Kifer, DomagojVillar García, JuditPosso, MargaritaHorcajada Gallego, Juan PabloCastells, XavierPascual, Julio (Pascual Santos)Barrios Barrera, ClaraLauc, Gordan2021-06-162021-06-162021Kifer D, Bugada D, Villar-Garcia J, Gudelj I, Menni C, Sudre C et al. Effects of environmental factors on severity and mortality of COVID-19. Front Med (Lausanne). 2021; 7:607786. DOI: 10.3389/fmed.2020.6077862296-858Xhttp://hdl.handle.net/10230/47896Background: Most respiratory viruses show pronounced seasonality, but for SARS-CoV-2, this still needs to be documented. Methods: We examined the disease progression of COVID-19 in 6,914 patients admitted to hospitals in Europe and China. In addition, we evaluated progress of disease symptoms in 37,187 individuals reporting symptoms into the COVID Symptom Study application. Findings: Meta-analysis of the mortality risk in seven European hospitals estimated odds ratios per 1-day increase in the admission date to be 0.981 (0.973-0.988, p < 0.001) and per increase in ambient temperature of 1°C to be 0.854 (0.773-0.944, p = 0.007). Statistically significant decreases of comparable magnitude in median hospital stay, probability of transfer to the intensive care unit, and need for mechanical ventilation were also observed in most, but not all hospitals. The analysis of individually reported symptoms of 37,187 individuals in the UK also showed the decrease in symptom duration and disease severity with time. Interpretation: Severity of COVID-19 in Europe decreased significantly between March and May and the seasonality of COVID-19 is the most likely explanation.application/pdfeng© 2021 Kifer, Bugada, Villar-Garcia, Gudelj, Menni, Sudre, Vučković, Ugrina, Lorini, Posso, Bettinelli, Ughi, Maloberti, Epis, Giannattasio, Rossetti, Kalogjera, Peršec, Ollivere, Ollivere, Yan, Cai, Aithal, Steves, Kantele, Kajova, Vapalahti, Sajantila, Wojtowicz, Wierzba, Krol, Zaczynski, Zycinska, Postula, Lukšić, Čivljak, Markotić, Brachmann, Markl, Mahnkopf, Murray, Ourselin, Valdes, Horcajada, Castells, Pascual, Allegri, Primorac, Spector, Barrios and Lauc. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.info:eu-repo/semantics/articlehttp://dx.doi.org/10.3389/fmed.2020.607786COVID-19HumidityMortalityMucinsSeasonalityinfo:eu-repo/semantics/openAccess