Cross correlation analysis
Table 1 shows results of Spearman Rank  contemporaneous correlations between GTD and incident case numbers  for different European countries. The increasing and decreasing phases of each country’s epidemics were also examined. For the increasing phase, the correlation was significant in all countries studied. With cross correlation analyses, in the majority of cases the correlation was strongest at a median of 0 day (IQR: 0-1) lag (Figure 1). In other words, GTD was contemporaneous with incident cases (with the notable exception of Ireland, where it preceded it by 18 days).
This pattern was also observed when data describing the decreasing phase of the epidemic was examined. All but two countries had significant moderate to strong positive correlations between GTD and incident case numbers (the exception being Sweden and France). With cross correlation analyses the median lag was also 0 days (IQR: -2.5-0, further details in the Supplementary material S1).