Addressing mpox disparities among MSM in Los Angeles County

1 This number accounts for first doses administered and data is available on the public facing dashboard. In total, LAC DPH has administered just over 115,000 doses – which includes both first and second doses.

This report summarizes findings from a partnership between the Southern California HIV/AIDS Policy Research Center (SCHPRC) and the University of Southern California (USC) Viterbi School
of Engineering with key input from LAC DPH. The goal of the project was to use microsimulation modeling to understand the trajectory of the mpox outbreak in LAC to inform ongoing public health efforts that focuses specifically on health equity.

Reduction in transmission is key to mpox control

Simulation results confirm that persistent reduction in transmission (possibly through reduction in the number of sexual contacts or other methods) is important to effective mpox infection control. Counterfactual scenarios examining what would have happened had transmission risen to levels seen in early summer after October 2, November 6, or December 4 indicate that case counts would have been substantially higher (see Figure 1). The figure shows that maintaining low partnership levels for longer results in better disease control. Efforts to limit transmission through effective public health messaging were likely critical to low levels of mpox cases observed in early December.

Model overview

We converted a published HIV simulation for MSM developed in partnership with LAC DPH to model the ongoing mpox outbreak. Leveraging an existing, peer-reviewed model for this new adaptation means many of the underlying population dynamics, including sexual partnerships by age and race/ethnicity, are already simulated and validated in detail. After discussion with policy experts, clinicians, and epidemiologists, we restructured the simulation to capture mpox disease progression, vaccination, treatment, and recovery.

Model structure, dynamics, and inputs

Specifically, the model simulates health and treatment states at each week, points at which individuals can be susceptible, asymptomatic, symptomatic, or have recovered from mpox. All individuals may seek testing; susceptible and asymptomatic individuals may become vaccinated, which reduces the likelihood of becoming infected for susceptible individuals or hastens recovery
for recently infected asymptomatic patients. Symptomatic individuals may receive treatment, which hastens recovery. The probabilities and rates of these transitions are informed by published literature, surveillance data from LAC DHSP, and discussions with public health experts.

The model is stratified by age, race/ethnicity and HIV status, which allows transition probabilities (and therefore infection, testing, vaccination, treatment, and recovery rates) to vary by these characteristics. Including these parameters in our model means we can capture differences observed in the empirical data between these groups; for instance, the model captures the higher burden of mpox among PLWH. We assume that infection is spread primarily through sexual contact, and we use age- and race/ethnicity-specific sexual partnership patterns as calculated from survey data on partnership demographics and frequency gathered by the Los Angeles LGBT Center (Drabo et al., 2022, Nguyen et al., 2022).

Because many model inputs were uncertain (e.g., the rate of test-seeking behavior by demographic group), we calibrated the model to match observed trends. Calibration parameters included the likelihood of infection given a discordant contact (force of infection), number of asymptomatic individuals at the beginning of the simulation, the relative risk of infection by age, race/ethnicity, and HIV status, the age-specific likelihood of seeking testing, and the relative risk of vaccination by age and race/ethnicity.

Of note is that the rate of sexual partnerships among MSM reportedly dropped sharply in mid- August, six weeks after the beginning of the simulation, due to increased awareness of mpox risk. This is suggested in empirical data by a reduction in the number of detected mpox cases and has been documented nationwide by the CDC (2022). The simulation accounts for this by using a time- varying force of infection that is reduced after the sixth week of the simulation.