Challenges in control of Covid-19: short doubling time and long delay to effect of interventions

Lorenzo Pellis; Francesca Scarabel; Helena B Stage; Christopher E Overton; Lauren H K Chappell; Katrina A Lythgoe; Elizabeth Fearon; Emma Bennett; Jacob Curran-Sebastian; Rajenki Das; Martyn Fyles; Hugo Lewkowicz; Xiaoxi Pang; Bindu Vekaria; Luke Webb; Thomas A House; Ian Hall

Although R 0 is commonly used to determine the reduction in person-to-person transmission needed to achieve control, this brings limited insight into how to control the COVID-19 pandemic. First, there is sufficient evidence worldwide that interventions must be draconian, particularly in countries where case numbers are high. Second, given the variability in R 0 estimates obtained from the same growth rate appear predominantly due to the assumed amount of pre-symptomatic transmission, the exact value of R 0 is only poorly correlated with the required aggressiveness of the intervention [38] . For example, if R 0 is four and most of the transmission occurs after symptoms onset, 75% of the transmission needs to be stopped and self-isolation when symptomatic can easily achieve this. Conversely, if R 0 is two but most of the transmission is pre-symptomatic, only 50% of the transmission needs to be prevented, but in practice this can only be achieved through interventions, like quarantining apparently healthy individuals, that are highly socially disruptive and likely enforced rather than spontaneous. Finally, R 0 informs how aggressive interventions should be, but not how quickly they should be implemented.