We do not observe i(t) and must infer it from the daily death and/or hospitalization data. In the case of deaths (which in §3.3 we show yields the best estimate of R(t)), we must account for the time delay (t) between infection and death. The time from symptom onset to death was remarkably similar across all age groups with a mode of 7 days (Armstrong 1920; figure 3 ). We add 1.5 days for the incubation period (Ferguson et al. 2005 ) and round to the nearest integer, so that tZ9.