Investigating asymptomatic SARS-CoV-2 transmission among Marines

Investigating asymptomatic SARS-CoV-2 transmission among Marines

Written by Lori Uildriks, Pharm.D., BCPS, BCGP on November 17, 2020 — Fact checked by Catherine Carver, MPH

A recent study demonstrated the transmission of the virus that causes COVID-19 among asymptomatic Marine recruits, despite strictly enforced quarantines and other recommended health measures.


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The results of the study could have important implications for the design of health measures aiming to limit the transmission of the virus, SARS-CoV-2, among young adults.

SARS-CoV-2 transmits through the inhalation of aerosolized or liquid respiratory droplets from the mouth or nose. The virus can enter the mouth, nose, or eyes when people are in direct or close contact, for example.

Stay informed with live updates on the current COVID-19 outbreak and visit our coronavirus hub for more advice on prevention and treatment.

Transmission can also occur through contact with objects or surfaces used by someone with the virus.

A person with SARS-CoV-2 can pass on the virus whether or not they have symptoms of the disease that it causes, COVID-19. Currently, scientists do not know how frequently asymptomatic transmission occurs.

Military personnel may have a particularly high risk of COVID-19 due to shared dining halls, cramped living areas, close contact during training, and contact with people from varied geographical areas.

Scientists from the Naval Medical Research Center, headquartered in Silver Spring, MD, and the Icahn School of Medicine at Mount Sinai, in New York City, decided to evaluate the effectiveness of strategies aimed to reduce the transmission of SARS-CoV-2 in a military setting. Their findings appear in The New England Journal of Medicine.

Safety protocols

Before arriving at Parris Island, SC, for Marine Corps basic training, potential recruits quarantine at home for 14 days. Next, those deemed mentally and physically suitable for enlistment are quarantined for another 2 weeks under supervision at a closed college campus.

Each class contains 350–450 recruits, subsequently divided into platoons of 50–60 recruits each. Classes arrived at the campus approximately every week, and each had designated housing, with separate training and dining schedules.

Within 48 hours of arrival on campus, 1,848 of the recruits enrolled in the longitudinal COVID-19 Health Action Response for Marines, or CHARM, study. Each participant underwent a baseline antibody test for past exposure to SARS-CoV-2. They then underwent weekly testing for SARS-CoV-2 infection.

During the on-campus quarantine, there was strict enforcement of public healthcare measures. Recruits wore double-layered cloth masks except while sleeping or eating, routinely washed their hands, practiced social distancing of at least 6 feet (2 meters), and remained on the campus, with no access to personal electronics.

The recruits slept in two-person rooms, shared bathrooms with sinks, and ate in communal dining areas. They used bleach wipes to clean their bathrooms after each use, cleaned their rooms daily, and ate pre-plated meals in a dining facility sanitized with bleach after each platoon’s dining period.

Instruction and exercise mostly took place outdoors. Six instructors, assigned to each platoon in 8-hour shifts, supervised unidirectional flow and enforced quarantine measures.

The instructors also stayed on campus, wore masks, and ate pre-plated meals. Neither recruits nor instructors interacted with campus support staff. They also underwent daily temperature checks and symptom screening.

Healthcare staff tested any recruits with symptoms using quantitative polymerase chain reaction (qPCR) tests. Recruits entered isolation pending the results. Instructors underwent rapid qPCR testing for SARS-CoV-2, with removal from duty if any recruit in their platoon tested positive.

Staff cleaned the bathrooms, showers, bedrooms, and hallways in the housing facilities, which remained unoccupied for at least 72 hours after the most recent class had completed quarantine.

Recruits only moved on to Parris Island after having completed the supervised quarantine and receiving a negative PCR test.

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