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Investigating asymptomatic SARS-CoV-2 transmission among Marines
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.
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.
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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.
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.
The study enrolled the 1,848 recruits from May 12 to July 15, 2020, with 40 study participants not returning for the follow-up.
The participants filled out a questionnaire requesting demographic information such as age, sex, ethnicity, race, place of birth, and country of residence. They also provided information about risk factors for COVID-19, any symptoms within the previous 14 days, and a brief medical history.
Of the study participants, 90.5% were male, 25.1% identified as Hispanic, 14.7% identified as Black, 7.2% were born outside the United States, and 83.5% were 18–20 years old.
The researchers performed serum antibody tests and nasal swabs for qPCR testing at baseline and on days 7 and 14, and they reported any symptoms that had occurred during the previous week. Recruits who tested positive were isolated. Otherwise, participants and nonparticipants all followed the same safety measures.
For the recruits who tested positive, scientists also analyzed their viral genetic material to identify transmission and outbreak events.
The researchers found that 105 of the 1,813 participants who underwent antibody testing at enrollment had SARS-CoV-2-specific antibodies.
Also, at enrollment, 16 of the 1,847 participants received positive qPCR test results, five of whom had also received positive antibody results at enrollment.
The 16 participants each reported no exposure to anyone with flu-like symptoms, had quarantined at home for 2 weeks prior to arrival, experienced no respiratory distress, and had not visited a healthcare facility in the past 14 days.
Of the 1,801 participants who had tested negative at enrollment, 24 tested positive for the virus on day 7, and four of this group had positive antibody results at baseline. Eleven tested positive on day 14, and none of this group had received positive antibody results at baseline.
Only 9.8% of the participants who tested positive at any time had experienced COVID-19 symptoms in the previous week.
At the end of the quarantine, 26 of the 1,554 nonparticipants tested positive for COVID-19. Overall, 31.2% of participants and nonparticipants who tested positive had a roommate with a SARS-CoV-2 infection.
The study did not identify any SARS-CoV-2 infections through qPCR testing performed due to daily symptom monitoring.
“This is a difficult infection to suppress in young people, even with close supervision of their mask-wearing, social distancing, and other mitigation measures. We find that regular testing not dependent on symptoms identifies carriers who can transmit SARS-CoV-2,” explained study co-author Dr. Stuart Sealfon, a professor of neurology at the Icahn School of Medicine.
The trial also identified six transmission clusters among 18 individuals by analyzing 36 SARS-CoV-2 specimens from 32 participants.
Honoratus van Bakel, Ph.D., a study co-author and assistant professor of genetics and genomic sciences at the Icahn School of Medicine, elaborated, “Identification of six independent transmission clusters defined by distinct mutations indicates that there were multiple independent SARS-CoV-2 introductions and outbreaks during the supervised quarantine.”
“The data from this large study indicates that in order to curtail coronavirus transmission in group settings and prevent spillover to the wider community, we need to establish widespread initial and repeated surveillance testing of all individuals, regardless of symptoms,” he stated.
The trial’s results may inform strategies for mitigating the transmission of SARS-CoV-2 and slowing the pandemic in the U.S. and globally.
“We hope this information helps in developing more effective measures to keep military installations and schools safe,” said Dr. Sealfon.
It is worth noting that the Centers for Disease Control and Prevention (CDC) still recommend interventions such as regularly washing the hands, wearing face coverings in public, and social distancing. These are instrumental ways to protect against the infection.
Also, it is worth noting that while 2% of the Marines did test positive after joining the supervised quarantine, 98% did not — and this study cannot tell us what the infection rates would have been if such rigorous safety measures had not been in place.
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