People younger than 30 account for a growing share of new COVID-19 cases in Maine, as a new surge is driving infections back up.

The surge comes as inoculations of the 50-plus age group are rapidly expanding, and with eligibility for all Mainers 16 and older beginning Wednesday.

By the end of March, people below 30 accounted for 34.6 percent of all COVID-19 cases diagnosed in Maine during the past 13 months. That is up slightly from 33.8 percent two weeks earlier and is a roughly 3 percent increase over the share that the under-30 crowd accounted for at the beginning of the year.

The age distribution of cases has changed dramatically in one year. On March 31, 2020, people younger than 30 accounted for only 11.7 percent of all Maine cases, while those 60 or older accounted for 44.2 percent.

Mainers in their 20s now account for 18.2 percent of all positive cases since the pandemic began. That’s the highest percentage of positive cases among all age groups. However, the most dramatic increase in case numbers in the first three months of 2021 came among those under age 20.

By Jan. 1, there had been 3,515 cases among those under 20. By March 31, that number had risen to 8,392 cases. That’s a 139 percent increase. The next largest jump, of 104 percent, was among Mainers in their 20s, followed closely behind by individuals in their 40s.

Case numbers among residents in their 70s and 80s grew the least – by 78 and 63 percent, respectively – during the first three months of the year, reflecting the state’s efforts to vaccinate those two age groups during that time. While Mainers 70 or older account for roughly 11 percent of COVID-19 cases in the state to date, they account for 85 percent of all deaths.

Case numbers are rising in Maine – and the average age of infected individuals skewing younger – despite a vaccination campaign that has reached roughly one-third of the state’s residents.

On Friday, the Maine Center for Disease Control and Prevention reported 402 additional cases of COVID-19, marking the first time since Feb. 2 that the daily case number surpassed 400. And it was the 11th time in 16 days that the number of cases the CDC reported crested 200.

The bulk of the 402 new cases date to Sunday but were added to Friday’s totals as Maine CDC epidemiologists work their way through a backlog of positive test results reported to the agency. Maine CDC spokesman Robert Long said 40 of the 402 new positive cases were reported on Thursday. But he said case numbers for that day, and previous days, also could increase as epidemiologists review reports to confirm they are new cases and not repeated positive results of people who have already been diagnosed with COVID-19.

“We are back in a situation in which a sustained increase in the number of positive test results means that it is taking between 24 and 48 hours to review incoming results, not the 24 hours or less that had been the norm other than during the spike from late November to early January,” Long said.

With the 402 new infections, Maine’s seven-day rolling average of new cases jumped to 253, up from 198 a week earlier and an average of 137 daily cases for the week ending Feb. 20. The state’s peak, seven-day average of 624 occurred on Jan. 15.

The Maine CDC also reported one additional death on Friday afternoon, identified as a York County man over 80.

Public health officials in Maine and across the country have raised concerns about the potential for another surge in COVID-19 cases threatening progress in controlling the deadly disease with vaccinations. Maine set a new record of shots administered on Thursday – 21,829 shots – for the second day in a row, with that rate likely to increase next week as the state expands eligibility to everyone over 16.

Dr. Nirav Shah, director of the Maine CDC, noted that the state’s positive test rate also is increasing in another potential sign that the virus is spreading more rapidly. While the daily, combined positivity rate for both molecular-based and antigen tests has ranged from a high of 4.9 percent to a low of 1.8 over the past two weeks, the line is trending upward. Friday’s combined positive rate was 3.5 percent.

“Broader eligibility is coming next week,” Shah said in a tweet. “Please, get your shot. We talk a lot about the light at the end of the tunnel. Vaccines are our vehicle out of that tunnel.”

To date, there have been 744 deaths linked to COVID-19 in the state and the Maine CDC has tracked a total of 51,189 confirmed or probable cases of the viral disease since March 2020.

Maine’s vaccination campaign is gaining momentum, however, and is expected to accelerate next week thanks to potentially large increases in shipments of doses from the federal government.

As of Friday morning, 435,700 individuals – or 32.4 percent of Maine residents – had received at least one dose of vaccine while second or final shots had been administered to 277,098 individuals, representing 20.6 percent of the population.

