PORTLAND, Maine (AP) The number of COVID-19 patients in hospital intensive care units reached alarming levels as Maine raced to vaccinate residents. More contagious COVID-19 variants are likely playing a role, officials said.

Central Maine Medical Center in Lewiston has been experiencing its heaviest COVID-19 inpatient burden in recent weeks. Sixteen of the 21 patients, or more than three-quarters, were in the ICU as of Wednesday, the Portland Press Herald reported.

“It’s a frightening shift. It’s not even subtle,” said Dr. Al Teng, chief of critical care at CMMC’s parent, Central Maine Healthcare. “Patients who were critically ill in previous COVID waves were in their 60s, 70s and 80s, but now we’re seeing them in their 20s. It’s quite a stark progression.”

A similar situation is happening at MaineHealth, the state’s largest hospital network and parent of Maine Medical Center in Portland. During the winter surge, about 3 in 10 of its patients hospitalized with COVID-19 required intensive care, but the proportion is now at least 6 in 10, said Dr. Joan Boomsma, chief medical officer.

There was a similar shift at Northern Light Health, the state’s second largest hospital network, with nearly half of all patients admitted to Eastern Maine Medical Center in Bangor requiring ventilators, said Dr. James Jarvis, the network’s physician incident commander.

“The number of COVID inpatients is smaller, but they are significantly sicker,” he said “We’re not talking about people in their 70s or 80s anymore, but individuals who are under the age of 50, and that’s concerning.”

The trend has emerged even as the state races to vaccinate. As of Thursday, more than 50% of the eligible population was fully vaccinated.

Boomsma urged people, including younger Mainers, to take the pandemic seriously. “Young people who think they can’t get sick or in the ICUs are being transported to our ICUs from all over the state,” she said.