key: cord-0793262-exl36jt3
authors: Rivera, Frida; Safdar, Nasia; Ledeboer, Nathan; Schaack, Grace; Chen, Derrick J; Munoz-Price, L Silvia
title: Prevalence of SARS-CoV-2 asymptomatic infections in two large academic health systems in Wisconsin
date: 2020-08-19
journal: Clin Infect Dis
DOI: 10.1093/cid/ciaa1225
sha: 519aa8ec75452f449e7b4ae7f38140330f2c8e19
doc_id: 793262
cord_uid: exl36jt3

SARS-CoV-2 asymptomatic infections may play a critical role in disease transmission. We aim to determine the prevalence of asymptomatic SARS-CoV-2 infection at two hospital systems in two counties in Wisconsin. The SARS-CoV-2 prevalence was 1% or lower at both systems despite the higher incidence of COVID-19 in Milwaukee county

The coronavirus disease 2019 (COVID-19) pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for more than two million COVID-19 cases in the United States. [1] As of June 23, 2020, Dane and Milwaukee counties in Wisconsin had reported a COVID-19 rate of 215 and 1,085.2 cases per 100,000 people, respectively. [2] Some studies suggest that asymptomatic infections could be responsible for amplifying the spread of the disease. [3] However, data regarding the prevalence of asymptomatic infections and their impact on transmission are still scarce. This study aims to determine the prevalence of asymptomatic SARS-CoV-2 infection at two hospital systems in two counties with markedly different rates of COVID-19.

This study was performed at Froedtert Health (879-bed system, Milwaukee, WI) and University of 

A c c e p t e d M a n u s c r i p t 4 Combined nasopharyngeal/oropharyngeal swab specimens or nasopharyngeal specimens only were collected at Froedtert Health and UW Health, respectively, according to standard institutional procedures. A nasopharyngeal swab was inserted into the nose until it reached the nasopharynx, rotated for 10 seconds, and removed. The oropharyrngeal swab was collected by swabbing the poster pharynx and tonsillar areas; the oropharyngeal swab was combined with the nasopharyngeal swab into a single tube of transport media and sent to the laboratory. Specimens were promptly tested using either the ThermoFisher TaqPath SARS-CoV-2 Assay or the Roche Cobas 6800 SARS- Demographic characteristics were retrospectively assessed from electronic medical records. An asymptomatic infection was defined as the presence of a positive SARS-CoV-2 test in the absence of any of the following symptoms: fever or chills, cough, shortness of breath or difficulty breathing, fatigue, muscle or body aches, headache, new loss of taste or smell, sore throat, congestion or runny nose, nausea or vomiting, or diarrhea. This study was approved by the institutional review boards at both institutions.

From April 6, 2020 to June 04, 2020, a total of 11,654 asymptomatic patients were tested for SARS-CoV-2, and 61 (0.52%) were positive [Froedtert Health, 38; UW Health, 23]. The percentage of positivity did not appear to increase over time. Among the total cohort, the median age was 42, IQR 37 and 27 (42%) were men. Demographic characteristics by hospital are shown in Table 1 Patients were followed-up 14 days after testing and seven patients (11.5%) out of 61 originally deemed to be asymptomatic infections developed symptoms consistent with COVID-19.

During the study period, we observed a low prevalence of asymptomatic SARS-CoV-2 infections in these two academic health systems in South Wisconsin. Even though both sites had a prevalence of 1% or less, Froedtert had higher prevalence of asymptomatic infections (1% vs 0.3%; p<0.001). This low prevalence of asymptomatic infections has been recently reported in other areas with high COVID-19 rates, such as Boston and Philadelphia [4, 5] ; however, these two studies included pregnant women and children. In contrast, two hospitals in New York City reported a prevalence of SARS-CoV-2 asymptomatic infections of 14% among women admitted for delivery. [6] Our results suggest that the prevalence of asymptomatic infections in individuals undergoing preadmission or preoperative screening may not be predictable based on the incidence of COVID-19 in the region. These data could bring into question the need to screen all asymptomatic patients in certain regions; however, in addition to guiding isolation precautions, these screening tests also serve as sentinel data for changes in the epidemiology of the virus. Our conclusions are hindered due to possible selection bias of the population surveyed. Additionally, rather than asymptomatic infections, these positive results could represent patients who previously experienced COVID-19 and are now long-term shedders.

Further studies are needed to determine the interplay between the prevalence of asymptomatic infections and COVID-19 in a region.

A c c e p t e d M a n u s c r i p t 6

No funding sources were used for this publication. N.S. reports grants from NIH.

All no conflicts of interests related to this publication.

A c c e p t e d M a n u s c r i p t M a n u s c r i p t 8 Table 1 . Demographic characteristics of SARS-CoV-2 asymptomatic infected patients stratified by health system. 

Wisconsin Department of Health Services. COVID-19: County Data

Household Transmission of SARS-CoV-2

Universal SARS-CoV-2 testing on admission to the labor and delivery unit: Low prevalence among asymptomatic obstetric patients

Incidence of COVID-19 in Pediatric Surgical Patients Among 3 US Children's Hospitals

Universal Screening for SARS-CoV-2 in Women Admitted for Delivery