On Thursday, Gov. Janet Mills expanded Maine’s eligibility window to all state residents 16 and older starting on Wednesday. The decision means that an estimated 470,000 Mainers will be eligible to receive a shot 12 days earlier than previously planned, although Mills urged residents to be patient with health care providers as they attempt to reserve appointments.

York, Androscoggin and Cumberland counties continue to have the highest per capita infection rates in the state, with York’s rate standing at 524 cases for every 10,000 residents. Waldo County has the lowest case rate at 181 confirmed or probable infections per 10,000 residents.

However, Maine CDC data show that more rural counties experienced the largest increases in cases percentage-wise during March.

The total number of cases in Piscataquis County, which has consistently had among the lowest infection rates, rose by 34 percent between March 1-31, while Knox and Waldo counties saw increases of 24 and 20 percent, respectively. Kennebec, Penobscot and Hancock counties also experienced increases of more than 15 percent during March.

Sporadic outbreaks continue to be a problem on some Maine college campuses and at higher education institutions around the country.

Bates College imposed a lockdown on all of its nearly 1,800 students on Thursday as the number of cases rose to 34, making it the worst outbreak to hit the Lewiston school during the pandemic. The University of Maine System also reported 65 known cases across the system’s campuses on Friday, which was down from a record of 95 cases last week.

Data from the Maine Department of Education, meanwhile, indicate that cases are rising in Maine schools, although the coronavirus remains less prevalent among students and school staff than in the state’s general population.

There were 724 confirmed or probable cases among students and staff in schools for the 30 days that ended March 31, compared to 481 cases for the 30-day period ending on March 10, according to weekly reports released by the department. Meanwhile, the case rate among school students and staff stood at 30 cases per 10,000 individuals, up from 22 per 10,000 several weeks earlier but still lower than the statewide rate of 41 per 10,000 residents.

Unlike in other states, many Maine schools have remained open – often with hybrid models – during parts of the year-long pandemic. All 16 Maine counties are currently designated as “green” under the state’s color-coded reopening model, meaning that schools within those counties can consider in-person instruction as long as they can meet health and safety guidelines.

Many Maine schools are expanding the number of in-person instruction days, with some returning to five days a week.

Roughly 200 doctors, nurses, social workers and other health care providers signed onto open letters to Gov. Janet Mills, Shah and the commissioners of the Department of Education and the Department of Health and Human Services supporting plans “for the full and safe reopening of Maine schools without further delay.”

The letter from the doctors said the pandemic “has had a devastating toll on children, including a significant increase in suicide attempts and an increase in disruptive behavior disorders due to social isolation and disruption of established routines.” Failure to return to full, in-person instruction “in a timely manner” could lead to those harms outweighing the benefits of minimizing risks of spread of COVID in schools.

“The science and a year of experience on the global, national and state level have demonstrated that schools with multiple mitigation strategies in place are simply not significant drivers of community transmission,” states the letter, a copy of which was released Friday by Rep. Laurel Libby, R-Auburn, a registered nurse who signed onto a joint letter endorsing the sentiments expressed by the physicians. “Maine schools are safe places for educators to work and for children to thrive.”

Hospitalizations remain relatively steady in Maine, with 75 people hospitalized statewide as of Friday morning and 25 of those in critical care beds. The rate of new deaths from COVID-19 in Maine has dropped significantly, however, as vaccinations increase among individuals 70 or older. As of Friday, 81.5 percent of the roughly 191,000 individuals in that age group had received at least a first dose of vaccine and 74.3 percent had received all necessary doses.

Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention, said that the seven-day average of new cases nationally continues to rise and surpassed 62,000 as of Friday. Hospitalizations also are rising, Walensky said, even as the country launches an unprecedented vaccination campaign.

“These data continue to be clear: despite the good news on the vaccination front, we cannot yet afford to relax the prevention strategies,” Walensky said during a briefing of the White House COVID-19 Response Team on Friday. “We must continue the practices of mitigation strategies that we know work, like wearing a mask and physical distancing, in order to slow the spread of COVID-19 and to see the end of this pandemic. This is a pivotal moment for our country.”