Background:
Mpox was first reported in France on May 19, 2022 and third-generation live Modified Vaccinia Ankara (MVA-BN) vaccination of multiple-partner MSM was recommended as of July 11, 2022. We assessed the impact of vaccination on incidence of mpox in MSM on PrEP participating in a clinical trial.
Methods:
ANRS-174 DOXYVAC is an ongoing trial enrolling MSM on PrEP with history of STI in the past year. We compared socio-behavioral characteristics of mpox cases and mpox-free controls in the pre-epidemic period (up to May 8 2022). Then we compared the incidence of mpox between May 9-July 10 2022 (before recommendation of vaccination for MSM) and July 11-September 20 2022 (after vaccination launch). Incidence rates (IRs) of mpox per 1000 person-months (p-m) over periods were calculated as the total number of mpox divided by the p-m of observation. IRs of mpox were compared between periods using a Poisson regression model with random intercepts to account for within-subject variability.
Results:
At the time of mpox outbreak, 546 MSM had been randomized including 472 with available data before and after May 9, 2022, with median age 39 years, median of 10 sexual partners during last 3 months, median of 5 condomless anal intercourses during last month and 20% received smallpox vaccine during childhood. Mpox occurred in 77/472 participants. Prior to the epidemic, cases were younger (37 vs 40, p=0.0179), had more sexual partners during last 3 months (15 vs 10, p=0.0022), more condomless anal intercourses during last month (7 vs 5, p=0.0244) and were less vaccinated during childhood (4% vs 23%, p< 0.0001) than controls. A significant change in sexual behavior was seen before and after May 9, 2022 only among the proportion of controls with > 10 partners during last 3 months (45% vs 38%, p=0.0035). Mpox incidence was 67.4 per 1000 p-m (95%CI 51.6-86.6) between May 9 and July 10, and 24.4 per 1000 p-m (95%CI 13.9-39.6) between July 11 and September 20 2022, with an IRR of 0.44 [0.24-0.81], with only one case reported after August 8, 2022. In multivariable Poisson regression model (see Table), only MVA-BN vaccination in 2022 remained significantly associated with mpox infection, with a 99% risk reduction (95%CI 96.6-99.7).
Conclusions:
In France, MVA-BN vaccination in summer 2022 conferred high-level protection against mpox infection in highly-at-risk MSM on PrEP. In this study population, sexual behavior change did not seem to play a role in reduction of mpox incidence.
Table: Poisson regression model