A new dominant variant pushed the viral load in Twin Cities wastewater up 13% in the week ending June 20, disrupting a steady decline in COVID-19 levels since mid-May.
Health officials cautioned against hasty interpretations of the reversal, one week after sewage sampling at the Metropolitan Wastewater Treatment Plant in St. Paul found a 12% decline in the presence of the coronavirus that causes COVID-19.
While wastewater levels have accurately forecast shifts in COVID-19 activity before infection numbers changed, they can be distorted by high or low readings on an individual day. The viral load found at the metro plant on June 19 was substantially higher than the rest over the past week.
"It's impossible to say whether this uptick represents the beginning of an increasing trend in the viral load at the Metro Plant, or if it's just a bump along the way," said Steve Balogh, a principal research scientist at the plant.
Another week of wastewater data could be more telling about whether COVID-19 levels are rising in Minnesota again or perhaps leveling off at a steady, endemic level into the future.
One thing that is clear: The fast-spreading BA.4 and BA.5 coronavirus subvariants made up 55% of the viral load identified in Twin Cities wastewater over the past week.
Health officials believe BA.5 will become the dominant source of COVID-19 in Minnesota this summer. Genomic sequencing of a sampling of positive specimens has found that 93 involved BA.5 and that the subvariant exists across the state, according to the Minnesota Department of Health.
Balogh said the variant has at least the potential to "drive the total viral RNA level higher" in wastewater samples in the coming weeks.
The two subvariants were first identified in South Africa, where they produced broad infection levels even in people with recent immunity but low rates of severe COVID-19 illness.
The spring COVID-19 wave in Minnesota has produced a lower rate of severe illness than previous waves as well. The 393 COVID-19 hospitalizations in Minnesota on Thursday included 33 people receiving intensive care — an ICU usage rate of 8% that is well below the peak of 30% earlier in the pandemic.
The state on Friday reported 1,708 more COVID-19 infections as well as five deaths in seniors. While the seven-day averages of infections and deaths have been declining across all age groups for the past month, the progress among seniors has been slower.
Seniors made up 73% of the COVID-19 deaths since last June — when a delta variant posed an increased threat to younger and mostly unvaccinated adults — but 88% of the deaths in the past two months. Minnesota has now reported 12,780 COVID-19 deaths in the entire pandemic.
The unclear Twin Cities wastewater data reflect the broader statewide picture. Minnesota for the first time in weeks has zero counties designated by the Centers for Disease Control and Prevention with high COVID-19 levels. The entire Twin Cities area is listed at low risk, meaning that COVID-19 doesn't present a substantial risk to hospital capacity in the region for now.
Sewage sampling reported by the University of Minnesota from 40 other treatment plants has shown level or declining viral loads in the Twin Cities and most of Minnesota. However, the latest readings as of June 20 showed increases in the northwest and south-central regions of the state.