Teens experience side effects after Pfizer's shot slightly more than adults do. A chart shows the most common.

• The US authorized Pfizer's coronavirus vaccine for 12- to 15-year-olds earlier this month.
• Adolescents in Pfizer's trials seemed to develop side effects more frequently than adults did.
• But even the most common side effects, like fatigue and headaches, were relatively minor.

During the initial week of Pfizer's vaccine rollout for 12- to 15-year-olds, roughly 600,000 adolescents received their first shots.

Scientists expected the vaccine to be effective in young people even before trial results were released - our immune systems get weaker with age, so children and adolescents typically develop strong protection from vaccines. Indeed, Pfizer's clinical trials showed that the vaccine was 100% effective among 12- to 15-year-olds: Out of more 1,100 adolescents who received the shot, none developed COVID-19.

Moderna, meanwhile, announced on Tuesday that its vaccine was also found to be 100% effective among 12- to 15-year-olds in clinical trials. The immune responses among adolescents appeared comparable to that in adults.

But adolescents seem to develop side effects more frequently after Pfizer's shot than adults. That's likely because kids' immune systems do an excellent job of revving up quickly.

"That feeling of yuckiness and fatigue and fever is your body making a great immune response," Dr. Sharon Nachman, chief of pediatric infectious diseases at Stony Brook Children's Hospital, told Insider.

The chart below shows the most common side effects among adolescents and young adults (people ages 16 to 25) after Pfizer's vaccine, depending on the dose.

For the most part, these side effects aren't too different from those recorded among older adults.

Fatigue and headaches were more prevalent among adolescents than adults

The most common side effect among adolescents in Pfizer's trial was injection-site pain: Nearly 91% of 12- to 15-year-olds reported that after either vaccine dose. Fatigue was the second-most common, with 78% reporting the symptom. About 76% reported headaches.

Nearly half of adolescents also reported chills after either dose of Pfizer's shot, while 42% reported muscle pain. Fever and joint pain weren't as prevalent, though: 24% and 20% of adolescents reported these side effects, respectively.

In general, almost all of the vaccine's side effects were more prevalent among adolescents than adults. About 84% of people ages 18 and older who participated in Pfizer's clinical trials reported injection site pain, while 63% reported fatigue and 55% reported headaches.

Pfizer's second shot amplifies an existing immune response, so people typically feel more run-down after that dose. That was true overall for both adults and adolescents in clinical trials. Both age groups also saw the side effects stop within a few days.

Clinical trials are studying the vaccines among kids younger than 12

Pfizer's shot isn't the first to be distributed to teens - the Food and Drug Administration authorized Moderna's vaccine for 16- and 17-year olds in December.

Moderna's chief executive officer, Stéphane Bancel, said in a statement that the company will ask the FDA to authorize its shot for 12- to 15-year-olds in early June. Johnson & Johnson, meanwhile, started testing its single-dose vaccine in adolescents ages 12 to 17 in April.

Moderna and Pfizer are both still testing their vaccines' safety and efficacy among younger children. Pfizer expects to have data about its shot's effectiveness among kids ages 2 to 11 by September, followed by data for children ages 6 months to 2 years in November. Moderna could produce similar data for kids between 6 months and 11 years soon after.

Public-health experts widely agree that vaccinating kids could help the US reach herd immunity - the threshold beyond which the virus can't spread easily from person to person - much faster.

"As you have new children enter the population, they're going to be susceptible," Rahul Subramanian, a data scientist at the University of Chicago, told Insider. "When we vaccinate children, it helps us maintain herd immunity."

This story has been updated with new information. It was originally published on May 19, 2021.