Troy Chang's breathing problems from COVID-19 were more severe than anyone realized.

When Troy's parents checked his blood oxygen, it had plummeted to the 30s, down from healthy levels in the 90s. The 9-year-old was quickly hospitalized, and his ordeal became one of the most severe in a rising tide of pediatric COVID-19 cases.

"It was getting worse and worse," said his mother, Linda Vang of Minneapolis.

Although no one knows which strain of coronavirus sickened Troy, his diagnosis on Dec. 7 and hospitalization on Dec. 14 came at the edge of a new pandemic wave fueled by the fast-spreading omicron variant. Doctors and public health officials are scrambling to understand the differences with this variant, but the data show it is sending more children to hospitals — particularly unvaccinated children.

The Centers for Disease Control and Prevention on Friday reported an increase since mid-December in hospitalizations of children 4 and under who aren't yet eligible for COVID-19 vaccination — from 2.5 per 100,000 to 4 per 100,000.

Minnesota's total pediatric COVID-19 hospitalizations have increased 20%, from 865 on Nov. 28 to 1,035 as of Thursday. Those involving children 4 and under increased 36% in that period, from 385 to 525.

"We haven't seen necessarily the same spike [in Minnesota] that we are seeing in some other parts of the country, but our numbers are going way up," said Kris Ehresmann, state infectious disease director.

Pediatric COVID-19 deaths remain rare even though the proportion of deaths involving people younger than 40 has increased since last spring. Three deaths of teenagers this fall increased Minnesota's COVID-19 toll for people 19 and younger to seven, but none has been reported in the past month. Only 35 COVID-19 deaths have been reported in Minnesotans 20-29, but five of them were reported last week alone.

Whether omicron will produce more severe COVID-19 hospitalizations such as Troy's is unclear. Dr. Janet Hume with M Health Fairview Masonic Children's Hospital said pediatric infections requiring intubation in intensive care have been rising since omicron became the dominant strain.

Children's Minnesota likewise has seen a tripling of pediatric COVID-19 cases requiring admission to its hospitals in Minneapolis and St. Paul — from 20 three weeks ago to 60 now, said Caroline Njau, chief nursing officer for the state's largest pediatric health system. However, she said there seems to be a lower proportion requiring intensive care.

"Ours are sick enough to be in the hospital right now but not to use critical care services," she said.

Omicron appears to produce a lower rate of severe COVID-19 illness than other variants, but health officials fear it will bring an increase in total infections that swamps hospitals and available beds. On Thursday, Minnesota reported that 498 of 527 inpatient pediatric beds were occupied by COVID and non-COVID patients. Of 146 pediatric ICU beds, 134 were filled — including one by Troy.

After Troy was transferred from North Memorial Health to HCMC, doctors tried to improve his breathing without placing him on a ventilator. Moving him onto his stomach helped, but when he was switched to his back, his oxygen levels plummeted.

Troy said he didn't like when health care workers came in and flipped him, and he knew enough about COVID-19 to not want to be placed on a ventilator. His mother didn't test positive and was able to stay at his bedside to provide a friendly face and comfort.

"This was the first one we've had here that knew from the news what it meant to be sick [with COVID-19] and going on a ventilator," said Dr. Ashley Bjorklund, an HCMC pediatric intensive care specialist. "He knew what that meant, and he was really scared."

Troy, a soft-spoken fourth-grader from New Millennium Academy, loves virtual reality video games, including an "infection" mode in one in which gorillas chase and infect one another. He just wanted to get home, so when he was told he would need to be placed on a ventilator, he gave a thumbs-up.

"To see a kid we know is that sick but able to sort of give us that thumps-up was inspiring for our staff to keep fighting and doing what we were doing," Bjorklund said.

Troy's parents had concerns about COVID-19 vaccination at first but got the shots in the summer when other relatives and friends showed no signs of side effects. His older siblings were vaccinated as well. Children younger than 12 became eligible only in November, and his parents hadn't scheduled him for his shot before he got COVID-19.

Pediatric hospital leaders said most of their COVID-19 patients haven't received shots — partly because the youngest preschoolers aren't eligible. The CDC's most recent data for children 12-17, who are eligible for the vaccine, showed a hospitalization rate of 2.2 per 100,000 among the unvaccinated and 0.2 per 100,000 among the vaccinated.

The U's Hume was part of a national group that reported Friday that COVID-19 vaccination lowered the rate of a severe pediatric COVID-19 complication called multisystem inflammatory syndrome in children. It's too soon to know whether MIS-C will be a problem in the omicron wave, she said, because it tends to emerge four to six weeks after initial waves of infection.

"MIS-C cases have been associated with all the other variants, however," said Hume, who strongly encouraged adults and children to get vaccinated to reduce their risk of complications and bad outcomes.

Vang said it was difficult to see her son on a ventilator, initially with paralyzing drugs to prevent him from moving his breathing tube. Mostly confined to the room because of COVID-19 isolation protocols, Vang stayed busy helping the nurses and changing Troy's bedding.

Even while Troy was sedated, Vang said she buried her fears when talking to him and to her older children back home. The older children quarantined because of their close contact with Troy and then got milder COVID-19 cases in late December anyway.

"You have to have hope," Vang said. "If you don't have hope and you're just like, 'Oh, he's not going to make it,' you know, it might go down that way."

Eventually, Troy stabilized enough to be conscious while on the ventilator. The family's hopes and dreams rose and fell with daily fluctuations in his breathing — which improved swiftly to the surprise of his doctors last week, allowing his breathing tube to be removed. Troy took his first bites of real food, a beef casserole, and dreamed of his favorite sushi with spicy mayo.

He now is able to sit up and walk and shower. Troy's father, Chaigh Chang, stood behind him and held a brace to prevent him from falling while he walked three full laps around the HCMC pediatric ICU.

Troy's father said he is astounded at how calm his son has been: "They put a new IV in him, and he was just like giggling. I couldn't do that."

Troy could have emotional and physical challenges back home after a month in intensive care, his parents said. He doesn't want to go back to school, even though he has immunity.

"Now that he got sick, he feels like he wants to distance, stay home and be in his own circle," Vang said. "He's scared to go back. He doesn't want to get sick again."