Pfizer said it had fully enrolled its Covid-19 vaccine trial in kids ages 12 to 15, a key step before the vaccine could be used in that age group.

The study, an extension of the one used to support the company’s Emergency Use Authorization of the vaccine in people 16 and older, has enrolled 2,259 kids between 12 and 15, Pfizer told CNBC Friday. Its entry on a government website for clinical trials was updated to note it’s no longer recruiting subjects.

The vaccine, developed with German partner BioNTech, was authorized for people ages 16 and older in December. Trials in younger age groups are necessary to ensure the correct dose, as well as safety and efficacy in those different groups, said Dr. Evan Anderson, a pediatrician at Emory University School of Medicine.

“I am very uncomfortable sending my children back to school, where — despite the best efforts of the school — there is a real risk for contracting Covid-19,” Anderson told CNBC in October.

While children are less affected by Covid-19 than adults, they do still catch the virus and get sick. Some have even died. More than 2.5 million cases of Covid-19 had been reported in children as of Jan. 14, about 13% of all cases, according to a report from the American Academy of Pediatrics and the Children’s Hospital Association.

“Children can still get sick and die from Covid-19,” said Dr. Paul Offit, director of the Vaccine Education Center and an infectious diseases physician at the Children’s Hospital of Philadelphia. “As many children this past year died from Covid-19 as died from influenza. And we recommend an influenza vaccine for children.”

Offit also pointed out that children can suffer from a disease called multisystem inflammatory syndrome associated with Covid-19, “which can be debilitating.”

There have been 1,659 cases of the syndrome in children, referred to as MIS-C, and 26 deaths associated with it as of Jan. 8, according to data from the Centers for Disease Control and Prevention. There have been 78 total deaths from Covid-19 among kids 4 and under, and 178 in kids 5 to 17, CDC data show, although those numbers don’t account for all deaths reported from the U.S.

Pfizer declined to say when it expected results from the trial, which are dependent on the rate of infections observed to be able to compare the rates in the placebo group to those receiving the vaccine. With higher infection rates across the U.S. since the fall — the seven-day average of daily cases is now about 187,500, according to a CNBC analysis of Johns Hopkins University data — vaccine efficacy trials have seen their readouts sped up.

But recruitment for trials in adolescents had been slower than hoped, at least for Moderna’s study in kids age 12 to 17, said Moncef Slaoui, chief adviser to Operation Warp Speed, the Trump Administration’s vaccines effort, at its last briefing Jan. 12.

As of then, about 800 kids had been enrolled in the trial over a month, he said, out of about 3,000 needed. Moderna’s vaccine was authorized for people 18 and over in December, weeks after it began its trial in adolescents. Pfizer lowered the age of its trial to 12 in October.

“While enrollment was lower over the holiday season, we expect to see an increase in the new year as planned,” Moderna spokeswoman Colleen Hussey said Friday. ”We are on track to provide updated data around mid-year 2021.”

AstraZeneca, whose Oxford University-developed vaccine is in late-stage trials in the U.S. and authorized in the U.K., told CNBC Friday it plans to continue U.K. trials in a new protocol for kids between ages 5 and 18 “beginning in the coming months.”

Johnson & Johnson, whose phase three results in adults are expected imminently, said it’s in discussions with regulators about including pediatric populations in its development plan. It noted the same technology it’s using for the Covid-19 vaccine has been used in vaccines given to more than 200,000 people, including people over 65, infants, children, HIV-positive adults and pregnant women.

Typically vaccine trials proceed into younger age groups after they’re proven safe and effective in older groups, and the manufacturers of Covid-19 vaccines have indicated they’ll follow that plan here as well.

Moderna’s chief executive, Stephane Bancel, said this month it’s unlikely the company would have data in kids age 11 and younger, which would involve a lower dose, before next year. He said he expects data for kids 12 and up may be available before September.

Public health officials in the U.S., like White House chief medical adviser Dr. Anthony Fauci, have said they hope as much as 75% to 80% of the U.S. population could be vaccinated by the fall to enable life to return to some form of normalcy.

About 78% of the U.S. population, or 255 million people, are over the age of 18, according to a CNBC analysis of Census data. Another 25 million people are aged 12 to 17.

Fauci didn’t immediately respond to an inquiry about whether including kids in vaccinations is necessary to reach his goal of 75% to 80% coverage.

“It’s important for all kids to get vaccinated and the manufacturers can’t do those trials quickly enough,” Angela Rasmussen, a virologist and affiliate at the Georgetown Center for Global Health Science and Security, told CNBC Friday. “The more people of any age vaccinated, the better.”