Basic estimation-prediction techniques for Covid-19, and a prediction for Stockholm

Tom Britton

To start, the evolution of the epidemic outbreak is simulated using the system defined above with λ = (ln(2)/3.5) * 2.5/1.5 = 0.330. The rate of recovery equals γ = λ−ln(2)/d = 0.132. Finally, the new contact rate λ E induced by the new bigger doubling time d E = 9 equals λ E = (ln(2)/d E ) + γ = 0.209. As a consequence, the iterative prediction model should have λ = 0.330 replaced by λ E = 0.209 on March 16 and onwards. As a side remark we note that this change of doubling time from 3.5 days to 9 days corresponds to changing R 0 = 2.5 to R E = 1 + (ln(2)/d E )/γ = 1.58 giving the magnitude of preventive effects of ρ = 1 − R E /R 0 = 0.37 so a 37% overall reduction in contact rates. Since the estimate d E = 9days of the new doubling time after preventive measures are put in place is highly uncertain we do similar calculations assuming the new doubling time instead equals 6 and 14 days respectively, and also for the situation where the preventive measures reduces R E to 0.8 implying that it directly starts decaying.