What determines mpox vaccination uptake? Assessing the effect of intent-to-vaccinate versus other determinants among men who have sex with men

Background: In response to the mpox outbreak, vaccination was offered in the Netherlands to men who have sex with men (MSM) at increased risk for mpox. Successful vaccination campaigns are leveraged by high intent-to-vaccinate, yet intent might not always lead to uptake. Therefore, we assessed the impact of intent-to-vaccinate and other factors on vaccination uptake among participants of the Amsterdam Cohort Studies (ACS). Method: In July 2022, prior to the mpox vaccination campaign, we distributed an online survey regarding mpox intent-to-vaccinate, as well as e.g. beliefs, attitude, subjective norms, and perception of risk among ACS participants (all MSM). Vaccination uptake was self-reported during study visits after August 2022. The association between vaccination intent and uptake, and determinants of intent, was jointly assessed using a structural equation model (SEM) based on components of the Theory of Planned Behavior (TPB). In a second SEM, determinants of intent were allowed to have a direct effect on vaccination uptake. Results: 492 MSM (median age = 46 years) were included in analyses. 380 (77%) had high intent-to-vaccinate and 238 (48%) received at least one vaccine dose. In the first model with a direct relation between intent and uptake only, TBP components predicted intent as expected, and high intent-to-vaccinate was significantly associated with getting vaccinated (beta = 1.1, 95%CI = 0.6-1.5). However, 175/380 (46%) participants with high intent-to-vaccinate did not get vaccinated. The second model had an improved model fit compared to the first model. The effect of intent on uptake was non-significant, and only perceiving to be at higher risk of infection significantly increased vaccination uptake later on (beta = 0.42, 95%CI = 0.26-0.59). Having a steady relationship decreased the probability of vaccination (beta = -0.59, 95%CI = -1.0- -0.18). Conclusions: While intent-to-vaccinate for mpox was high among MSM, high intent did not necessarily result in vaccine uptake. Mpox risk perception might have played a more pivotal role in getting vaccinated, which may be related to the evolution of vaccination eligibility criteria and accessibility to the vaccine.