background: A longstanding puzzle in efforts to control the spread of HIV, is why HIV treatment-as-prevention efforts in sub-Saharan Africa showed modest reductions in the occurrence of new infections despite achieving substantial gains in viral suppression. Methods: Using data from a community-randomized trial of HIV prevention in sub-Saharan Africa we offer for the first time an evidence-based explanation of this longstanding puzzle. More specifically, we deep-sequenced whole genome HIV viral sequences from 5,114 trial participants in the 30-community BCPP (Botswana Combination Prevention Project) trial in Botswana to track the directional spread of HIV infections between communities. After, we combined the deep-sequenced HIV viral genome data with population census data to quantify the relative contribution of transmissions to intervention communities from individuals with HIV in the same community, other intervention communities, control communities, and (the vast majority of the country) non study communities. Results: We found that individuals with HIV in non-study communities accounted for most of the transmissions to trial communities 90% [81 – 93], and that the impact of the BCPP intervention in reducing transmissions to trial communities could have been considerably larger if the intervention had been applied nationwide 59% [3 – 87] compared to the observed 30% reduction. Conclusions: This work transforms our thinking about the effectiveness of TasP for HIV prevention in general populations to a highly effective intervention when applied nationwide to maximize the capture of sources of transmission.

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