key: cord-0961003-zag5f4hq authors: Blanco-Suárez, Ana; Pérez-Jové, P.; Escribano-Castillejo, Nerea; Ballestero-Tellez, Mónica title: Retrospective search of SARS-CoV-2 in respiratory samples in Vallès Occidental (Barcelona, Spain) before the first case was reported date: 2020-06-11 journal: Enferm Infecc Microbiol Clin DOI: 10.1016/j.eimc.2020.05.019 sha: 5ced993e0635e1c340352f08f3de29ac3d074a59 doc_id: 961003 cord_uid: zag5f4hq nan Vallès Occidental region is an area near Barcelona (Spain) with 925,237 inhabitants in 2019 (12% of Catalonia's total population) (6) . It is estimated that there are 580,000 daily trips from and to Barcelona, 43% of which are made on public transport. The first case in this area was reported on February 25 th . According to a Spanish phylogenetic study of SARS-CoV-2, there are two clusters that originated in Spain around February 14 th -18 th 2020 and generated local transmission in the country (7). That is, 10 days before the first community transmitted case was detected in Spain. Regarding these data we hypothesize that SARS-CoV-2 might have been circulating in Vallès Occidental before the first case was detected. A retrospective study was performed re-analyzing -80°C frozen nucleic acid extract from In early March, Spain suffered an explosion of COVID-19 that lead to a national lockdown, but it is believed that the virus was already present in the country days before the first case was confirmed (7) . Moreover, the virus has been detected in retrospective wastewater analysis of samples dated before the first case was declared in some regions of Spain (8), but similar studies have not been performed yet in our geographic area. Similar findings have been reported in other countries (9) and it would be of great interest to perform this analysis widely. In a Chinese study similar to ours, it was described that SARS-CoV-2 virus was undetectably circulating camouflaged among flu cases (10) . In our case, the virus was not detected in any of the tested samples. This may be related to different possibilities, such as patients showing mild symptoms that did not require medical attention, patients having medical care in another institution or an explosive spreading scenario starting with a very low number of cases and a sample size not big enough. To our knowledge this is the first study of these characteristics in Spain. The fact that an area with such proximity to a big focus of transmission and such a high number of displacements per day may have had no cases before the first confirmed detection may help to understand the dynamics of SARS-CoV-2. We encourage our colleagues from other institutions to perform similar researchs. A new coronavirus associated with human respiratory disease in China First cases of coronavirus disease 2019 (COVID-19) in France: surveillance, investigations and control measures Calidad e Innovación. Actualización nº 30. Enfermedad por el coronavirus (COVID-19). Madrid: Ministerio de Sanidad China/documentos/Actualizacion_30_COVID-19_China.pdf [Accessed 1st Una italiana resident a Barcelona, primer cas per coronavirus a Catalunya Dirección general de Salud Pública Madrid: Ministerio de Sanidad El municipio en cifras. Vallès Occidental RNA titers in wastewater anticipated COVID-19 occurrence in a low prevalence area Presence of SARS Coronavirus-2 in sewage SARS-CoV-2 detection in patients with influenza-like illness