The end of the social confinement in Spain and the COVID-19 re-emergence risk Leonardo L ´ opez 1,*,+ and Xavier Rod ´ o 1,2,*,+ 1 Climate and Health Program, Barcelona Institute for Global Health 2 ICREA * leonardorafael.lopez@isglobal.org * xavier.rodo@isglobal.org + these authors contributed equally to this work ABSTRACT After the spread of SARS-CoV-2 epidemic out of China, the world approaches the 2 million declared infected cases and death toll rises well above the 100 thousand. The course of pandemic evolution has shown great differences among countries and not much is yet known about the level of generated immunity, which might appear not to be long-lasting. In this situation, management of a recurrent disease seems to be a plausible scenario that countries worldwide will have to face, before effective drugs or a vaccine appear. Spain in Europe, appears to be the first country deciding to partly lift the strict social distancing regulations imposed. Whether this action may lead to further epidemic recrudescence, to a following second wave of cases or conversely, help return to previous normality, is a subject of great debate and interest to all other countries affected by COVID-19. Here we applied a modified SEIR compartmental model accounting for the spread of infection during the latent period, in which we had also incorporated effects of social confinement. We now modify this previous model configuration to mimic potential post-confinement scenarios, by simulating from instant massive liberation of different portions of the confined population, up to a more gradual incorporation of people to work. Results show how current lockdown conditions should be extended at least two weeks more to prevent a new escalation in cases and deaths, as well as a larger second wave occurring in just a few months. Conversely, best-case scenario in terms of lower COVID-19 incidence and casualties should gradually incorporate workers back in a daily proportion at most 30 percent higher than that of previous confinement. The former should begin not earlier than by the end of April and it would represent approximately 600 thousand people or a 3.75% rate for the whole of Spain. Introduction COVID-19 pandemic has entered a new stage in which the United States appears to be at the epicenter and Europe seems to have passed the peak of SARS-CoV-2 outbreak 1 . In this situation and whereas Italy decided not to yet lift lockdown measures, Spain after a large outbreak of still tens of thousand active cases, already plans for immediate deconfinement actions. Data on the last similar scale pandemic more than one century ago remains limited, despite aftermath reports exist 2 . The fact that the 1918 Spanish flu pandemic had a second deadly epidemic wave, presumably caused by strain mutations 3 , has also stimulated a vivid debate on whether actions to take now should or not incorporate this uncertain future. Under this scenario, optimal interventions on the COVID-19 pandemic aiming at a relaxation of strict social distancing enforcement are hard to fathom. More so, when the extent of pre-symptomatic and asymptomatic infections is not narrowly constrained (e.g. may be up to 86 percent 4 ). Europe’s worst hit countries, Italy and Spain, are now seeing a flattening of their respective death tolls. On March 29, the Government of Spain published a decree that paralyzed all sectors considered non-essential in a draconian lockdown 5, 6 . The objective was to take advantage of Easter holidays to transfer the low level of mobility from Saturdays and Sundays to the rest of the days of the week, thereby reducing new infections and thus giving hospitals and ICUs a break when on the brink of collapse. Under an impending economic crisis, the Spanish executive government now believes it is time for the economy to recover a certain dynamism 7 . However, whether soon or later the return to normality should be attempted, the decision of when and how best decide to optimally move in that direction is at the center of worldwide debate 8 . This decision unfortunately presents a double problem in which two factors are related. First, there is a risk that the return to activity will end up causing a spike in infections that will aggravate the sanitary collapse at a time when the situation is still delicate. Second, there is no way to assess this risk, given the lack of existing information on the actual number of people infected or the real extent of immunity developed among the population. In both Spain and Italy active infections are still in the thousands, and a lift of lockdown makes the risk of a flare up in cases again a serious threat. According to the WHO, it is premature for Europe to think about a lack of confinement, given the number of active cases and waiting longer would be more prudent 9 . In the UK, for example, All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted April 17, 2020. . https://doi.org/10.1101/2020.04.14.20064766 doi: medRxiv preprint