Background The origin of divergent SARS-CoV-2 spike sequences found in wastewater, but not in clinical surveillance, remains unclear. These “cryptic” wastewater sequences have harbored many of the same mutations that later emerged in Omicron lineages. We first detected a cryptic lineage in municipal wastewater in Wisconsin in January 2022. Named the “Wisconsin Lineage”, we sought to determine this virus’s geographic origin and characterize its persistence and evolution over time.
Methods We systematically sampled maintenance holes to trace the Wisconsin Lineage’s origin. We sequenced spike RBD domains, and where possible, whole viral genomes, to characterize the evolution of this lineage over the 13 consecutive months that it was detectable.
Findings The persistence of the Wisconsin Lineage signal allowed us to trace it from a central wastewater plant to a single facility, with a high concentration of viral RNA. The viral sequences contained a combination of fixed nucleotide substitutions characteristic of Pango lineage B.1.234, which circulated in Wisconsin at low levels from October 2020 to February 2021, while mutations in the spike gene resembled those subsequently found in Omicron variants.
Interpretation We propose that prolonged detection of the Wisconsin Lineage in wastewater represents persistent shedding of SARS-CoV-2 from an infected individual, with ongoing within-host viral evolution leading to an ancestral B.1.234 virus accumulating “Omicron-like” mutations.