The impact of insularity on SARS-CoV-2 diffusion: Recapitulating three years of COVID-19 pandemic in the island of Sardinia.

Background: Italy has been the first European Country dealing with SARS-CoV-2, whose diffusion on the territory has not been homogeneous. Among Italian regions, Sardinia represented one of the lowest incidence areas, likely due to its insular nature. Despite this, the impact of insularity on SARS-CoV-2 genetic diversity has not been comprehensively described.

Methods: In the present study, we performed the high throughput sequencing of 888 SARS-CoV-2 genomes collected in Sardinia during the first 23 months of pandemics. In addition, 1439 high-coverage SARS-CoV-2 genomes circulating in Sardinia along three years (December 2019 - January 2023) were downloaded from GISAID, for a total of 2327 viral sequences that were characterized in terms of phylogeny and genomic diversity.

Results: Overall, COVID-19 pandemic in Sardinia showed substantial differences with respect to the national panorama, with additional peaks of infections and uncommon lineages that reflects the national and regional policies of re-opening and the subsequent touristic arrivals. Sardinia has been interested by the circulation of at least 87 SARS-CoV-2 lineages, including some that were poorly represented at national and European level, likely linked to multiple importation events. The relative frequency of Sardinian SARS-CoV-2 lineages has been compared to other Mediterranean Islands, revealing a unique composition.

Conclusions: The genomic diversity of SARS-CoV-2 in Sardinia has been shaped by a complex interplay of insular geography, low population density, and touristic arrivals, leading on the one side to the importation of lineages remaining rare at the national level, and resulting on the other side in the delayed entry of otherwise common variants.