Epidemiological Risk in Italy Increases During the Cold Season and Heatwaves: Considerations for Health Policies During COVID-19 and Future Crises.

Abstract

Background. The seasonality of COVID-19 is a debated topic within the scientific community. The goal of this paper is to bring SARS-CoV-2 into the factual context in Italy, investigating the interaction with seasonal factors that can aggravate the health crisis.Methods. The study is longitudinal retrospective. The relationship between the number of deaths in the period 2015-2019 and the average monthly temperatures was investigated. The excess deaths and confirmed deaths from COVID-19 in 2020, 2021, and 2022 were examined to estimate the impact of the COVID-19 crisis and its relationships with temperatures.Results. Cold months lead to a considerable and surprising increase in epidemiological risk and mortality in Italy from 2015 to 2019 (+45,000 annual deaths, SD = 4,700, S = 21). COVID-19 crisis has further aggravated this scenario during 2020 (+115,000) and 2021 (+63,000, S > 52). Mortality was boosted by low average minimum temperatures, although the death curve rose moderately during the four warmest months (Spearman r = -0.75, 95% CI = [-0.87; -0.56], S = 23). COVID-19 deaths also showed a pronounced seasonality, although the latter was decreasing over time (Spearman r = -0.85, 95% CI = [-0.92; -0.70], S = 20). Monthly excess deaths during COVID-19 were extremely high and surprising (+4,200, IQR = [2,800; 8,000], Wilcoxon signed rank test S = 28) but didn’t show a clear seasonality during 2021 and 2022. Overall COVID-19 mortality was strongly and surprisingly correlated with regional latitude (Spearman r = 0.86, 95% CI = [0.68; 0.94], S = 20). Discrepancies between COVID-19 and excess deaths during 2021 and 2022 suggest the occurrence of seasonal estimation errors.Discussion and conclusion. Based on these findings, it can be concluded that: i) the epidemiological risk is seasonal since cold seasons lead to higher mortality, ii) COVID-19's impact on public health is strongly influenced by both environmental/seasonal and virological factors, iii) temperatures' increase due to climate change is able to create summer mortality peaks. Future research should investigate the interrelation between all these epidemiological variables.