Shifting slightly in his seat, Hector Knox bounced his leg and stretched before indicating his decision: He wanted the shot in his left arm.

The eighth-grader was one of the first 12-to-15-year-olds to receive his initial dose of a coronavirus vaccine at Children’s National Hospital in the District on Thursday morning, less than a day after the Centers for Disease Control and Prevention authorized the Pfizer-BioNTech vaccine for his age group.

He shared a commonality with several other children in the room, and even some of the adults: He’s not the biggest fan of needles.

But did it hurt?

“No, not at all,” said Hector, 14, who wore a shirt embroidered with the D.C. flag. “I was actually surprised — I didn’t feel it.”

Children’s was planning to inoculate about 100 younger adolescents on Thursday. The District also opened its city-run walk-up sites that are using the Pfizer vaccine to residents ages 12 and older. That age group can get appointments in the District at commercial pharmacies and several hospitals and health clinics as well.

At Children’s, many of the youngsters appeared to be smiling through their masks after receiving their initial shot, overjoyed at the thought of returning to previously restricted activities such as traveling and playing with friends. Some took selfies with their new vaccination cards.

Cooper More, 13, was brimming with excitement. He has multiple birth defects and a chronic condition that required him to stay more vigilant than most, spending nearly all of his time at home in the past year to avoid the virus.

He was relishing the thought of attending baseball games and spending more time out of the house with his brother, 15-year-old Turner, who also received his first shot Thursday.

“I feel very privileged . . . and have more anticipation for the second [shot] to get back to normalcy, and get back to my life,” Cooper said. Embracing his brother, he added: “I’m also glad I did it with him.”

Claire Boogaard, the medical director for the Children’s Hospital coronavirus vaccination program, said more than 6,000 12-to 15-year-olds had registered in advance to be vaccinated at the hospital in D.C. and at a clinic in Prince George’s County. Her team launched a social media campaign and sent text messages to patients letting them know about the opportunity.

She said the outreach effort was particularly focused on Wards 7 and 8 — historically underserved D.C. neighborhoods with some of the most high-risk populations. But residents from those areas were nevertheless in the minority of patients who registered in advance, she said.

“In all communities, but especially that particular community, there are a lot of people who just want to be thoughtful about it, and who need time to think and ask questions,” said Boogaard, who is also a pediatrician at Children’s National at the Town Hall Education Arts Recreation Campus in Ward 8. “We’re aware of that and will be doing dedicated outreach to those communities.”

Stephanie Sims, a nurse who lives in Ward 8, brought her 12-year-old son, Alphonso, to the hospital on Thursday to be vaccinated. She was vaccinated in December. She said it took several weeks to get her 74-year-old mother a shot at a Giant grocery store, even when seniors were given priority.

“It was disappointing to go to the Giant and see individuals who were not normally in that neighborhood, who are not Ward 8 residents, trying to get a vaccine,” she recalled. “But you’re not coming to the neighborhood any other time?”

With that in mind, Sims made it a priority to get her son inoculated as early as possible.

“I’m always talking about myself being vaccinated, and I’m just fine,” she added. “Now I can spread the word that my son is vaccinated, and my mom is vaccinated, so just trying to be an example.”

One youth said he wasn’t feeling well soon after getting the shot. A few minutes later, he fainted. He was taken into a room for observation, according to his father, John See, and soon was sitting up and doing well.

“He’s fine. I’ll just say he has a history of not enjoying getting shots,” See said. “This could’ve been any vaccination, could’ve been any shot.”

Officials in Maryland and Virginia have also started vaccinating younger adolescents. School systems are working with public health officials to arrange voluntary, in-school vaccination clinics for students, and elected leaders say getting more young people vaccinated will speed the timeline for lifting capacity and mask restrictions.

On Wednesday, Maryland Gov. Larry Hogan (R) said all businesses in the state, including restaurants and entertainment and sporting venues, may resume normal operations on Saturday, after more than a year of pandemic-related capacity restrictions. Hogan also announced plans to lift the state’s indoor mask mandate when 70 percent of adult residents have received at least one dose of a coronavirus vaccine, which he said should happen by Memorial Day weekend.

Eric Toner, a senior scholar with the Johns Hopkins Center for Health Security, said the decision appears to be premature. “I think wearing masks should be [the] last thing to go” to lower the spread of the virus, he said, adding that the metric to decide to lift the mask mandate should be based on the number of cases, not the number of people who are vaccinated.

Maryland’s seven-day average of new daily cases per 100,000 residents is eight, according to The Washington Post tracker. Toner said it should be five or less before the mask mandate is lifted.

Removing masks could slow progress in reducing cases, Toner said. “That is what is potentially in sight, and sooner or later we are going to get there,” he added. “But hopefully we get there before a lot more people get sick.”

Local jurisdictions have the option of keeping restrictions in place, however, and officials in Montgomery County said Thursday that Montgomery would stick to its own reopening framework, which says the county will remove capacity restrictions two weeks after 50 percent of its residents have been fully vaccinated.

The county is on track to reach that milestone Saturday, meaning that businesses should be allowed to resume normal operations by May 29.

“Montgomery County leaders continue to follow the guidance of our public health team and take all the actions necessary to ensure that our residents stay as healthy as possible,” Montgomery County Executive Marc Elrich (D) said in a statement. “Our actions have lowered the COVID-19 positivity rate in our community to 1.57 percent and our case rate to less than four per 100,000 residents.”

Prince George’s County will allow most businesses to operate at full capacity as of Monday evening, but will keep social distancing and mask requirements in place and will still limit banquet halls, concert venues and social and fraternal clubs to 50 percent maximum capacity. Indoor gatherings are still limited to 20 people and outdoor gatherings to 50.

D.C. Mayor Muriel E. Bowser (D) said this week that she plans to lift capacity and other restrictions on most businesses and public venues by May 21. She said entertainment and sports venues will be able to return to full capacity on June 11.

Virginia Gov. Ralph Northam (D) is planning to lift most remaining limits on gatherings and mandates for social distancing on June 15. He anticipates mask requirements to remain in place until the end of June, he said, and possibly longer if federal guidelines recommend it.

Rebecca Tan, Ovetta Wiggins and Julie Zauzmer contributed to this report.