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

The delay between infection and case detection is crucial in determining how long cases have been growing unobserved. Early detection of cases during the incubation period, before individuals have become symptomatic, is typically not possible once containment has failed since it relies on full contact tracing and testing of asymptomatic individuals. Detecting cases at symptom onset is potentially more feasible, but in practice depends on the case-finding strategy. For example, in the UK, symptomatic individuals are instructed to self-isolate at home, and are only tested if they subsequently require hospitalisation. Thus the delay between infection and case detection includes the incubation period, the time between symptom onset and hospitalisation, and the time it takes to receive a positive test result. Similar effects will be visible in other countries where case counts are dominated by hospitalisations.