I don't know but I've been told; Covid-19's is just a cold . . .

Interesting and suggestive and with one more element to the study could have been really, really interesting.  

I saw this study mentioned in New study: Lockdowns & masks are useless and might even increase COVID-19 spread by Robert Zimmerman.  

A recently completed research study by Icahn School of Medicine at Mount Sinai in cooperation with the Naval Medical Research Center and published in the New England Journal of Medicine has found that strict quarantine, tightly controlled social distancing, and continuous use of masks did absolutely nothing to contain the spread of COVID-19, and might even have increased its spread.
 
First, the study used 1,843 Marine volunteers, individuals well trained to follow orders as well as the required procedures. Second, their quarantine took place at Marine facility under the supervision of the military. Both factors meant that the volunteers were going to follow procedures much more correctly than the general public.
 
Third, no one could enter the study without undergoing 14-days of quarantine beforehand, plus a test to prove they were negative of COVID-19 at the study’s start. The study itself was held in a tightly controlled quarantine campus run by the Marines.

The study itself is SARS-CoV-2 Transmission among Marine Recruits during Quarantine by Andrew G. Letizia, M.D, et al.

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.

METHODS

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 asymptomatic. 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.

Crucially, as Zimmerman points out, the cumulative number of participants with Covid-19 during the study was 2.8% for those adhering to the strictest regimen of masking, distancing and cleaning and 1.7% for those Marines in the control group not participating in the study. 

Another finding, to be expected at this point, is that those infected who are healthy and young experience very few effects.  Nobody was hospitalized in serious condition and nobody died.  

Interesting material in both the report and Zimmerman's reporting.  

My only wish is that they had included a comparably sized, similarly healthy and youthful random cohort from outside the Marines.

The issue is that the Marines already have pretty rigid requirements in terms of masks, social distancing, personal cleanliness, quarantining, etc.  What they are really doing is comparing otherwise similar populations of near 2,000, one of which are reasonably disciplined on lockdown/social distancing/masking adherence and those who are exceptionally disciplined.

What they have really found is that there is not much distance between excellent adherence and pretty good adherence.

What might the civilian equivalent look like?  Ten minutes of searching tells me that this could be an hour long quest for data.  I would imagine the infection rate is higher than that for the Marines.  While the Marines are orders of magnitude more fit than their civilian equivalents, I suspect that youth is the predominant factor and that the health consequences are going to be similar.  Civilians will have more hospitalizations then the Marines but the effect size will be tiny at these sample sizes.

Had that third control group been added we would have had even stronger evidence that the masking and social distancing are only peripheral to outcomes and might potentially accidentally exacerbate the issue.

It also suggests that we really ought to be encouraging all who are healthy, and especially if young, to get out there, be active, be productive and be sensible.  Yes, many will get infected but virtually none will suffer ill consequences and we might approach herd immunity much faster than currently seems to be happening.