Originally Published: 12 AUG 21 07:45 ET

Updated: 12 AUG 21 12:51 ET

By Jen Christensen, CNN

(CNN) -- Dr. Bud Weidermann runs one of the US pediatric trial sites for a Covid-19 vaccine and the question coming from parents now isn't about the vaccine itself, but rather, what exactly is taking so long?

Eager parents want to vaccinate young children, and they want to do it now -- but no vaccine is authorized for people younger than 12.

There's a 2,000-person waiting list to get into Wiedermann's Covid-19 vaccine trial at Chlldren's National Hospital, where he's an infectious disease specialist.

"I certainly understand where people are coming from. I think we're dealing with the Delta variant, which is a different ballgame than we've had before with Covid-19 and school is just around the corner," Wiedermann said. "So things are starting to ramp up and parents and kids are getting anxious about that."

Parents, doctors, and even President Joe Biden have voiced their concern about the growing number of children who are getting Covid-19. Children have largely been spared the worst of Covid-19 -- hospitalizations and deaths are still rare -- but cases have been rising, and without a vaccine, they remain vulnerable.

Pediatricians say FDA needs to act faster

The American Academy of Pediatrics understands parents' anxiety and impatience. AAP President Dr. Lee Savio Beers said there needs to be more urgency around the authorization of the Covid-19 vaccine for young children in the United States.

"It's really important to make sure that we are approaching authorization of the Covid vaccine for our youngest children with the same urgency that we did in adults," Beers told CNN's Wolf Blitzer on Tuesday. "It really is a very urgent situation."

Beers' organization wrote a letter to Acting US Food and Drug Administration Commissioner Dr. Janet Woodcock last week urging the FDA not to slow down authorization.

Last month, the FDA asked Pfizer and Moderna to double the number of children ages 5 to 11 in clinical trials. The FDA also asked for six months of follow-up safety data, instead of the two months it asked for with adults.

"While we appreciate this prudent step to gather more safety data, we urge FDA to carefully consider the impact of this decision on the timeline for authorizing a vaccine for this age group," the letter said.

The AAP argued the FDA should authorize the vaccines for kids ages 5 to 11 based on the initial trial data already available. Two months of safety data should also be enough, it said.

"Waiting on a 6-month follow-up will significantly hinder the ability to reduce the spread of the hyper infectious Covid-19 delta variant," the letter said.

Vaccine advisers to the US Centers for Disease Control and Prevention said in June there is a likely association between the mRNA Covid-19 vaccines and extremely rare cases of heart inflammation in adolescents and young adults, but the benefits of vaccination still clearly outweigh the risks. The inflammation cases appeared to be mild, and they resolved quickly on their own or with minimal treatment.

"There is no biological plausibility for serious adverse immunological or inflammatory events to occur more than two month after Covid-19 vaccine administration," the letter said.

US Surgeon General Dr. Vivek Murthy said Wednesday the FDA will "move fast" to evaluate data from vaccine companies once it's ready, and it's possible a Covid-19 vaccine will be available for kids under the age of 12 before the end of 2021.

"If everything were to go well, and everything were to fall into place, I think it's possible that we could see a vaccine before the end of the calendar year for kids under 12," Murthy told CNN's Wolf Blitzer.

"Make no mistake, the FDA will move quickly on this because they recognize what's at stake. It's the health of our children, and there's really nothing more important than that."

No 'luxury' of time for additional data

Still, Dr. Yvonne Maldonado, a Stanford professor of pediatric infectious diseases who was on the AAP committee that helped draft the letter, is concerned the FDA is slowing down authorization unnecessarily.

"We don't have the luxury of being able to sit back and wait for additional data like this to roll in," said Maldonado, who is also a lead investigator at the Stanford site of Pfizer-BioNTech's pediatric vaccine trial. "We have to make some decisions based on best practices."

Maldonado is concerned the FDA is listening too closely to its critics.

"We can't stop now because people are second guessing what the FDA is doing," said Maldonado.

Dr. Paul Offit, a member of the FDA's vaccine advisory committee, said he can't understand why the FDA requested more data. "The longer follow up for safety doesn't make sense. You just really need just two months," Offit said.

He hopes his committee will be given data from the trials to review soon.

