The Conundrum of Giglio Island: unraveling the dynamics of an apparent resistance to COVID-19 – A descriptive study

Bognanni, Antonio; Schiaffino, Armando; Pimpinelli, Fulvia; Donzelli, Sara; Celesti, Ilaria; Strano, Sabrina; Solari, Elena; Schiaffino, Giorgia; Solari, Gabriele; Solari, Domenico; Delbue, Serena; Rigoni, Marta; Nollo, Giandomenico; Muti, Greta E.; Muti Schünemann, Giovanna E.U.; Schünemann, Holger J; Blandino, Giovanni; Morrone, Aldo; Muti, Paola

OBJECTIVES: Despite an extensive risk of exposure to COVID-19, the residents of Giglio Island, Italy, did not develop any symptom of SARS-CoV-2. The present study aims to characterize the nature of exposure and to describe the local population dynamics underlying its apparent resistance to COVID-19. METHODS: Descriptive study of an islander partially-segregated population cohort based on a seroprevalence screening conducted from Aprile 29 to May 3, 2020 and including SARS-CoV-2 seroprevalence and viral prevalence in samples of saliva assessed through RT-qPCR. Serologic testing was performed using a COVID-19 IgG/IgM rapid test while molecular analyses were carried out by Allplex 2019-nCoV Assay (Seegene). RESULTS: A total of 634 residents out of 748 (84.8%) present at the time, and 89 non-residents underwent serological testing. 364 males and 359 females with a median age of 58.5 years. The serological screening identified one positive, asymptomatic subject. The Nucleic Acid Amplification Tests (NAAT) did not yield any positive result. CONCLUSION: Despite extensive exposure to SARS-CoV-2, possibly only one new asymptomatic infection occurred in this population, as documented by IgM positivity not confirmed by RT-qPCR. This may be due to unknown protective factors or chance. On the basis of this baseline study, using its population as a reference model, further investigations will be conducted to test the advanced hypotheses, focusing on the evaluation of a possible cross-reactivity to SARS-CoV-2 from exposure to endemic viruses.