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Severe Acute Respiratory Syndrome Coronavirus 2 among Asymptomatic Workers Screened for Work Resumption, China

Our take —

This study screened 22,299 workers in China for SARS-CoV-2 infection, and found 18 active infections (0.06%). 22,633 were screened via antibody tests, and 853 (3.8%) had evidence of prior infection. In this setting and transmission context, healthy workers returning to work were rarely infected with SARS-CoV-2, thought this may not be generalizable to other settings.

Study design

Case Series; Cross-Sectional

Study population and setting

This study’s goal was to assess the rate of acute SARS-CoV-2 infection among a group of 29,299 asymptomatic workers and prior SARS-CoV-2 infection among 22,633 workers screened for antibodies while returning to work following lockdown from March 13 to April 25, 2020 in Wuhan, China. Sociodemographic information on sex, age, and chest imaging were also collected.

Summary of Main Findings

Eighteen of 29,299 workers had evidence of active SARS-CoV-2 infection (0.06%) by RT-PCR. Of the 22,633 workers screened for antibodies, 617 (2.7%) had detectable IgG but no IgM, 196 (0.87%) had detectable IgG and IgM, and 40 (0.18%) had no detectable IgG but did have IgM. Among 18 workers with viral RNA detected in swabs, all had high cycle threshold (Ct) values (median 38); 14 were also tested for antibodies, and 7 (50%) had IgG antibodies, suggesting they were the later stages of infection. Workers with detectable viral RNA by RT-PCR had variable follow-up ranging from 3 to 41 days, during which they were retested–13 no longer had detectable viral RNA within 8 days after their first positive test, suggesting they likely cleared the active viral infection. Of the 18 actively positive workers, 7 (38.9%) were reportedly office workers, while others worked in journalism (n=2), marketing (n=2), or were cleaners (n=2).

Study Strengths

This study screened a large sample of workers returning to work, and employed both PCR testing for active viral infection, as well as antibody testing to screen for prior infection, providing information on the proportion of healthy workers who might be expected to be infectious. It also collected occupational information for everyone who tested positive, and followed them through time to understand the duration of infection.


The study reported a very low prevalence in its overall population, which may be the result of testing issues such as false negatives. The small number of cases led to overall limitations in assessing the clinical characteristics of those asymptomatic carriers, a primary study objective. Workers were followed-up for variable lengths of time (ranging from 3 to 41 days), and not all participants were followed until they test negative, making it difficult to accurately estimate the duration of infection. The study also occurred during a return-to-work screening, and individuals who have not been at work or otherwise leaving the house may be less likely to be infected than had restrictions not been put in place; the findings from this study may not be generalizable to other populations with different levels of community transmission.

Value added

This is one of the largest universal screening tests of mostly healthy workers conducted, including both testing for active viral infection and antibodies indicating prior infection. The findings indicate that in this community context, asymptomatic workers were infected, but rarely so (18/29,299).

This review was posted on: 21 July 2020