(A) Spearman rank correlations between R0 estimations, population size, peak sizes, peak times (for all cities), and latitudes (for European cities). Significance levels: P < 10−5 for |r| > 0.45; P < 10−4 for |r| > 0.40; P < 10−3 for |r| > 0.35, P < 0.05 for |r| > 0.20. (B) Correlation between 1918 (12) and 1889 R0 values (this study) for the 11 US cities studied in common (Ba, Baltimore; Bo, Boston, Ci, Cincinnati; Cl, Cleveland; De, Detroit; In, Indianapolis; Mi, Minneapolis; NY, New York; Pr, Providence; SF, San Francisco; Wa, Washington). Pearson's correlation = 0.62 (P < 0.05). Dotted line corresponds to linear regression.