Slowly declining growth rates and dynamic reporting delays characterise the Monkeypox epidemic in the UK over May-August 2022

The monkeypox epidemic in the UK began in May 2022, and subsequently and rather quickly, rates of new cases have declined during August 2022. Identifying the causes of this decline requires accurate estimates of the time-varying epidemic growth rate r(t), which in turn depend upon the reporting delays (defined as the time from onset of symptoms to presenting to healthcare). Using a custom nowcasting method which allows for time-varying delays (EpiLine), we show that the reporting delay for Monkeypox in the UK decreased from an average of 22 days in early May 2022 to 10 days by early June and 7 days in August 2022. Accounting for these dynamic delays shows that the time-varying r(t) declined gradually in the UK over this period. Not accounting for varying time delays would have incorrectly characterised r(t) by a sharp increase followed by a rapid drop. We discuss the importance of this gradual decline, which helps identify the potential mechanisms responsible for the decline in the rate of spread of Monkeypox, which was gradual and started well before vaccines were widely used.