COVID-19—Lessons Learned and Questions Remaining

Abstract

In this article, the editors of Clinical Infectious Diseases review some of the most important lessons they have learned about the epidemiology, clinical features, diagnosis, treatment and prevention of SARS-CoV-2 infection and identify essential questions about COVID-19 that remain to be answered.

Mechanistic transmission modeling of COVID-19 on the Diamond Princess cruise ship demonstrates the importance of aerosol transmission

“…Therefore, we tested three logarithmically spaced assumptions for the ratio of the effective ID 50 for SARS-CoV-2 for aerosol inhalation (assuming deposition in the LRT) and fomite and droplet deposition (assuming deposition in the URT): ID 50 URT:LRT = 1:1, 10:1, and 100:1. Our assumptions for equal or higher SARS-CoV-2 median infectious doses for fomite and droplet deposition in comparison to aerosol inhalation are generally in line with existing studies showing SARS-CoV-2 preferentially replicates deeper in the lungs ( 15 ) and leads to clinical symptoms at lower doses of aerosol exposures compared to ocular or intranasal routes in animal models such as African green monkeys ( 47 , 48 ) and golden hamsters ( 49 ). We rely on our model approach to back-calculate effective ID 50 values (using a basis of RNA copies) by analyzing successful model results, as described in SI Appendix , section 1.3 .…”

“…Instead, the model framework produces average and uniform outcomes, which remains a limitation. Third, we relied on a conventional discrete size cutoff to define aerosols and droplets (i.e., ∼10 µm); however, respiratory droplets and aerosols actually exist on a continuum of particle sizes influenced by inertia, gravitational settling, and evaporation, and there is increasing recognition that the distinction between droplet and aerosol is a false dichotomy that is inconsistent with our understanding of the physics of respiratory aerosols ( 15 ). We recognize and understand this as well, although we still find value in defining our model framework around this conventional definition because it aligns with the definitions in current public health guidance.…”

“…Since the beginning of the pandemic, numerous researchers ( 5 – 15 ) and professional societies [e.g., American Society of Heating, Refrigerating and Air-Conditioning Engineers ( 16 )] have raised concerns that transmission of SARS-CoV-2 can occur from both symptomatic and asymptomatic (or presymptomatic) individuals to others beyond close-range contact through a combination of larger respiratory droplets that are carried further than 1 to 2 m via airflow patterns and smaller inhalable aerosols that can remain suspended and easily transport over longer distances. These concerns arise from a growing understanding of human respiratory emissions ( 17 , 18 ), known transmission pathways of other respiratory viruses ( 19 ), recent empirical evidence detecting SARS-CoV-2 in aerosol and surface samples in health care settings ( 20 – 25 ), and recent case studies demonstrating the likely importance of longer-range aerosol transmission in some settings ( 26 – 28 ).…”

Healthcare workers and protection against inhalable SARS-CoV-2 aerosols

“…This becomes particularly relevant with the emergence of more transmissible strains of SARS-CoV-2 and another wave of disease. Despite overwhelming evidence supporting aerosol transmission of SARS-CoV-2 [ 1–3 ], this has not translated into appropriate, consistent policies on RPE for HCWs. Partly, this is attributable to nations and organizations not stockpiling RPE despite the predicted occurrence of an influenza pandemic.…”

Filling the Gap Until Full Vaccine Deployment in the War on Coronavirus Disease-19

“…Considering the most recent information about the dynamic of virus spreading, and the fact that asymptomatic and pre-symptomatic individuals represent an important proportion of the potential spreaders and super-spreaders [25,26], personal protective measures such as physical distancing and mask-wearing could still be required for the next 2 years, while awaiting the establishment of sufficient immunization by vaccination. It may be necessary to extend such a requirement even beyond 2 years because it has not yet been tested if vaccinated people can still be infected and transmit the infection.…”