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Suppression of a SARS-CoV-2 outbreak in the Italian municipality of Vo’

Our take —

The study shows that with a 14-day lockdown in the Italian city of Vo’, the prevalence of SARS-CoV-2 infection decreased from 2.6% to 1.2% in the general population. 42.5% of those that tested positive reported minimal or no symptoms consistent with COVID-19. These results suggest that screening for SARS-CoV-2 should be expanded beyond those that report symptoms, and are consistent with a large role for transmission by infected individuals who do not feel sick.

Study design

Prospective Cohort

Study population and setting

Vo’, a small municipality in Italy (population = 3,275), underwent a two-week lockdown following the death of a resident due to COVID-19 in February 2020. To assess the prevalence of SARS-CoV-2 infection in the general population, nasopharyngeal swabs were collected and community surveys of residents conducted at the start of the lockdown (February 21 to 29, 2020) and at the end of lockdown (March 7, 2020). Samples were run on RT-PCR. Surveys collected a range of information including symptoms, previous health conditions, size of households, and close contacts. Participation in the testing and survey was high, with 2,812 (85.9%) individuals sampled in the first survey and 2,343 (71.5%) in the second.

Summary of Main Findings

The prevalence of COVID-19 decreased from 2.6% (95%CI 2.1-3.3%) at the start of lockdown to 1.2% (95%CI 0.8-1.8%) at the end of lockdown. Based on the authors’ definition of symptomatic cases, 34 out of 80 (42.5%) of those that tested positive at either survey were asymptomatic. Reconstructed transmission chains showed potential infector and infectee pairs based on household contact data. Based on these pairs, the average time between symptom onset in the infector and infectee was 7.2 days. The viral load (quantity of virus in a given volume) did not significantly differ between asymptomatic and symptomatic cases.

Study Strengths

Testing for SARS-CoV-2 infection was conducted irrespective of clinical presentation or symptoms. The authors combined empirical data with mathematical modeling to reconstruct COVID-19 transmission chains and better understand transmission dynamics among households and other close contacts.

Limitations

The authors used an unusually narrow definition of symptomatic cases: “a participant who required hospitalization and/or reported fever (yes/no or a temperature above 37 degrees C) and/or cough and/or at least two of the following symptoms: sore throat, headache, diarrhoea, vomit, asthenia, muscle pain, joint pain, loss of taste or smell, or shortness of breath” (page 12). This definition implies that someone reporting one symptom, like loss of taste or sense of smell, would be classified as asymptomatic. This definition would likely inflate the number of “asymptomatic” cases. Furthermore, it is unclear how data on symptoms were collected (e.g., interviews vs. health records). It is unclear how presymptomatic cases were assessed in the second survey, given that data collection stopped after the second survey. Additionally the paper reports high participation rates for both surveys, but does not define who the eligible population is. Finally, viral load is assumed to but may not directly correspond to infectivity.

Value added

This study provides valuable information on COVID-19 positivity rates in the general population irrespective of symptom presentation.

This review was posted on: 10 August 2020