Even the U.S. Marines couldn't stop the silent spread of Covid-19, study shows

"This is a very infectious virus that requires a concerted effort to control transmission."

Between May and July of 2020, hundreds of young Marine recruits arrived at a college campus for a two-week quarantine before heading to basic training in South Carolina. But despite measures embodying the strictest military discipline, a new study shows their preparedness still couldn’t vanquish Covid-19.

The findings are a stark reminder of how the virus can silently spread through asymptomatic carriers and the risks of communal living. As civilian college campuses continue to grapple with their own coronavirus response, these results also spell out what works and what doesn't.

Published Monday in The New England Journal of Medicine, the study details 1,848 Marine recruits who were regularly tested for the novel coronavirus. Although they stuck to strict coronavirus guidelines, 35 of the recruits, representing about 2 percent of the group, still tested positive for the coronavirus by the end of quarantine.

Most of these recruits didn’t have symptoms at any point during the study, the study’s lead author Stuart Sealfon, a professor of Neurology at Mt. Sinai, tells Inverse.

Despite wearing masks at all times, cleaning rooms daily, sanitizing bathrooms with bleach wipes, maintaining six feet of distance, never leaving campus, eschewing electronics or other items that might lead to surface-based transmission, and eating pre-plated meals, the virus still spread among platoons.

Recruits were placed into strict quarantine, but 35 of them still got the coronavirus.

Getty Images/ David Wells

Life in military quarantine – If winter lockdown feels isolating, it has nothing on U.S. Marine quarantine.

Their coronavirus plan was a multi-step process involving multiple quarantines: First, the recruits (all between the age of 18 and 20) quarantined at home for two weeks.

Subsequently, and immediately upon arriving at the campus, the 1,848 recruits were tested for coronavirus. Sixteen tested positive and were placed in isolation. The rest proceeded with their strict quarantines and received daily temperature checks and brief screening for symptoms.

But by the end of the first week, 24 more recruits tested positive for the virus. They were placed in isolation. By the end of week two, 11 more recruits were positive.

At the end of the study, only 10 percent of all Covid-positive recruits reported symptoms in the week prior to their tests or on the day they were tested. The rest showed no symptoms — hinting at asymptomatic spread — but it's possible that they may have developed them later.

Drivers of transmission — In the paper, the authors reported that the two biggest drivers of transmission appeared to be platoon membership (whether infected individuals were in the same platoon) and sharing rooms. Each recruit had one roommate.

If the U.S. Marines couldn’t keep the virus at bay under these strict measures, it doesn’t spell good news for college campuses, who also have similar-aged people congregating but far less of an ability to control behavior, Sealfon says. Ultimately communal living and coronavirus are simply at odds.

He says that the closest civilian comparison to this Marine living situation would be a college campus.

“The social distancing, hand washing, mask-wearing, and other infection control measures at our study site were supervised around the clock by marine instructors,” Sealfon explains. “Presumably our study has better compliance with these measures than what is achieved in the civilian world.”

Take Notre Dame for example. Though the University does regular surveillance testing of its student body, there have still been over 1,200 cases on campus. This weekend students rushed the field after clinching a football victory and are now undergoing mandatory testing.

What can civilians take away? — Some studies have suggested that asymptomatic cases make up the "majority of infections." A review of 79 studies published in PLOS Medicine suggests that between 20 and 30 percent of infections are asymptomatic. However, the exact percentage remains elusive.

This study can demonstrate the costs of any amount of asymptomatic spread and show us how hard it is to control.

"This is a very infectious virus that requires a concerted effort to control transmission."

Most of the recruits on campus who tested positive were asymptomatic, says Sealfon. And even with routine screening, temperature checks, and symptom monitoring — all the tools that we currently use to try to gather safely in places like college campuses or workplaces — the virus still spread.

“Reliance on temperature screening and symptom screening has limited success in detecting infectious in this population of young adults," says Sealfon. "This is a very infectious virus that requires a concerted effort to control transmission."

The study places additional weight on the importance of routine testing – even without clear symptoms, the authors emphasize.

Places that have tried to recapture a semblance of normal life by using a “bubble format” do rely on routine testing of even non-symptomatic people with no known exposure or symptoms. There are examples of successes. During the NBA playoffs, the players in the NBA bubble were tested daily for the coronavirus —and recorded no cases at all.

The season ended, and the NBA bubble held. The Marines' bubble didn't.

It's hard to know why that's the case. Ideally, there’s no “right” answer for how much we need to test to control a silent spread of the virus, says Sealfon. Ultimately, it depends on how much outside contact the group you’re trying to protect has, and how strictly social distancing measures are enforced in that group.

The short answer is the more testing the better, he says. Even with temperature checks, and strict protocols, the virus can still find a way to slip by.

“The more testing and the better public health infection control measures are maintained the more effectively we suppress this pandemic,” Sealfon says.

Background: The efficacy of public health measures to control the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has not been well studied in young adults.
Method: We investigated SARS-CoV-2 infections among U.S. Marine Corps recruits who underwent a 2-week quarantine at home followed by a second supervised 2-week quarantine at a closed college campus that involved mask-wearing, social distancing, and daily temperature and symptom monitoring. Study volunteers were tested for SARS- CoV-2 by means of quantitative polymerase-chain-reaction (qPCR) assay of nares swab specimens obtained between the time of arrival and the second day of supervised quarantine and on days 7 and 14. Recruits who did not volunteer for the study underwent qPCR testing only on day 14, at the end of the quarantine period. We performed phylogenetic analysis of viral genomes obtained from infected study volunteers to identify clusters and to assess the epidemiologic features of infections.
Results: A total of 1848 recruits volunteered to participate in the study; within 2 days after arrival on campus, 16 (0.9%) tested positive for SARS-CoV-2, 15 of whom were asymp- tomatic. An additional 35 participants (1.9%) tested positive on day 7 or on day 14. Five of the 51 participants (9.8%) who tested positive at any time had symptoms in the week before a positive qPCR test. Of the recruits who declined to participate in the study, 26 (1.7%) of the 1554 recruits with available qPCR results tested positive on day 14. No SARS-CoV-2 infections were identified through clinical qPCR testing performed as a result of daily symptom monitoring. Analysis of 36 SARS-CoV-2 genomes obtained from 32 participants revealed six transmission clusters among 18 participants. Epidemiologic analysis supported multiple local transmission events, including transmission between roommates and among recruits within the same platoon.
Conclusions: Among Marine Corps recruits, approximately 2% who had previously had negative results for SARS-CoV-2 at the beginning of supervised quarantine, and less than 2% of recruits with unknown previous status, tested positive by day 14. Most recruits who tested positive were asymptomatic, and no infections were detected through daily symptom monitoring. Transmission clusters occurred within platoons. (Funded by the Defense Health Agency and others.)
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