key: cord-313099-rpdlk1b6
authors: Han, Xiaoyu; Wei, Xiong; Alwalid, Osamah; Cao, Yukun; Li, Yumin; Wang, Li; Shi, Heshui
title: Severe Acute Respiratory Syndrome Coronavirus 2 among Asymptomatic Workers Screened for Work Resumption, China
date: 2020-09-17
journal: Emerg Infect Dis
DOI: 10.3201/eid2609.201848
sha: 
doc_id: 313099
cord_uid: rpdlk1b6

After the outbreak in Wuhan, China, we assessed 29,299 workers screened for severe acute respiratory syndrome coronavirus 2 by reverse transcription PCR. We noted 18 (0.061%) cases of asymptomatic infection; 13 turned negative within 8.0 days, and 41 close contacts tested negative. Among 6 contacts who had serologic tests, none were positive.

A s the population of Wuhan, China, returns to work, asymptomatic cases of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are being discovered among workers receiving health checkups for work resumption. Previous studies have shown that asymptomatic cases can be a public health threat and might lead to another outbreak (1, 2) . However, little is known about the clinical characteristics of asymptomatic infections. We report on cases of asymptomatic SARS-CoV-2 infection among persons during work resumption screening in Wuhan.

At Wuhan Pingan Healthcare Diagnostic Center, we reviewed 29,299 asymptomatic persons who were screened for SARS-CoV-2 by reverse transcription PCR (RT-PCR) and 22,633 asymptomatic persons tested for SARS-CoV-2 antibodies during March 13-April 25, 2020. Throat swab specimens were tested for SARS-CoV-2 by using Real-Time Fluorescent-PCR Kits (DAAN GENE Co., LTD, https://www. en.daangene.com; Appendix, https://wwwnc.cdc. gov/EID/article/28/9/20-1848-App1.pdf). We used colloidal gold-based immunochromatographic strip assay, Novel Coronavirus (SARS-CoV-2) IgM/IgG Antibody Detection Assay (Vazyme Biotech Co. Ltd., http://vazyme.bioon.com.cn) to perform antibody testing (Appendix). We recorded the demographic features, exposure history, RT-PCR and serology results, and imaging reports at the time of testing. We obtained follow-up data from persons screened by telephone.

Among 29,299 persons screened by RT-PCR, we confirmed 18 (0.061%) cases of SARS-CoV-2 infection.

Of 22,633 persons tested for SARS-CoV-2 antibodies, 617 (2.7%) cases had positive IgG but negative IgM; 196 (0.87%) cases had positive IgG and IgM; and 40 (0.18%) cases had negative IgG but positive IgM.

The median age of 18 asymptomatic case-patients (10 male, 8 female) was 30.5 years (Table) . Six (33.3%) cases had clear contact history with a confirmed case of SARS-CoV-2 infection. The median cycle threshold (C t ) values on the day of first positive RT-PCR were 38.2 (C t range 37.2-39.3) for ORFa1b gene and 38.1 (C t range 36.81-38.5) for N gene (Table) . All antibody tests were obtained on the day of first positive RT-PCR except in 1 case (obtained 6 days later). Half (7/14) the cases had negative IgM and IgG; the other half had positive IgG but negative IgM results (Table) . Among 8 case-patients who had computed tomography imaging of the chest, none had remarkable findings. We closely observed the cases for 3-41 (median 16.5) days; 13 cases had negative RT-PCR within a median of 8 (range 3-14) days (Table) , and none had symptoms. Among 41 close contacts, all had 2 consecutive negative RT-PCR tests >24 hours apart. (3), 275,400 RT-PCR tests were performed for universal screening during April 9-15, 2020. Among those, 182 (0.066%) asymptomatic persons were identified as SARS-Cov-2-positive, which is consistent with our study. Half the cases in our study showed negative IgM and IgG at the time of positive RT-PCR, suggesting recent infections (<14 days). Seven (50%) cases in our study had positive IgG but negative IgM, indicating a late stage infection, 4 of which had a long interval of exposure (30-75 days). In addition, 13 cases had negative RT-PCR assays <8 (range 3-14) days, suggesting a favorable prognosis for persons with asymptomatic infections.

Epidemiologic, virologic, and modeling evidence support the possibility of SARS-CoV-2 transmission from persons who are presymptomatic (SARS-CoV-2 detected before symptoms onset) or asymptomatic (never develop symptoms) (4). None of the 18 asymptomatic persons in our study developed symptoms.

CoV-2 infections in persons with no or mild symptoms was similar to the viral load of symptomatic patients (5, 6) , which could contribute to rapid transmissions (5) . However, other studies demonstrated that asymptomatic patients had a lower viral load than symptomatic and presymptomatic patients (7, 8, 9) , which might indicate less transmissibility from asymptomatic persons. The median cycle threshold values for the 18 cases were 38.2 for ORFa1b gene and 38.1 for the N gene, indicating a relatively low viral load. In addition, all 41 close contacts of the asymptomatic case-patients tested negative by RT-PCR. Possible explanations for this finding include that: the asymptomatic infected persons had relatively low viral load and were less infectious; that asymptomatic persons did not have clinical symptoms, such as sneezing or coughing, that could cause virus spread; and that, due to the strict isolation and preventive measures taken in Wuhan for >3 months, the population was generally protected from the spread of infection by mask-wearing and self-quarantine. Our report has limitations. Our sample size of asymptomatic cases is small, and follow-up was short. Recall bias of exposure history is another limitation; in the absence of clear symptom onset, asymptomatic persons might be less likely to accurately recall exposures than persons with symptoms. Finally, that the study took a place during the post-peak period of the epidemic in Wuhan, so contacts could have been seropositive already; those tested were seronegative, but most contacts did not have serologic testing.

In conclusion, as the population returns to the workplace, asymptomatic SARS-CoV-2-infected persons could be among workers. Although we did not detect transmission among 41 contacts of persons who were SARS-CoV-2-positive, such transmission cannot be excluded. Therefore, continued testing, self-quarantine, and mask-wearing should be encouraged to reduce the risk for additional outbreaks.

Covert coronavirus infections could be seeding new outbreaks

Association of public health interventions with the epidemiology of the COVID-19 outbreak in Wuhan, China

Wuhan Municipal Health Commission. CN-HEALTHCARE: Hubei health

Evidence supporting transmission of severe acute respiratory syndrome coronavirus 2 while presymptomatic or asymptomatic. Emerg Infect Dis

Presymptomatic SARS-CoV-2 infections and transmission in a skilled nursing facility

SARS-CoV-2 viral load in upper respiratory specimens of infected patients

Viral kinetics of SARS-CoV-2 in asymptomatic carriers and presymptomatic patients

The relative transmissibility of asymptomatic COVID-19 infections among close contacts

Viral dynamics in asymptomatic patients with COVID-19

We thank all our colleagues for helping us during the current study.This study was supported by Zhejiang University special scientific research fund for COVID-19 prevention and control, the Huazhong University of Science and Technology (HUST) COVID-19 Rapid Response Call (grant no. 2020kfyXGYJ019).

Dr. Han is a clinician in the Department of Radiology, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China. Her research interests including image diagnosis of pneumonia and lung cancer.