Fig. 5

Contour plots of (a) the resurgence risk \(P\left({\phi }_{c},{t}_{N}\right)\) in June 2022, (b) the cumulative number of daily reported cases in June 2022, with respect to the resumed social activity level ratio \({\phi }_{c}\) and the swift relaxation timing \({t}_{N}\). The green dotted curve in (a) below which the resurgence risk \(P({\phi }_{c},{t}_{N})\) was always 0. Taking strategy 4 as a case, the red points represented the location of (\({\phi }_{c},{t}_{N}\)), namely, \({\phi }_{c}=0.6\), \({t}_{N}\) was 1 June