Acute SARS-CoV-2 infections harbor limited within-host diversity and transmit via tight transmission bottlenecks

Author summary RNA viruses generate diversity within individual, infected hosts. This genetic diversity can be used to trace how viruses evolve during the course of infection within individuals, and transmission between them. To investigate how SARS-CoV-2 diversity is generated and propagated, we deep sequenced 133 SARS-CoV-2 genomes isolated from acutely infected individuals in Wisconsin. We capitalize on a large dataset of consensus genomes from Wisconsin to investigate how variants are transmitted within the surrounding community, and use a unique household dataset to estimate the number of viruses that are transmitted between epidemiologically linked individuals. We find that most SARS-CoV-2 infections are characterized by limited within-host diversity, and that the vast majority of intra-host single nucleotide variants (iSNVs) are lost during transmission. We do not find evidence that variation is frequently propagated along phylogenetically linked infections, and estimate that most infections are founded by very few unique virions. The combination of limited within-host diversity and tight transmission bottlenecks may slow the pace of SARS-CoV-2 evolution in the future, and suggests that extensive within-host evolution is likely rare. The emergence of divergent SARS-CoV-2 lineages has raised concern that novel variants eliciting immune escape or the ability to displace circulating lineages could emerge within individual hosts. Though growing evidence suggests that novel variants arise during prolonged infections, most infections are acute. Understanding how efficiently variants emerge and transmit among acutely-infected hosts is therefore critical for predicting the pace of long-term SARS-CoV-2 evolution. To characterize how within-host diversity is generated and propagated, we combine extensive laboratory and bioinformatic controls with metrics of within- and between-host diversity to 133 SARS-CoV-2 genomes from acutely-infected individuals. We find that within-host diversity is low and transmission bottlenecks are narrow, with very few viruses founding most infections. Within-host variants are rarely transmitted, even among individuals within the same household, and are rarely detected along phylogenetically linked infections in the broader community. These findings suggest that most variation generated within-host is lost during transmission.

Automated summary

The study shows that within-host diversity of SARS-CoV-2 is low and transmission bottlenecks are narrow, with most infections founded by very few viruses. Within-host variants are rarely transmitted, even within the same household, and are seldom detected along phylogenetically linked infections within the broader community. This suggests that most variation generated within-host is lost during transmission.