The dominant source of COVID-19 in Minnesota right now might have a mythic nickname, but it has driven down severe illness levels rather than scare up another pandemic wave.

The kraken variant, or XBB.1.5, raised fears when it let loose in January because it was a hybrid of prior coronavirus variants with the potential to evade immunity from vaccines and prior infections. But its dominance has coincided with a decline in COVID-19 hospitalizations in Minnesota from 579 at the start of 2023 to 52 on Tuesday.

The rate of COVID-19 deaths in that timeframe has declined from eight per day in Minnesota to fewer than three per day, according to Thursday's weekly state pandemic update.

"It's still circulating out there," said Michael Osterholm, director of the University of Minnesota's Center for Infectious Disease Research and Policy. "It's just acting more as a virus that is willing to live with us rather than try to kill us."

XBB variants in general have made up more than 90% of the viral material found in sewage samples for nearly five months, according to Friday's update from the Metropolitan Wastewater Treatment Plant in St. Paul. That is the same length of time as delta variant dominance in fall 2021, which was among the most lethal in the pandemic.

The total amount of virus found in the latest sewage samples is very low and hasn't been seen since July 2021, according to a statement from the Metropolitan Council. Results of statewide sampling by more than 35 plants show similar declines.

Add XBB's extended run at the end of the pandemic to the list of mysteries about how coronavirus spread in the past three years. Epidemiologists still are puzzled about why an alpha variant seized on Minnesota and Michigan in spring 2021 when those states had above-average distribution rates of the first COVID-19 vaccine.

The natural evolution of a virus would predict that a variant like XBB would come along that would infect people at a steady rate but cause fewer illnesses, said Sara Vetter, director of the Minnesota Department of Health's infectious disease lab.

"As with any parasite-host relationship the parasite (in this case the virus) needs to find a balance with its host in order to survive," Vetter said in an email. "If it's too virulent and makes its host too sick, it won't be able to spread as fast. Also if it spreads too fast, it will run out of susceptible hosts."

Federal officials have banked on XBB remaining dominant. Advisers told the U.S. Food and Drug Administration last week to make the kraken variant the sole target of the next round of COVID-19 vaccine booster shots.

Sampling of positive specimens at the state public health lab has shown the emergence of new XBB variants. The kraken variant made up at least 88% of its samples in mid-March but only 60% in early May, when XBB.1.16 and XBB.1.9 started to emerge.

Osterholm said he is confident a booster specific to XBB.1.5 will provide protection against these subvariants. Even boosters formulated against older omicron variants still are reducing risks of COVID-19 hospitalizations and deaths, he said.

Osterholm is a living reminder of the risks, though, having tested positive for COVID-19 earlier in March and suffering fatigue and other long COVID symptoms several weeks later.

"My fatigue is almost over," he said on Friday.

The state Health Department on Thursday urged Minnesotans to check with their doctors to see if they are due for COVID-19 boosters.

The department also cut back on its weekly reporting of vaccination progress, eliminating statistics on the percentage of Minnesotans who completed the original vaccine series or first round of boosters.

Less than 27% of Minnesotans are considered up to date with the COVID-19 booster recommendations, although that rate increases to 69% in seniors who are at elevated risk of severe illness.

Seniors made up 83% of Minnesota's 14,861 COVID-19 deaths, but 92% of the 800 deaths reported so far in 2023.