Lowering SARS-CoV-2 viral load might affect transmission but not disease severity in secondary cases - Authors' reply

Virulence Profiles of Wild-Type, P.1 and Delta SARS-CoV-2 Variants in K18-hACE2 Transgenic Mice

Since December 2019, the world has been experiencing the COVID-19 pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and we now face the emergence of several variants. We aimed to assess the differences between the wild-type (Wt) (Wuhan) strain and the P.1 (Gamma) and Delta variants using infected K18-hACE2 mice. The clinical manifestations, behavior, virus load, pulmonary capacity, and histopathological alterations were analyzed. The P.1-infected mice showed weight loss and more severe clinical manifestations of COVID-19 than the Wt and Delta-infected mice. The respiratory capacity was reduced in the P.1-infected mice compared to the other groups. Pulmonary histological findings demonstrated that a more aggressive disease was generated by the P.1 and Delta variants compared to the Wt strain of the virus. The quantification of the SARS-CoV-2 viral copies varied greatly among the infected mice although it was higher in P.1-infected mice on the day of death. Our data revealed that K18-hACE2 mice infected with the P.1 variant develop a more severe infectious disease than those infected with the other variants, despite the significant heterogeneity among the mice.

Bimodal distribution pattern associated with the PCR cycle threshold (Ct) and implications in COVID-19 infections

SARS-CoV-2 is notable for its extremely high level of viral replication in respiratory epithelial cells, relative to other cell types. This may partially explain the high transmissibility and rapid global dissemination observed during the COVID-19 pandemic. Polymerase chain reaction (PCR) cycle threshold (Ct) number has been widely used as a proxy for viral load based on the inverse relationship between Ct number and amplifiable genome copies present in a sample. We examined two PCR platforms (Centers for Disease Control and Prevention 2019-nCoV Real-time RT-PCR, Integrated DNA Technologies; and TaqPath COVID-19 multi-plex combination kit, ThermoFisher Scientific) for their performance characteristics and Ct distribution patterns based on results generated from 208,947 clinical samples obtained between October 2020 and September 2021. From 14,231 positive tests, Ct values ranged from 8 to 39 and displayed a pronounced bimodal distribution. The bimodal distribution persisted when stratified by gender, age, and time period of sample collection during which different viral variants circulated. This finding may be a result of heterogeneity in disease progression or host response to infection irrespective of age, gender, or viral variants. Quantification of respiratory mucosal viral load may provide additional insight into transmission and clinical indicators helpful for infection control.