Transmission of SARS-CoV-2 among US marine recruits after quarantine: A case study

By Dr. Ananya Mandal, MDNov 15 2020

The coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has infected over 52.7 million people around the world to date, killing over 1.29 million. This highly infectious virus spreads rapidly from person to person, and public health measures to prevent the spread of the infection include strict isolation and quarantine measures. A team of researchers at the Naval Medical Research Center, Silver Spring, USA, analyzed the risk of transmission of SARS-CoV-2 among marine recruits. Their study titled, “SARS-CoV-2 Transmission among Marine Recruits during Quarantine,” was published in the latest issue of the New England Journal of Medicine.

Study: SARS-CoV-2 Transmission among Marine Recruits during Quarantine. Image Credit: NIAID / Flickr.

Background

Public health measures to prevent the spread of SARS-CoV-2 include rigorous testing, isolating active cases and quarantining those exposed to an active case. Several large population surveys have attempted to see the efficacy of these measures in breaking the transmission chain, though the efficacy of these measures has not been clearly studied among the young population.

The US Department of Defence has similarly implemented such public health interventions. However, the military and marine recruits face several hurdles in observing such measures. These include “confined living spaces, close contact among persons during training regimens and other activities, shared dining facilities, and mixing of persons from across the United States.” According to the researchers, these put them at a greater risk of contracting the infection.

The public health program implemented by the U.S. Marine Corps for the new marine recruits includes a period of home quarantine, which needs to be followed by two weeks of strictly supervised quarantine at a closed campus. This is undertaken to prevent the spread of the infection among new recruits. During this second quarantine, the recruits have to follow certain measures, including:

• Wearing masks (double-layered cloth) indoors and outdoors
• Social distancing of at least six feet
• They did not have access to personal electronics and other items to prevent surface transmission
• Routine hand washing
• Sleeping in double-occupancy rooms
• Cleaning and sanitization of rooms, bathrooms
• Instruction and exercises were conducted outdoors
• Movement supervised, and unidirectional flow was implemented to minimize contact among persons
• Daily monitoring of temperature and symptoms

This new study looked at the virus transmission among marine recruits using the serial real-time quantitative polymerase-chain-reaction (qPCR) assays among the population.

Study Design

For this study, recruits who had undergone a two week home quarantine and a supervised two week quarantine at a closed college campus. They were also asked to home quarantine immediately before they traveled to the campus.

All the recruits enrolled in the study were tested for SARS-CoV-2 via qPCR using swabs from the nasopharynx. The samples were initially collected on their arrival and then were followed by further sampling on their seventh and fourteenth days on campus. Those that did not volunteer for the study underwent a single qPCR test at the end of the quarantine period on day 14. At the end of the supervised quarantine, they were allowed to enter Parris Island.

A further phylogenetic analysis was undertaken of the viral genes obtained from the viral samples of the volunteers in order to assess the features of the infections. Blood samples were collected to check for IgG antibody against SARS CoV-2.

SARS-CoV-2 sequencing was performed using Illumina sequencing protocol and Ion Torrent sequencing protocol. SARS-CoV-2 genomes obtained from patients around the world were obtained from the Global Initiative on Sharing All Influenza Data EpiCoV database (79,840 sequences), and phylogenetic analysis of the samples obtained here was done.

Results

Overall findings:

• This study included a total of 1,848 recruits
• From the first test that took place within two days after the recruits’ arrival on campus (after spending two weeks of home quarantine), 0.9 percent (16 recruits) tested positive for SARS-CoV-2
• Of the 16 who tested positive, 15 were asymptomatic
• 35 participants (1.9 percent) tested positive either on day seven or day fourteen
• Thus a total of 51 recruits tested positive
• Five of these 51 (9.8 percent) had symptoms in the week before a positive qPCR test
• 1,554 recruits had declined to participate in the study and underwent a mandatory test at the end of 14 days supervised quarantine.
• Of these, 26 (1.7 percent) tested positive on day 14.
• 36 SARS-CoV-2 genomes were analyzed from the 32 participants
• This showed six transmission clusters among 18 participants
• There were multiple local transmissions according to the analysis. This included transmission between roommates and among recruits within the same platoon.

Local Transmission of SARS-CoV-2 during Quarantine. Participants who were associated with the two largest transmission clusters (clusters 2 and 5) were identified by means of sequencing and were either roommates or members of the same platoons, which indicates that double-occupancy rooming and shared platoon membership were important contributors to transmission. Other infected members of these platoons whose samples were not sequenced may have been infected with the same cluster strains. One recruit in each platoon was found to be infected at the beginning of quarantine and represents the potential source of each cluster strain.

Conclusions and implications

This study showed that the positivity rate for SARS-CoV-2 was 2 percent among recruits who had earlier tested negative at the start of supervised quarantine. Less than 2 percent who had not tested earlier, tested positive by day 14. Most recruits with a positive test were asymptomatic, and transmissions were seen mostly among the recruits within their platoon. This study reveals the transmission dynamics among Marine recruits who underwent supervised and home quarantine and could act as a useful case study in assessing the efficacy of the public health measures in place to prevent such transmissions.