In the current epidemiological situation of COVID-19 in countries such as Italy, quantifying the contribution of asymptomatic infections to SARS-CoV-2 transmission is of crucial importance for pandemic control. We conducted a retrospective epidemiological study to characterize asymptomatic COVID-19 cases occurred in the Apulia region, Italy, during the first epidemic wave of COVID-19 outbreak.
We analyzed data collected in a regional surveillance platform developed to manage the emergency through investigation and follow-up of cases and contacts, contact tracing, laboratory and clinical data collection. We included asymptomatic laboratory-confirmed cases defined as persons infected with SARS-CoV-2 who did not develop symptoms/clinical signs of the disease.
Between February 29 and July 7, 2020, a total of 4,536 cases were diagnosed with COVID-19 among 193.757 tests performed. The group of persons with asymptomatic SARS-CoV-2 infection consisted of 903 cases; the asymptomatic proportion was 19.9% (95%CI: 18.8-21.1%); this decreased with increasing age (OR: 0.89, 95%CI: 0.83-0.96; p=0.001), in individuals with underlying comorbidities (OR: 0.55, 95%CI: 0.41-0.73; p<0.001), and males (OR: 0.69, 95%CI: 0.54-0.87; p=0.002). The median asymptomatic infectious period was 19 days (IQR: 14-31) and the cumulative proportion of persons with resolution of infection 14 days after the first positive PCR test was 74%.
In Europe and globally, there is again an increase in the number of new cases, mainly asymptomatic. As the public health community debates the question of whether asymptomatic and late spreaders could sustain virus transmission in the communities, such cases present unique opportunities to gain insight into SARS-CoV-2 adaptation to human host.