Inferring the effective start dates of non-pharmaceutical interventions during COVID-19 outbreaks

Kohanovski, I.; Obolski, U.; Ram, Y.

During February and March 2020, several countries implemented non-pharmaceutical interventions, such as school closures and lockdowns, with variable schedules, to control the COVID-19 pandemic caused by the SARS-CoV-2 virus. Overall, these interventions seem to have successfully reduced the spread of the pandemic. We hypothesise that the official and effective start date of such interventions can significantly differ, for example due to slow adoption by the population, or due to unpreparedness of the authorities and the public. We fit an SEIR model to case data from 12 countries to infer the effective start dates of interventions and contrast them with the official dates. We find both late and early effects of interventions. For example, Italy implemented a nationwide lockdown on Mar 11, but we infer the effective date on Mar 16 ({+/-}0.47 days 95% CI). In contrast, Spain announced a lockdown on Mar 14, but we infer an effective start date on Mar 8 ({+/-}1.08 days 95% CI). We discuss potential causes and consequences of our results.