In a highly anticipated announcement, Pfizer said on Monday a Phase 2/3 trial showed its Covid-19 vaccine was safe and generated a "robust" antibody response in children ages 5 to 11.

These are the first such results released for this age group for a US Covid-19 vaccine, and the data has not yet been peer-reviewed or published. Pfizer said it plans to submit to the US Food and Drug Administration for emergency use authorization soon. FDA officials have said that once data is submitted, the agency could authorize a vaccine for younger children in a matter of weeks.

The trial included 2,268 participants ages 5 to 11 and used a two-dose regimen of the vaccine administered 21 days apart. This trial used a 10-microgram dose -- smaller than the 30-microgram dose that has been used for those 12 and older.

"The 10 microgram dose was carefully selected as the preferred dose for safety, tolerability and immunogenicity in children 5 to 11 years of age," Pfizer said in a news release.

Participants' immune responses were measured by looking at neutralizing antibody levels in their blood and comparing those levels to a control group of 16- to 25-year-olds who were given a two-dose regimen with the larger 30-microgram dose. Pfizer said the levels compared well with older people who received the larger dose, demonstrating a "strong immune response in this cohort of children one month after the second dose."

"Further, the COVID-19 vaccine was well tolerated, with side effects generally comparable to those observed in participants 16 to 25 years of age," the company said.

A Pfizer spokesperson also confirmed that were no instances of myocarditis, a type of heart inflammation that has been linked with mRNA vaccines.

Pfizer said these data will be included in a "near-term submission" for EUA and the companies will continue to accumulate the data needed to file for FDA approval for people ages 5 to 11.

The Pfizer/BioNTech vaccine is currently approved for people age 16 and older, and authorized for use in people ages 12 to 15.

Pfizer said it is expecting trial data for children as young as 6 months "as soon as the fourth quarter of this year."

"Since July, pediatric cases of COVID-19 have risen by about 240 percent in the U.S. -- underscoring the public health need for vaccination. These trial results provide a strong foundation for seeking authorization of our vaccine for children 5 to 11 years old, and we plan to submit them to the FDA and other regulators with urgency," Albert Bourla, chairman and chief executive officer of Pfizer, said in the statement.

What happens next

Dr. Scott Gottlieb, a former FDA commissioner and current Pfizer board member, has said a vaccine for children ages 5 to 11 could be available by Halloween.

"Pfizer could be in a position to file very quickly," Gottlieb said Monday on CNBC. "The data came a little earlier than some were expecting, and depending on how long the FDA takes to review the application, whether it's a four week review or a six week review, you could have a vaccine available to children as early as probably by the end of October, perhaps it slips a little bit into November."

Acting FDA Commissioner Dr. Janet Woodcock and Dr. Peter Marks, director of FDA's Center for Biologics Research and Evaluation, said in a statement this month that the agency would review data for a vaccine for younger children "as quickly as possible, likely in a matter of weeks rather than months," once it was submitted for authorization.

"However, the agency's ability to review these submissions rapidly will depend in part on the quality and timeliness of the submissions by manufacturers," they wrote.

Calls for a Covid-19 vaccine for younger children have grown louder in recent months as cases surged among children.

Coronavirus infections have risen "exponentially" among children across the United States, and now account for nearly 29% of all cases reported nationwide, the American Academy of Pediatrics reported last week.

Still, US health officials have emphasized that children are not just small adults, and even those approaching age 12 should not be given the larger vaccine dose available for older people.

"We don't want children to have adverse effects. Granted, we want them to be able to get vaccinated as quickly as possible, but let's do it right," FDA's Marks said in a fireside chat hosted by the ResearchAmerica Alliance last week.

"There is a difference here because they're not just getting the same-old, same-old dose as a 12 and up person will. They have to get a reduced dose. And that's why it's not a good idea for doctors to take things in their own hands at this point."

Millions more would be eligible for vaccination

If the Pfizer/BioNTech Covid-19 vaccine is authorized for children ages 5 to 11, more than 28 million additional people will be eligible to be vaccinated against Covid-19, according to a CNN analysis of data from the US Census Bureau.

Currently, about 85% of the US population is eligible to receive a Covid-19 vaccine. But the 5 to 11 age group represents about 9% of the total United States population, which would bump it up to 94%.

According to the latest data from the US Centers for Disease Control and Prevention, 63.8% of the total US population has received at least one dose of Covid-19 vaccine and 54.6% of the population is fully vaccinated.

Adolescents are the least vaccinated group. About 57% of adolescents age 12 to 17 have received at least one dose and 46% are fully vaccinated, according to a CNN analysis of CDC data. Children 16 and up were included in the initial emergency use authorization of the Pfizer/BioNTech vaccine and children ages 12 to 15 were added in May, more than four months ago.

A Kaiser Family Foundation poll last month found that about a quarter of parents of children ages 5 to 11 say they will vaccinate their children "right away" once a vaccine is authorized. Four in 10 parents said they would "wait and see" how the vaccine is working before they get younger children vaccinated.

Meanwhile, one-quarter of parents in the poll said they would "definitely not" have their 5-to-11-year-old children vaccinated against Covid-19.

The-CNN-Wire

™ & © 2021 Cable News Network, Inc., a WarnerMedia Company. All rights reserved.

CNN's Deidre McPhillips, Jacqueline Howard and Naomi Thomas contributed to this report.

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