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Risk Factors Associated With COVID-19 Transmission Among US Air Force Trainees in a Congregate Setting

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To identify the key insights or developments described in this article
1 Credit CME
Key Points

Question  In congregate settings with the introduction of coronavirus disease 2019 (COVID-19), what clinical and laboratory findings are associated with an outbreak?

Findings  In this cohort study of 10 613 US Air Force basic trainees living congregately in 263 cohorts, 3% were diagnosed with COVID-19 infection. Cohorts with trainees with more symptoms and lower cycle threshold values on reverse transcription–polymerase chain reaction assay were significantly associated with greater risk of transmission of COVID-19 within their cohorts.

Meaning  In this study, a higher number of symptoms and lower cycle threshold values were associated with subsequent clusters of outbreaks within cohorts and may be useful as risk factor measures if validated in future studies.


Importance  Owing to concerns of coronavirus disease 2019 (COVID-19) outbreaks, many congregate settings are forced to close when cases are detected because there are few data on the risk of different markers of transmission within groups.

Objective  To determine whether symptoms and laboratory results on the first day of COVID-19 diagnosis are associated with development of a case cluster in a congregate setting.

Design, Setting, and Participants  This cohort study of trainees with COVID-19 from May 11 through August 24, 2020, was conducted at Joint Base San Antonio–Lackland, the primary site of entry for enlistment in the US Air Force. Symptoms and duration, known contacts, and cycle threshold for trainees diagnosed by reverse transcription–polymerase chain reaction were collected. A cycle threshold value represents the number of nucleic acid amplification cycles that occur before a specimen containing the target material generates a signal greater than the predetermined threshold that defines positivity. Cohorts with 5 or more individuals with COVID-19 infection were defined as clusters. Participants included 10 613 trainees divided into 263 parallel cohorts of 30 to 50 people arriving weekly for 7 weeks of training.

Exposures  All trainees were quarantined for 14 days on arrival. Testing was performed on arrival, on day 14, and anytime during training when indicated. Protective measures included universal masking, physical distancing, and rapid isolation of trainees with COVID-19.

Main Outcomes and Measures  Association between days of symptoms, specific symptoms, number of symptoms, or cycle threshold values of individuals diagnosed with COVID-19 via reverse transcription–polymerase chain reaction and subsequent transmission within cohorts.

Results  In this cohort study of 10 613 US Air Force basic trainees in 263 cohorts, 403 trainees (3%) received a diagnosis of COVID-19 in 129 cohorts (49%). Among trainees with COVID-19 infection, 318 (79%) were men, and the median (interquartile range [IQR]) age was 20 (19-23) years; 204 (51%) were symptomatic, and 199 (49%) were asymptomatic. Median (IQR) cycle threshold values were lower in symptomatic trainees compared with asymptomatic trainees (21.2 [18.4-27.60] vs 34.8 [29.3-37.4]; P < .001). Cohorts with clusters of individuals with COVID-19 infection were predominantly men (204 cohorts [89%] vs 114 cohorts [64%]; P < .001), had more symptomatic trainees (146 cohorts [64%] vs 53 cohorts [30%]; P < .001), and had more median (IQR) symptoms per patient (3 [2-5] vs 1 [1-2]; P < .001) compared with cohorts without clusters. Within cohorts, subsequent development of clusters of 5 or more individuals with COVID-19 infection compared with those that did not develop clusters was associated with cohorts that had more symptomatic trainees (31 of 58 trainees [53%] vs 43 of 151 trainees [28%]; P = .001) and lower median (IQR) cycle threshold values (22.3 [18.4-27.3] vs 35.3 [26.5-37.8]; P < .001).

Conclusions and Relevance  In this cohort study of US Air Force trainees living in a congregate setting during the COVID-19 pandemic, higher numbers of symptoms and lower cycle threshold values were associated with subsequent development of clusters of individuals with COVID-19 infection. These values may be useful if validated in future studies.

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Article Information

Accepted for Publication: January 7, 2021.

Published: February 25, 2021. doi:10.1001/jamanetworkopen.2021.0202

Correction: This article was corrected on March 17, 2021, to fix typographical errors.

Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2021 Marcus JE et al. JAMA Network Open.

Corresponding Author: Joseph E. Marcus, MD, Infectious Diseases Service, Brooke Army Medical Center, Infectious Disease Service (MCHE-ZDM-I), 3551 Roger Brooke Dr, JBSA Fort Sam Houston, TX 78234-4505 (

Author Contributions: Drs Marcus and Yun had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: Marcus, Frankel, Cybulski, Okulicz, Yun.

Acquisition, analysis, or interpretation of data: All authors.

Drafting of the manuscript: Marcus, Okulicz, Yun.

Critical revision of the manuscript for important intellectual content: All authors.

Statistical analysis: Marcus, Enriquez, Yun.

Administrative, technical, or material support: Marcus, Frankel, Pawlak, Casey, Cybulski, Okulicz.

Supervision: Frankel, Okulicz, Yun.

Conflict of Interest Disclosures: None reported.

Disclaimer: The statements and opinions expressed on this subject are those of the participants and not necessarily those of the US Air Force or the US government.

Additional Contributions: We appreciate the assistance of the basic military training trainees and instructors, the 737th Training Group, the 559th Medical Group, and the 59th Medical Wing.

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