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

Objectives

To investigate the occurrence of seasonal variations of COVID-19 deaths in Italy over a period during which the SARS-CoV-2 Omicron and post-Omicron variants were predominant: September 2021 - September 2024.

Design

We examined the seasonal profiles of weekly COVID-19 deaths data, over a three-year long period, using a segmented linear regression model. Comparing the slopes of the regression segments, we are able to discuss the variation in steepness of the Italian COVID-19 mortality trend, identifying the corresponding growth/decline profile for each considered season.

Settings

Public data of weekly COVID-19 deaths over a three-year long period in Italy.

Participants

Only publicly available data were utilized for this study, patients and/or the public were not involved in any phase of this study.

Interventions

Not applicable, not being proposed any interventions.

Primary and secondary outcome measures

Not applicable, being this study not based on a protocol with preliminary planned measures.

Results

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, we have identified increasing variations of the COVID-19 deaths for all winters and summers of that period. These increasing mortality variations were more pronounced in winters (average slope coefficient of the regression segments: 55.75, average and SD values of the coefficient of determination r ² : 0.74 and 0.05) than in summers (average slope coefficient of the regression segments: 22.90, average and SD values of the coefficient of determination r ² : 0.63 and 0.19). We found that COVID-19 deaths were, instead, less frequent in the intermediate periods between winters and summers (average slope coefficient of the regression segments: −38.01, average and SD values of the coefficient of determination r ² : 0.70 and 0.06).

Conclusion

Favored by a declining COVID-19 mortality trend in Italy in the period from September 2021 to September 2024, the occurrence of increasing alterations (from serious to moderate) received little attention. These transient rises of the trend have been present both in winters and in summers, but more pronounced in winters. Since these increasing seasonal alterations of the COVID-19 mortality trend have been always compensated by consistent downward drifts occurring during the intermediate periods between winters and summers, they have been considered cause for concern, only occasionally.

Article Summary

Strengths and Limitations of this Study

• Using a segmented linear regression offers the advantage of a temporal decomposition of the time series of the COVID-19 deaths data, allowing to identify the seasonal variations of mortality over a relatively long period.

• Applying a segmented linear regression represents an appropriate method to differentiate between severe and moderate variations of the COVID-19 deaths trend, distinguishing seasonal alterations from minimal upward/downward drifts of the time series.

• We avoided to investigate the reasons behind the seasonal variations of the COVID-19 death time series, which could be the reflection of several causes, including: climatic and environmental factors, dense gathering of people in specific circumstances, decreasing immunity from previous infections and vaccinations.

• The time series of the weekly COVID-19 deaths data was not normalized or corrected and it comprised a relatively extended period characterized by the circulation of a long list of Omicron and post-Omicron subvariants, started in the high fall of 2021.

• The period covered by the data extends from the end of 2021 to the end of 2024, including three winters and three summers; however, there is reason to believe that the associations we found still exist.