Figure 2

Monitoring regional homogeneity in England from December 2020 to May 2022. The upper panel shows the Bonferroni control chart of the normalized R_t’s of English regions during 18 months. In this period, seven major events, labelled from A to G, are visible, whose plausible explanations are conjectured below. A– Alpha variant: although no crossing is observed, the curves form two clusters, and the upper is formed by the three regions where the alpha variant first became dominant. No crossing occurs because, rather than starting in a unique region, the variant colonized three regions at the same time, hindering out-of-control detection. B–Outbreaks in Yorkshire: during Spring 2021, the number of infection cases in the region was slowly decreasing and then it had a huge peak, due to very high numbers of manufacturing jobs and related high-exposure workplaces, leading to outstanding outbreaks such as the one in a Selby warehouse with more than 700 employees. C– Delta variant first arrival in the North West: in May 2021 the Delta variant, originated in India, started its colonization of England from the North West region. As a characteristic feature of the control chart, the curve that first crosses the upper limit in correspondence of a new VoC, later on is often going to cross also the lower limit if its R_t is the first one to decrease, as happened to the North West. D– Immensa scandal: see Fig. 4 for a discussion of the massive lab malfunctioning that perturbed case recording in the South West and South East. Compared to the rise of a new VoC, a specular pattern is observed: when the malfunctioning is fixed, a fake outbreak is observed and the red curve crosses the upper control limit. E – Omicron 1 variant first arrival in the London region: see Fig. 3k–o for a discussion. F – In mid February, the UK Government announced that on April 1 free tests would be suspended, thus triggering social behaviors that may explain the anomalous trends in the control chart. G – Omicron 4 variant first arrival in the London region. For validation purposes, the four middle panels report the number of weekly detected cases, in each region, of the main VoCs³⁶, Alpha (B.117, Q) , Delta (B.1.617.2, AY), Omicron 1 (BA.1) and Omicron 4 (BA.4), during selected time windows. In each case, the control chart correctly identifies the region where the spread starts. The number of detected cases for Omicron 4 is far less than for the other VoCs, due to cuts to genomic surveillance: the value of the control chart as a surveillance tool is even more evident. Finally, the bottom panel reports the time profile of the national R_t, to facilitate the connection with the different phases of the pandemic in UK.