On 21 February 2020, a resident of the municipality of Vo’, a small town near Padua (Italy), died of pneumonia due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection1. This was the first coronavirus disease 19 (COVID-19)-related death detected in Italy since the detection of SARS-CoV-2 in the Chinese city of Wuhan, Hubei province2. In response, the regional authorities imposed the lockdown of the whole municipality for 14 days3. Here we collected information on the demography, clinical presentation, hospitalization, contact network and the presence of SARS-CoV-2 infection in nasopharyngeal swabs for 85.9% and 71.5% of the population of Vo’ at two consecutive time points. From the first survey, which was conducted around the time the town lockdown started, we found a prevalence of infection of 2.6% (95% confidence interval (CI): 2.1–3.3%). From the second survey, which was conducted at the end of the lockdown, we found a prevalence of 1.2% (95% CI: 0.8–1.8%). Notably, 42.5% (95% CI: 31.5–54.6%) of the confirmed SARS-CoV-2 infections detected across the two surveys were asymptomatic (that is, did not have symptoms at the time of swab testing and did not develop symptoms afterwards). The mean serial interval was 7.2 days (95% CI: 5.9–9.6). We found no statistically significant difference in the viral load of symptomatic versus asymptomatic infections (P = 0.62 and 0.74 for E and RdRp genes, respectively, exact Wilcoxon–Mann–Whitney test). This study sheds light on the frequency of asymptomatic SARS-CoV-2 infection, their infectivity (as measured by the viral load) and provides insights into its transmission dynamics and the efficacy of the implemented control measures.
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Acknowledgements We thank the Mayor of the municipality of Vo’, G. Martini, for his unreserved support throughout the study; a special thanks to the population of Vo’ who volunteered en masse to this study; M. Perilli and S. Guglielmo for assistance in data collection and consistency check; and F. Bosa and G. Rupolo from the Italian Red Cross for the support in patient samplings. This work was supported by the Veneto region and was jointly funded by the UK Medical Research Council (MRC; grant MR/R015600/1), the UK Department for International Development (DFID) under the MRC/DFID Concordat agreement, the Abdul Latif Jameel Foundation and is also part of the EDCTP2 programme supported by the European Union. I.D. acknowledges research funding from a Sir Henry Dale Fellowship funded by the Royal Society and Wellcome Trust (grant 213494/Z/18/Z). C.C. acknowledges funding from the Wellcome Trust (grant 203851/Z/16/Z). L.C.O. from the Imperial College COVID-19 Response Team and G.C.-D. acknowledge research funding from The Royal Society. L.B., E.L. and S.T. acknowledge research funding from the European Union's Horizon 2020 research and innovation programme, under grant agreement no. 874735 (VEO).
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