Study population and setting
This cross-sectional study used 165 clinical specimens (e.g., nasal swabs) identified to be positive for SARS-CoV-2 at the University of Washington Virology Laboratory on March 25, 2021, August 3, 2021, or August 25, 2021. Aliquots of all included specimens were frozen within two days of collection. RT-PCR was used to measure total and subgenomic E RNA levels of SARS-CoV-2. A micro-focus forming assay, which involves growing live virus, was used to measure infectious viral titers of SARS-CoV-2.
Summary of Main Findings
For each variant, the amount of total and E subgenomic viral RNA levels (i.e., cycle threshold values) were positively correlated with the amount of replication competent virus. Compared to the Alpha variant, the Epsilon and Delta variants had a greater number of infectious units per quantity of total RNA (i.e., infectivity) and subgenomic E RNA.
The study measured infectious viral titers and total and subgenomic viral RNA levels, and compared these values across variants.
The major limitation of the study is the lack of clinical and epidemiological data that may impact viral titers (e.g., age, time since symptom onset and vaccination status). Additionally, since not all variants were widely circulating at the same time in the U.S., the included samples were not collected from the same population, nor would sample characteristics that may impact viral titers and culturable virus likely be the same across time when the samples were collected (ex: vaccination status).
This novel investigation using a micro-focus forming assay demonstrated that the Delta and Epsilon variants of SARS-CoV-2 may be more biologically infectious than the Alpha variant. This information can be useful in understanding how interventions such as masking, ventilation, etc. may need to be increased in response to increased infectivity of specific variants.
This review was posted on: 8 December 2021