Monitoring of the SARS-CoV-2 Omicron BA.1/BA.2 lineage transition in the Swedish population reveals increased viral RNA levels in BA.2 cases.

Background: Throughout the SARS-CoV-2 pandemic, multiple waves of variants of concern have swept across populations, leading to a chain of new and yet more contagious variants dominating COVID-19 cases. Here, we tracked the remarkably rapid shift from Omicron BA.1 to BA.2 sublineage dominance in the Swedish population in early 2022 at a day-by-day basis.

Methods: Using a custom SARS-CoV-2 Omicron BA.1 lineage-typing RT-PCR assay, we analyzed 174,933 clinical upper airway samples collected during January to March 2022.

Findings: Our study demonstrates the feasibility and reliability of parallel lineage assignment of select variants at population scale, tracking the dominant sublineage transition from BA.1 to BA.2 at day-to-day resolution and uncovering nearly 2-fold higher levels of viral RNA in cases infected with Omicron BA.2 relative to BA.1.

Conclusions: Our data provide unique insights into the Omicron BA.1 to BA.2 transition that occurred in Sweden during early 2022, and later, across the world. This may help to understand the increased transmissibility of the BA.2 variant.