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Asymptomatic Transmission, the Achilles’ Heel of Current Strategies to Control Covid-19

By May 5, 2020 No Comments

Traditional infection-control and public health strategies rely heavily on early detection of disease to contain spread, Monica Gandhi et al writes in New England Journal of Medicine (NEJM).

When Covid-19 burst onto the global scene, public health officials initially deployed interventions that were used to control severe acute respiratory syndrome (SARS) in 2003, including symptom-based case detection and subsequent testing to guide isolation and quarantine.

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.

What explains these differences in transmission and spread? the authors ask.

A key factor in the transmissibility of Covid-19 is the high level of SARS-CoV-2 shedding in the upper respiratory tract, even among presymptomatic patients, which distinguishes it from SARS-CoV-1, where replication occurs mainly in the lower respiratory tract. 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.

They authors refer, among other things, to the spread of Covid-19 in a Washington State nursing home, where up to 56 percent were asymptomatic when tested. They were contagious 1 to 7 days before they got symptoms.

They therefore believe that a new approach is needed.

Mass testing in these facilities could also allow cohorting and some resumption of group activities in a nonoutbreak setting. Routine rRT-PCR testing in addition to symptomatic screening of new residents before entry, conservative guidelines for discontinuation of isolation, 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 article also mentions that general public should use face masks when in crowded outdoor or indoor spaces. 

Read the whole article in NEJM