Monday, July 13, 2020

Exaggerated Risk of Transmission of COVID-19 by Infected Surfaces

thelancet |  A clinically significant risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission by fomites (inanimate surfaces or objects) has been assumed on the basis of studies that have little resemblance to real-life scenarios.

The longest survival (6 days) of severe acute respiratory syndrome coronavirus (SARS-CoV) on surfaces was done by placing a very large initial virus titre sample (107 infectious virus particles) on the surface being tested. Another study that claimed survival of 4 days used a similarly large sample (106 infectious virus particles) on the surface.  A report by van Doremalen and colleagues found survival of both SARS-CoV and SARS-CoV-2 of up to 2 days (on surfaces) and 3 days (in aerosols generated in the laboratory), but again with a large inoculum (105–107 infectious virus particles per mL in aerosols, 104 infectious virus particles on surfaces).

Yet another study found long survival (5 days) of human coronavirus 229E on surfaces with what I would still consider a substantially large viral load (103 plaque-forming units) in a cell lysate. However, using a cell lysate rather than purified or semipurified virus might enable initial viral proliferation or protection from the effects of the sample drying out.
None of these studies present scenarios akin to real-life situations. Although I did not find measurements of coronavirus quantities in aerosol droplets from patients, the amount of influenza virus RNA in aerosols has been measured, with a concentration equivalent to 10–100 viral particles in a droplet, with even fewer infectious influenza virus particles capable of growth in a plaque assay.

By contrast, one study found human coronavirus 229E to survive for only 3 h, and human coronavirus OC43 to survive for 1 h, after drying on various surfaces including aluminum, sterile latex surgical gloves, and sterile sponges. In a study in which the authors tried to mimic actual conditions in which a surface might be contaminated by a patient, no viable SARS-CoV was detected on surfaces.

A 2020 literature review included most of the studies I have cited here (and others), but adds no new research, and in my view, does not critically evaluate previously published studies. I am not disputing the findings of these studies, only the applicability to real life. For example, in the studies that used a sample of 107, 106, and 104 particles of infectious virus on a small surface area,, these concentrations are a lot higher than those in droplets in real-life situations, with the amount of virus actually deposited on surfaces likely to be several orders of magnitude smaller.

Hence, a real-life situation is better represented in the work of Dowell and colleagues in which no viable virus was found on fomites.