Background
The mpox epidemic in the UK began in May 2022. Unexpectedly and rapidly, rates of new cases declined during August 2022. Interpreting trends in infection requires disentangling the underlying growth rate of cases from the delay from symptom onset to presenting to health care. Quantifying both separately is required to correctly identify the cause of the rapid decline in case counts, and yet both together contribute to observed reported case counts and delays. In addition, they may be causally linked since reducing delays from symptom onset to presenting to healthcare may be a mechanism that slow epidemic spread, as undiagnosed individuals may be more likely to transmit the pathogen. In this paper, we apply a novel Gaussian statistical model to mpox data from the UK to quantify the changes in the onset-to-presentation delay and the underlying the epidemic growth rate of mpox over the period May-August 2022 in the UK.
Methods
We developed a nowcasting Bayesian method which incorporates time-varying delays (EpiLine), simulating the growth rate of symptomatic cases and the parameters of delay distributions following Gaussian processes. We applied our model to the mpox data from the UK, sampling the posterior distribution of all parameters using Markov Chain Monte Carlo methods, to quantify the delay between mpox symptom onset to healthcare presentation and the growth rate over the study period.
Results
Our results suggest that the mean delay between symptom onset and healthcare presentation for mpox in the UK decreased 22 days in early May 2022 to 10 days by early June and 8 days in August 2022. When we account for these dynamic delays, the time-varying growth rate declined gradually and continuously in the UK during the May-August 2022 period. Not accounting for varying time delays would have incorrectly characterised the growth rate by a sharp increase followed by a rapid decline in mpox cases.
Conclusions
We show that the delay in accessing healthcare was quickly reduced from 22 days to 8 days from symptom onset, which will have contributed to preventing onward transmission, and allowed prompt use of antivirals post infection. In addition, our results highlight the importance of correctly quantifying the delay between symptom onset to healthcare presentation when characterising the epidemic growth of mpox in the UK. The gradual reduction in the rate of epidemic spread, which pre-dated the roll-out of vaccination, is most easily explained by gradual risk reduction, as well as acquired immunity amongst the highest risk individuals. Our study highlights the importance and need for public health agencies to focus on reducing delays from symptom onset to healthcare presentation early in an outbreak.