Transmission Characteristics of SARS-CoV-2 That Hinder Effective Control

Abstract

The most important characteristics of coronavirus disease 2019 (COVID-19) transmission that makes it difficult to control are 1) asymptomatic and presymptomatic transmission, 2) low incidence or lack of dominant systemic symptoms such as fever, 3) airborne transmission that may need a high infectious dose, and 4) super-spread events (SSEs). Patients with COVID-19 have high viral loads at symptom onset or even a few days prior to symptom onset, and most patients with COVID-19 have only mild respiratory symptoms or merely pauci-/null-symptoms. These characteristics of the virus enable it to easily spread to the community because most patients are unaware of their potential infectivity, and symptom-based control measures cannot prevent this type of transmission. Furthermore, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is also capable of airborne transmission in conditions such as aerosol-generating procedures, under-ventilated indoor spaces, and over-crowded areas. In this context, universal mask-wearing is important to prevent both outward and inward transmission until an adequate degree of herd immunity is achieved through vaccination. Lastly, the SSEs of SARS-CoV-2 transmission emphasize the importance of reducing contacts by limiting social gatherings. The above-mentioned transmission characteristics of SARS-CoV-2 have culminated in the failure of long-lasting quarantine measures, and indicate that only highly effective vaccines can keep the communities safe from this deadly, multifaceted virus.

Keywords: COVID-19; SARS-CoV-2; Transmission.

Figure 1. Viral load kinetics in the nasopharyngeal swab samples of patients with severe (group 1, n=3) and mild-to-moderate (group 2, n=4) symptoms of COVID-19 who were admitted to Asan Medical Center between February 2020 and June 2020. SARS-CoV-2 viral loads were quantified by in-house real-time RT-PCR assay targeting the S gene and the N gene of SARS-CoV-2.

Figure 2. Comparisons of viral loads between asymptomatic and mild symptomatic individuals with SARS-CoV-2 infection for the E gene, RdRp gene, and N gene of SARS-CoV-2 (10). The nasopharyngeal and oropharyngeal swab samples were obtained from asymptomatic individuals and mild symptomatic individuals at a median of 13 days from the diagnosis. SARS-CoV-2 viral loads (Ct values) were quantified by a commercial real-time RT-PCR using the Allplex 2019-nCoV Assay kit (Seegene, Seoul, Korea).