  In May 2022, a cluster of mpox cases were detected in the UK that could not
be traced to recent travel history from an endemic region. Over the coming
months, the outbreak grew, with over 3000 total cases reported in the UK, and
similar outbreaks occurring worldwide. These outbreaks appeared linked to
sexual contact networks between gay, bisexual and other men who have sex with
men. Following the COVID-19 pandemic, local health systems were strained, and
therefore effective surveillance for mpox was essential for managing public
health policy. However, the mpox outbreak in the UK was characterised by
substantial delays in the reporting of the symptom onset date and specimen
collection date for confirmed positive cases. These delays led to substantial
backfilling in the epidemic curve, making it challenging to interpret the
epidemic trajectory in real-time. Many nowcasting models exist to tackle this
challenge in epidemiological data, but these lacked sufficient flexibility. We
have developed a novel nowcasting model using generalised additive models to
correct the mpox epidemic curve in England, and provide real-time
characteristics of the state of the epidemic, including the real-time growth
rate. This model benefited from close collaboration with individuals involved
in collecting and processing the data, enabling temporal changes in the
reporting structure to be built into the model, which improved the robustness
of the nowcasts generated.
