  One of the difficulties in monitoring an ongoing pandemic is deciding on the
metric that best describes its status when multiple intercorrelated
measurements are available. Having a single measure, such as the effective
reproduction number R, has been a simple and useful metric for tracking the
epidemic and for imposing policy interventions to curb the increase when R >1.
While R is easy to interpret in a fully susceptible population, it is more
difficult to interpret for a population with heterogeneous prior immunity,
e.g., from vaccination and prior infection. We propose an additional metric for
tracking the UK epidemic which can capture the different spatial scales. These
are the principal scores (PCs) from a weighted Principal Component Analysis. In
this paper, we have used the methodology across the four UK nations and across
the first two epidemic waves (January 2020-March 2021) to show that first
principal score across nations and epidemic waves is a representative indicator
of the state of the pandemic and are correlated with the trend in R.
Hospitalisations are shown to be consistently representative, however, the
precise dominant indicator, i.e. the principal loading(s) of the analysis, can
vary geographically and across epidemic waves.
