Seasonal variations and time trends of deaths from COVID-19 in Italy, September 2021 - September 2024: a segmented linear regression study

Using a segmented linear regression model, we examined the seasonal profiles of weekly COVID-19 deaths data, in Italy, over a three-year long period during which the SARS-CoV-2 Omicron and post-Omicron variants were predominant (September 2021 - September 2024). Comparing the slopes of the regression segments, we were able to discuss the variation in steepness of the Italian COVID-19 mortality trend, identifying the corresponding growth/decline profile for each considered season. Our findings show that, although the COVID-19 weekly death mortality has been in a declining trend in Italy since the end of 2021 until the end of 2024, there have been increasing alterations of the COVID-19 deaths for all winters and summers of that period. These increasing mortality variations were more pronounced in winters than in summers, with an average progressive increase of the number of COVID-19 deaths, with each new week, of 55.75 and 22.90, in winters and in summers, respectively. We found that COVID-19 deaths were, instead, less frequent in the intermediate periods between winters and summers, with an average decrease of -38.01 COVID-19 deaths for each new week. Our segmented regression model has fit well the observed COVID-19 deaths, as confirmed by the average value of the determination coefficients: 0.74, 0.63 and 0.70, respectively for winters, summers and intermediate periods, in a scale from 0 to 1. In conclusion, favored by a general declining COVID-19 mortality trend in Italy in the period of interest, transient rises of the mortality have occurred both in winters and in summers, but received little attention. The reason why these increasing alterations of the COVID-19 seasonal deaths have been considered cause for concern only occasionally is that they have been always compensated by consistent downward drifts occurring during the intermediate periods between winters and summers.