a, National lockdown (including stay-at-home order in US states). b, A selection of three NPIs displaying ‘the earlier the better’ behaviour—that is, their impact is enhanced if implemented at earlier epidemic ages. c, Enhance laboratory testing capacity and Surveillance. d, Tracing and tracking and Activate case notification. Negative (positive) values indicate that the adoption of the NPI has reduced (increased) the value of Rt. Shaded areas denote s.d.