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

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, hypothesizing the presence of several co-factors that may have protected against SARS-CoV-2 infection. Methods: Descriptive study of an isolated population cohort, including SARS-CoV-2 seroprevalence and viral prevalence in samples of saliva assessed through RT-qPCR. We conducted seroprevalence screening from April 29 to May 3, 2020 across the three main settlements on the island. We invited the adult resident population, present on the island to undergo testing by rapid serologic assay and to provide a sample of saliva, which was cryopreserved for molecular validation. We monitored the participation through the official municipality residents list. 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, 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 first descriptive study, using its population as a reference model, further investigations will be conducted to characterize the summer period exposure and to test the advanced hypotheses, focusing on the evaluation of a possible cross-reactivity to SARS-CoV-2 from exposure to endemic viruses.