The seasonality of Covid-19 in 2021 in the metropolitan region of Natal (RN), Brazil

Abstract

Introduction: Based on the well established existence of a seasonality for influenza and other respiratory diseases, this paper analyses the hypothesis of the existence of a seasonality for Covid-19 as well. Methodology: The study focuses on the Metropolitan Region of Natal and on the year 2021, when vaccination coverage has not yet reached large population contingents. It was used as dependent variable the Patients Admitted to Critical Hospitalisation that had as a source the database of the Epidemiological Surveillance System for Severe Acute Respiratory Syndromes (SIVEP - SARS) of the Ministry of Health.  Such variable behaves as a reliable indicator, given the accuracy of diagnosis, of the spread of the disease at a given time, since for severe hospitalizations are a small part of the total number of people who became ill in the period and it were counted by the date of onset of symptoms of hospitalized patients. The independent variables, climatological normals, (Insolation, Accumulated Rainfall, Atmospheric Pressure, Maximum Temperature and Humidity) came from the National Institute of Meteorology (INMET). Each climatological variable was associated with the dependent variable in search of statistical significance. The climatological variables were combined in all possible combinations from two to five variables, multiplied among themselves and associated with the variable that measured the Patients Admitted to Critical Hospitalisation to identify patterns of synergy between such variables that could point to increased seasonal risk for Covid-19. Results: Except for the variables Atmospheric Pressure and Maximum Temperature, all other variables showed strongly significant associations with the variation of Patients Admitted to Critical Hospitalisation throughout 2021. Among the variables created by the combination of the others, it was identified a Seasonal Risk Index (SRI) for now applicable only to the studied region. Such index, which grouped the variables Insolation, Accumulated Rainfall, Maximum Temperature and Air Humidity, showed a significant association with the variation of critical hospital admissions with multiple R of 0.865322 and p=0.000277. The validity of this index for other climatic regions will be the object of the next work of the authors.