The role of vaccination and public awareness in forecasts of Mpox incidence in the United Kingdom

Abstract

Beginning in May 2022, Mpox virus spread rapidly in high-income countries through close human-to-human contact primarily amongst communities of gay, bisexual and men who have sex with men (GBMSM). Behavioural change arising from increased knowledge and health warnings may have reduced the rate of transmission and modified Vaccinia-based vaccination is likely to be an effective longer-term intervention. We investigate the UK epidemic presenting 26-week projections using a stochastic discrete-population transmission model which includes GBMSM status, rate of formation of new sexual partnerships, and clique partitioning of the population. The Mpox cases peaked in mid-July; our analysis is that the decline was due to decreased transmission rate per infected individual and infection-induced immunity among GBMSM, especially those with the highest rate of new partners. Vaccination did not cause Mpox incidence to turn over, however, we predict that a rebound in cases due to behaviour reversion was prevented by high-risk group-targeted vaccination.

Fig. 1. GBMSM reproductive number and model-based case projections.

Top row: Inferred basic reproductive numbers for transmission via sexual partnership among GBMSM people (a). The fitted reproductive numbers are shown as basic (solid black curve), not accounting for population immunity, and effective (dashed black curve), accounting for population immunity. Reversion towards baseline behaviour is shown by basic reproductive numbers, this begins on 1st September and reaches 99% of pre-outbreak behaviour either 4 weeks (red curve) or 12 weeks (blue curve). Bottom row: Weekly Mpox confirmed case data from UKHSA linelist with inferred GBMSM status (black markers; GBMSM b, non-GBMSM c). The last week of available data (black squares) was not used in inference. Error bars on data points indicate 95% confidence intervals for GBMSM status inference. The posterior median over model projections are shown for 4 weeks reversion and 12 weeks reversion to baseline behaviour (solid curves). The posterior median model projections for the counterfactual scenario where vaccines are either absent or ineffective generate substantial secondary peaks (dashed curves). Background shading indicates 50, 80 and 95% credible or prediction intervals; prediction intervals for the counterfactual results are omitted for clarity. Model predictions are based on n = 2000 parameter sets drawn from their posterior distribution.

Fig. 2. Population exposure and medium-term cumulative case projections.

a The proportion of the whole UK population in each risk group: non-GBMSM and ten GBMSM sexual activity groups (bars). The posterior median of proportion in each risk group uninfected (blue) and infected (red) by 1st October 2022 is shown as proportion shaded in each bar. Bottom row: Posterior median model projections of cumulative cases among GBMSM (b) and non-GBMSM (c) for 4-week (red) or 12-week (blue) reversion to baseline behaviour with background shading for 50, 80 and 95% prediction intervals. Also shown are counterfactual results without vaccination but with behaviour change (dashed red and blue), and without either vaccination or behavioural change (dashed green). Shaded and cross-hashed regions show the 50% prediction intervals for the counterfactual projections. Model predictions are based on n = 2000 parameter sets drawn from their posterior distribution.

Fig. 3. Schematic diagram of compartmental model with daily transition probabilities.

Top: Compartmental model for GBMSM individuals. GBMSM population structure is indicated by subscripts for sexual activity group g and metapopulation group m. Force of infection on GBMSM individuals is sum of infection rate from within GBMSM sexual contacts and background homogeneous transmission (inset equation). Bottom: Compartmental model for other non-GBMSM individuals. Force of infection on other individuals is only due to background homogeneous transmission.

Fig. 4. Multi-stage model for incubation period.

Standard deviation for multi-stage discrete time incubation model when mean is fixed to be 7.7 days by number of stages. The standard deviation for incubation period fitted from data is given as a horizontal red line. The closest match (n = 2 stages) was used in simulations. Inset: the incubation period distribution used in simulations.

Fig. 5. The next generation time distribution used in the model.

Next generation distribution used in this model (blue dots) compared against serial interval in Ward et al. (red dots) and the same serial interval with reduced standard deviation to match generation distribution used in this model (green dots).

Fig. 6. Proportion of GBMSM in each sexual activity group, and daily rate of new sexual partnership formation.

The proportion of the active GBMSM population in each sexual activity group (a); NB this is shown on a log-scale. The mean rate of forming new sexual partnerships by sexual activity group (b). Sexual activity groups were defined by equipartition of the GBMSM by total group rate of sexual activity (see section Population structure for details). The main scenario in this paper assumes that the GBMSM individuals being offered vaccination are those who typically have at least one new sexual partner per month (shown as red line). For this model structure this corresponds to sexual activity groups 3–10 (the 9.4% of the active GBMSM population most sexually active).

Fig. 7. Projected number of cumulative first vaccine doses given in the United Kingdom.

Projected cumulative numbers of first vaccine doses given is shown (black lines) compared against UKHSA reported cumulative vaccine first doses (red dots).