"We’re the poo crew"

Using this data to make sense of how much virus is circulating in England’s cities and towns is by no means straightforward. The amount of poo carried by sewers varies according to the time of day, and the surge in solids may come later in university towns – where students get up later – compared with more rural locations. It can also be diluted by large amounts of rainwater, so all of this must be carefully factored into the scientists’ calculations. Wastewater analysis is not accurate enough to tell us how many individuals are infected with Covid in any given area at any given time. However, it can provide an early warning of escalating cases in specific geographical areas that can be followed up with additional community testing and messaging, or the sewage equivalent of surge testing – where manhole covers are lifted up and samples taken from sewers in specific areas of a city, say, to try to narrow down the source of the outbreak. Sewage from individual households or tower blocks is not monitored – although separate research is investigating whether sewage from school wastepipes could be used as an early warning system of outbreaks among pupils, many of whom may be asymptomatic.