Fig. 1.

Schematic diagrams of serial and generation intervals from symptom- and infection-based infector cohorts. (A) Forward serial intervals are measured from the cohort of infectors who develop symptoms at the same time. In this case, infectors will have shorter incubation periods than their infectees on average; the corresponding generation intervals will be also short because infectors with short incubation periods will transmit earlier. (B) Generation intervals for the cohort of infectors who are infected at the same time are not biased by dynamical effects on incubation periods.