When the COVID-19 epidemic reached global dimensions, many countries took the traditional measures that had been effective in containing the previous outbreak responsible for the severe acute respiratory syndrome (SARS) in 2003. These included symptom-based case detection and subsequent testing to guide isolation and quarantine. It was the logical way to react while studying the new virus, but in this case, the approach failed to achieve the same results.
Now an Editorial in the most recent issue of the New England Journal of Medicine entitled Asymptomatic Transmission, the Achilles’ Heel of Current Strategies to Control Covid-19 discusses the differences in transmission and spread between both viruses. We excerpt some of the points that, in our opinion, have more relevance.

Trajectories of the two epidemics

The report provides a critical review of the findings of the spread and characteristics of the SARS-CoV-2 virus—responsible for COVID-19—published in many scientific papers. The authors say that, “[the] initial approach was justified by the many similarities between SARS-CoV-1 [SARS 2003] and the new SARS-CoV-2, including high genetic relatedness, transmission primarily through respiratory droplets, and the frequency of lower respiratory symptoms (fever, cough, and shortness of breath) with both infections developing a median of 5 days after exposure. However, despite the deployment of similar control interventions, the trajectories of the two epidemics have veered in dramatically different directions. Within 8 months, SARS was controlled after SARS-CoV-1 had infected approximately 8100 persons in limited geographic areas. Within 5 months, SARS-CoV-2 has infected more than 2.6 million people and continues to spread rapidly around the world.”

They continue, explaining these differences in transmission and spread. “A key factor in the transmissibility of Covid-19 is the high level of SARS-CoV-2 shedding in the upper respiratory tract (read HERE), even among presymptomatic patients, which distinguishes it from SARS-CoV-1, where replication occurs mainly in the lower respiratory tract (read HERE). Viral loads with SARS-CoV-1, which are associated with symptom onset, peak a median of 5 days later than viral loads with SARS-CoV-2, which makes symptom-based detection of infection more effective in the case of SARS CoV-1 (read HERE). With influenza [the flu], persons with asymptomatic disease generally have lower quantitative viral loads in secretions from the upper respiratory tract than from the lower respiratory tract and a shorter duration of viral shedding than persons with symptoms (read HERE), which decreases the risk of transmission from paucisymptomatic persons (i.e., those with few symptoms).”

Covid-19 testing priority would save more lives worldwide

The Editorial refers in particular to a paper published in the same issue of the Journal: “Arons et al. now report an outbreak of Covid-19 in a skilled nursing facility in Washington State where a health care provider who was working while symptomatic tested positive for infection with SARS-CoV-2 on March 1, 2020 (read HERE). Residents of the facility were then offered two facility-wide point-prevalence screenings for SARS-CoV-2 by real-time reverse-transcriptase polymerase chain reaction (rRT-PCR) of nasopharyngeal swabs on March 13 and March 19–20, along with a collection of information on symptoms the residents recalled having had over the preceding 14 days. Symptoms were classified into

  • typical (fever, cough, and shortness of breath),
  • presymptomatic
  • atypical, and
  • none [further subdivided into presymptomatic and asymptomatic].”

“Among 76 residents in the point-prevalence surveys,

  • 48 (63%) had positive rRT-PCR results, with
  • 27 (56%) essentially asymptomatic, although symptoms subsequently developed in
  • 24 of these residents (within a median of 4 days) and they were reclassified as presymptomatic. Quantitative SARS-CoV-2 viral loads were similarly high in the four symptom groups (residents with typical symptoms, those with atypical symptoms, those who were presymptomatic, and those who remained asymptomatic).”

It is notable, state the authors, that 17 of 24 specimens (71%) from presymptomatic persons had viable virus by culture 1 to 6 days before the development of symptoms. Finally, the mortality from Covid-19 in this facility was high; of 57 residents who tested positive, 15 (26%) died.

“Asymptomatic persons are playing a major role in the transmission of SARS-CoV-2”

The authors highlighted that “the important finding of this report is that more than half the residents of this skilled nursing facility (27 of 48) who had positive tests were asymptomatic at testing. Moreover, live coronavirus clearly sheds at high concentrations from the nasal cavity even before symptom development. Although the investigators were not able to retrospectively elucidate specific person-to-person transmission events and although symptom ascertainment may be unreliable in a group in which more than half the residents had cognitive impairment, these results indicate that asymptomatic persons are playing a major role in the transmission of SARS-CoV-2. Symptom-based screening alone failed to detect a high proportion of infectious cases and was not enough to control transmission in this setting. The high mortality (>25%) argues that we need to change our current approach for skilled nursing facilities in order to protect vulnerable, enclosed populations until other preventive measures, such as a vaccine or chemoprophylaxis, are available.”

A new approach is needed: Covid-19 testing priority including asymptomatic persons

They, therefore, recommend focusing on a particular risk group, “A new approach that expands Covid-19 testing to include asymptomatic persons residing or working in skilled nursing facilities needs to be implemented now. Despite ‘lockdowns’ in these facilities, coronavirus outbreaks continue to spread, with 1 in 10 nursing homes in the United States (>1300 skilled nursing facilities) now reporting cases, with the likelihood of thousands of deaths (read HERE). Mass testing of the residents in skilled nursing facilities will allow appropriate isolation of infected residents so that they can be cared for and quarantine of exposed residents to minimize the risk of spread. Mass testing in these facilities could also allow cohorting (read HERE) and some resumption of group activities in a nonoutbreak setting. Routine rRT-PCR testing in addition to asymptomatic screening of new residents before entry, conservative guidelines for discontinuation of isolation (read HERE), and periodic retesting of long-term residents, as well as both periodic rRT-PCR screening and surgical masking of all staff, are important concomitant measures.”


The report concludes with a call to change current strategies to fight the coronavirus pandemic, which our Observatory supports: Ultimately, the rapid spread of Covid-19 across the United States and the globe, the clear evidence of SARS-CoV-2 transmission from asymptomatic persons (read HERE), and the eventual need to relax current social distancing practices argue for broadened SARS-CoV-2 testing to include asymptomatic persons in prioritized settings. These factors also support the case for the general public to use face masks (read HERE) when in crowded outdoor or indoor spaces. This unprecedented pandemic calls for unprecedented measures to achieve its ultimate defeat.


Justo Aznar Ph.D. MED and Manuel Zunin

Bioethics Observatory – Institute of Life Sciences