Asymptomatic transmission of SARS-CoV-2 in young Marine Corps recruits

1. Shared rooms and platoon membership within young Marine Corps recruits were risk factors for transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.

2. Most Marine Corps recruits were asymptomatic when they tested positive for the infection.

Evidence Rating Level: 1 (Excellent)

Study Rundown: The U.S. Department of Defense implements public health interventions to reduce the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), specifically in young adults. However, specific military challenges such as confined living spaces, shared dining facilities, and congregating people from across the country increase the risk of contracting coronavirus disease 2019 (Covid-19). Furthermore, minimal research has been conducted on the transmission of SARS-CoV-2 in military settings. As such, this study evaluated the effectiveness of public health measures to control SARS-CoV-2 infection in young military recruits. The study results determined transmission of the virus occurred in shared platoons, while most recruits were asymptomatic at the time of their positive laboratory test and diagnosis. This prospective cohort study was limited by the inability to determine the infection rate during the supervised quarantine period. Without the initial infection rate, a baseline was not established for subsequent comparison of infection rates throughout the study. Nonetheless, the study determined the epidemiological factors associated with SARS-CoV-2 transmission in a military setting.

Click to read the study in NEJM

Relevant Reading: Rapid asymptomatic transmission of COVID-19 during the incubation period demonstrating strong infectivity in a cluster of youngsters aged 16-23 years outside Wuhan and characteristics of young patients with COVID-19: a prospective contact-tracing study

In-Depth [prospective cohort]: This prospective cohort study enrolled 1,848 recruits in the COVID-19 Health Action Response for Marines (CHARM) study at the Marine Corps Recruit Depot in Parris Island, South Carolina. Participants included in the study were ≥18 years of age and available for follow-up visits. Participants declining to participate in the study were excluded from this study. Prior to arrival, recruits quarantined at home for two weeks, and another 14-day supervised quarantine was instituted upon arrival to campus. All recruits were required to have negative quantitative polymerase chain reaction (qPCR) test prior to entering Parris Island. On the island, recruits were assigned to a platoon of 50 to 60 members and roommates independent of study participation. The study consisted of participating recruits answering a questionnaire along with obtaining blood and mid-turbinate nares swab samples. On day 7 and 14, the participants were followed-up to report any symptoms that occurred within the last seven days. Also, mid-turbinate nares swab samples were obtained at follow-up visits. At the time of enrollment, 16 of 1847 participants (0.9%) tested positive for SARS-CoV-2. The 16 participants followed appropriate quarantine protocols prior to arrival and were asymptomatic. At day 7, a total of 24 of the remaining negatively tested 1801 participants (1.3%) had a positive qPCR test for SARS-CoV-2. At day 14, a total of 11 of 1760 participants (0.6%) tested positive for the virus. Altogether, 51 participants had at least one positive qPCR test with 35 of these participants having a negative qPCR result within the first two days after arrival. Furthermore, complete viral genomes from 36 specimens were obtained from 32 of 51 participants that tested positive. Phylogenetic analyses of the genomes determined six independent monophyletic clusters, and 14 participants shared platoon assignments with other members who were in the same the cluster. Additionally, 12 infected participants had a room assignment in the same hallway with other members who were in the same cluster. Taken together, the spread of SARS-CoV-2 was attributed to local transmission through shared rooms and platoon membership in U.S. Marine Corps recruits, and most participants who tested positive were asymptomatic.

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