"You know I'm frustrated," Offit said. "As we move further into the fall this puts children at greater risk."

Dr. Bill Muller, who has been working on the Moderna pediatric trials, thinks some parts of the trial process have gone too slowly, but he is in favor of additional data.

"We have to be really cautious about expanding the authorization to really large population of patients," said Muller, scientific director at community and clinical trials, Stanley Manne Children's Research Institute.

"That doesn't mean we need to be overly cautious," said Muller, who is an attending physicians in pediatric infectious diseases at Ann & Robert H. Lurie Children's Hospital of Chicago. "The question is really going to be one of balance."

With the more contagious Delta variant, Covid-19 cases among children have been on the rise since early July after months of decline. Almost 94,000 cases among children were added in the past week, the American Academy of Pediatrics said in what the group described as a "continuing substantial increase."

The vast majority of children don't end up in the hospital, but as of Saturday, the number of children who were newly hospitalized saw a 21% increase week over week, according to the CDC.

Why child trials are necessary

Before a vaccine can be tested in children, scientists test it on adults.

"It's to protect children and to protect their safety and well-being," said Dr. Daniel B. Blatt of Norton Children's and University of Louisville School of Medicine, who has been working on the Covid-19 pediatric trials for the Pfizer vaccine.

Typically, investigators use what's called an age deescalation model, he said. Scientists start with adults, and once they see that it is safe to do so, they move the study onto older children, and then younger children. It's a time consuming, but necessary process.

While the Covid-19 vaccines have been proven to be safe and protective in millions of people, scientists can't just extrapolate that out to younger children.

"Kids aren't just miniature-sized adults, their immune systems are different," said Dr. Katherine Luzuriaga, professor of molecular medicine, pediatrics and medicine at UMass Medical School, who is working on the Moderna pediatric trial. "The way they respond can sometimes be different."

In a trial, scientists start the research by essentially making their best educated guess on what dose would be safe and generate an immune response to protect the child from getting Covid-19.

"It's that kind of Goldilocks approach that's not too big, not too small," Luzuriaga said.

All along the way, there's an evaluation of the data to make sure there's no concerns about the safety of the product.

Vaccine timeline for younger children

"I think by late fall, we will have, if everything goes well, hopefully, enough information and data to support us for that age group," Luzuriaga said.

Data for children 5 to 11 could come sometime in September, depending on the findings, Pfizer told CNN it could ask the FDA to authorize the vaccine that same month.

Data for 2-to-5-year-olds could arrive soon after. For the youngest children, Pfizer said it could potentially get data in October or November, and shortly thereafter ask the FDA to authorize emergency use.

Emergency use considerations by the FDA can take several weeks, meaning a vaccine for younger children likely won't be available until late fall or even next year.

Pediatricians never feel like things are moving fast enough, especially when they have a patient sitting in front of them that needs something, Luzuriaga said -- but research takes time.

"I think we're getting close," Luzuriaga said.

Moderna told CNN it expects data later this year or toward the beginning of 2022. There may also be interim data at various stages in the trial, but that timeline is hard to predict.

Johnson & Johnson told CNN that it has used its small trial with 16- and 17-year-olds from earlier in the year to design four late stage studies for younger children, and the company is in "active discussions with regulatory authorities regarding our development plan and trial designs." The company anticipates those trials will start in the fall.

"To keep children safe, and ultimately to achieve herd immunity, it is imperative that COVID-19 vaccine clinical trials continue to move forward in this population, and we remain deeply committed to the critical work needed to make our COVID-19 vaccine equitably accessible for all age groups," a J&J spokesperson told CNN Wednesday.

Protecting children before vaccines

While younger children don't have the protection of the vaccine yet, others can help protect them.

"If we're going to give our fighting chance to have a successful school year, we're going to need everyone who is vaccine eligible to get vaccinated and everybody masked," Dr. Peter Hotez, dean for the National School of Tropical Medicine at Baylor College of Medicine told CNN on Monday.

Physical distance, good ventilation, good hand hygiene and masks are all essential, especially as kids head back to school.

"These things, while they're not ideal, we'd love to get to the point where we can vaccinate kids and hopefully pull back some of these measures," Luzuriaga said. "But we're not at that level yet."