Low dose inocula of SARS-CoV-2 B.1.1.7 variant initiate more robust infections in the upper respiratory tract of hamsters than earlier D614G variants

There is a lack of experimental evidence to explain how the B.1.1.7 variant spreads more quickly than pre-existing variants in humans. We found that B.1.1.7 displays increased competitive fitness over earlier D614G lineages in an in-vitro system. Furthermore,, we demonstrated that B.1.1.7 variant is able to replicate and shed more efficiently in the nasal cavity than other variants with lower dose and shorter duration of exposure.

SARS-CoV-2 spike D614G variant exhibits highly efficient replication and transmission in hamsters

SARS-CoV-2 causes disease varying in severity from asymptomatic infections to severe respiratory distress and death in humans. The viral factors which determine transmissibility and pathogenicity are not yet clearly characterized. We used the hamster infection model to compare the replication ability and pathogenicity of five SARS-CoV-2 strains isolated from early cases originating in Wuhan, China, in February, and infected individuals returning from Europe and elsewhere in March 2020. The HK-13 and HK-95 isolates showed distinct pathogenicity in hamsters, with higher virus titers and more severe pathological changes in the lungs observed compared to other isolates. HK-95 contains a D614G substitution in the spike protein and demonstrated higher viral gene expression and transmission efficiency in hamsters. Intra-host diversity analysis revealed that further quasi species were generated during hamster infections, indicating that strain-specific adaptive mutants with advantages in replication and transmission will continue to arise and dominate subsequent waves of SARS-CoV-2 dissemination.

A Territory-wide study of COVID-19 cases and clusters with unknown source in Hong Kong community: A clinical, epidemiological and phylogenomic investigation

Initial cases of COVID-19 reported in Hong Kong were mostly imported from China. However, most cases reported in February 2020 were locally-acquired infections, indicating local community transmissions. We extracted the demographic, clinical and epidemiological data from 50 COVID-19 patients, who accounted for 53.8% of the cases in Hong Kong by February 2020. Whole-genome sequencing of the SARS-CoV-2 were conducted to determine the phylogenetic relatedness and transmission dynamics. Only three (6.0%) patients required ICU admission. Phylogenetic analysis identified six transmission clusters. All locally-acquired cases harboured a common mutation Orf3a G251V and were clustered in two subclades in global phylogeny of SARS-CoV-2. The estimated time to the most recent common ancestor of local COVID-2019 outbreak was December 24, 2019 with an evolutionary rate of 3.04x10-3 substitutions per site per year. The reproduction number value was 1.84. Social distancing and vigilant epidemiological control are crucial to the containment of COVID-19 transmission. Article summary linesA combined epidemiological and phylogenetic analysis of early COVID-19 outbreak in Hong Kong revealed that a SARS-CoV-2 variant with ORF3a G251V mutation accounted for all locally acquired cases, and that asymptomatic carriers could be a huge public health risk for COVID-19 control.