This study of coronavirus infections that covered almost everyone in the quarantined north Italian town of Vò found that 40% of cases showed no symptoms – suggesting that asymptomatic cases are important in the spread of the pandemic.
On February 21, 2020 a resident of Vo’ died of pneumonia due to SARS-CoV-2 infection, leading to a lockdown of the whole municipality for 14 days. During this time, the authors collected information on the demography, clinical presentation, hospitalization, contact network and 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.
The first survey, which was conducted around the time the town lockdown started, revealed a prevalence of infection of 2.6% (95% confidence interval (CI) 2.1-3.3%); the prevalence was 1.2% (CI: 0.8-1.8%) on the second survey, which was conducted at the end of the lockdown.
Notably, 42.5% (CI 31.5-54.6%) of confirmed infections 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). In addition, the authors found no statistically significant difference in the viral load of symptomatic versus asymptomatic infections.