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

Our estimate of the delay between infection and detection is consistent with observations of the UK and Italian epidemics ( Figure 3 ). For both countries, we plot the numbers of new cases and notice a visible drop occurring 8-9 days after the first, relatively soft, control measures were implemented (in the UK, recommended self-isolation if symptomatic on 13 March, and in Italy, lockdown of infected towns and school and university closure in Northern Italy, on 22-23 February). After the first control measure in the UK ( Figure 3A ), cases continued to increase exponentially with an estimated growth rate of ~0.22/day (corresponding to a doubling time of just over 3 days) for 9 days. During this period numbers of daily confirmed cases rose approximately 8-fold. Subsequently, the number of new infections started to tail off. A similar pattern is observed in Italy ( Figure 3B ). Because nonpharmaceutical interventions, with unknown compliance, cannot be evaluated until their effects emerge in the data, a pattern of introduction of increasingly strong measures has repeated across Europe, with long delays to control. Even with immediate hard interventions halting all community transmission, within-household transmission will continue to occur, creating an additional delay between the beginning of the intervention and its effect. This is consistent with the approximate 2-week delay from lockdown to peak in new cases observed in Hubei [4] . Further delays in case-confirmations, hospitalisations, potential ICU admissions and deaths mean the latter figures keep on growing well after transmission control is achieved.