How coronavirus vaccines will make their way from adults to children

As the pace of COVID-19 vaccinations among adults picks up, many parents are wondering when the vaccine will be deemed safe enough for children.

Most new drugs and vaccines are tested first in adults to ensure they are safe and effective before moving on to children and other vulnerable groups, and while studies in children are now ongoing, the COVID-19 vaccines that are currently available have yet to be authorized for use in young children.

Now, doctors say it is increasingly urgent to authorize a COVID-19 vaccine for children.

As of Feb. 4, approximately more than 2.9 million U.S. children had tested positive for the virus since the onset of the pandemic, representing 12.9% of all cases in states that report cases by age, according to data collected by the American Academy of Pediatrics and the Children's Hospital Association.

And now, experts from Cincinnati Children's Hospital Medical Center report that the infection rate in children is equivalent to the infection rate in adults.

"If you wipe out the infection in the younger children, they don't spread it to the adults, and so then, you can get a big handle on disease just by targeting the younger children and getting the infection out of that age group," Dr. Robert Frenck, lead investigator of the COVID-19 vaccine trials at Cincinnati Children's Hospital, explained during a briefing on Friday.

About a quarter of the country's population is younger than 18, according to the 2010 Census.

"Achieving the population-level immunity that will be required to control this pandemic will be much more difficult if 25% of our population is not eligible for vaccination," Dr. Kristin Moffitt, an infectious disease specialist at Boston Children's Hospital, told ABC News.

Earlier this week, White House chief medical adviser Dr. Anthony Fauci told ProPublica that children as young as first graders might be able to get the vaccine by the time school is scheduled to start in the fall.

Fauci previously said he expected the 12-to-15-year-old age group to be authorized by late spring or early summer. The timeline is based on safety trials of the vaccine done slowly in younger age groups, beginning with the 12-to-15 range and moving down from there.

"We're in the process of starting clinical trials in what we call age de-escalation, where you do a clinical trial with people 16 to 12, then 12 to 9, then 9 to 6," Fauci said. When asked what was the youngest age group that might be authorized for the vaccine by September, he said, "I would think by the time we get to school opening, we likely will be able to get people who come into the first grade."

Nationwide, children have not been as severely affected by COVID-19 as have many adults, but they can still catch and spread the virus. Additionally, because of the pandemic, fewer children received regularly scheduled health care, such as checkups and routine immunizations.

"While COVID-19 tends to be a minor infection in most children, some children do require hospitalization, and over 200 children have died," Moffitt said.

"Teenagers, particularly those with underlying medical conditions, are more likely to have a clinically significant disease that is similar to adults," said C. Buddy Creech, director of the Vanderbilt Vaccine Research Program and associate professor of pediatric infectious diseases.

At least 2,060 U.S. children also have been diagnosed with MIS-C, which is short for multisystem inflammatory syndrome in children. The rare and extreme immune system response is linked to COVID-19 and has killed at least 30 people through Feb. 8, according to the CDC.

"Although this virus is not an obviously common killer of children ... certainly children can suffer from multi-system inflammatory disease," said Dr. Paul Offit, a professor of pediatrics in the Division of Infectious Diseases at Children's Hospital of Philadelphia and a member of the Food and Drug Administration's vaccine advisory board.

"Any virus that can cause children to suffer or be hospitalized, and even rarely, die, if you can safely prevent it, prevent it," Offit added, speaking during a JAMA Network Q&A on Thursday.

The FDA has so far authorized Pfizer's vaccine for people 16 or older and Moderna's for those 18 or older. Both pharmaceutical companies have begun adolescent trials in children as young as 12, collecting and analyzing data on how best to move forward.

Pfizer, in a statement to ABC News, said the company has "fully enrolled the 12-15-year old cohort with 2,259 participants and expect to have results ready in the first half of 2021" and anticipates "starting a trial in the first half of 2021." But those protocols still need to be approved by regulators.

Moderna began trials in December with subject aged 12 to 17, in several states, the company said in a statement. Moderna said it's seen an increase in enrollment and interest in participating in adolescent study trials around the country and is "on track to provide updated data around mid-year 2021."

"If we want to have children in school reliably, and if we want grandparents to be able to visit with grandchildren most safely," Creech explained, "having our children vaccinated is an important part of the equation."

ABC News' Sony Salzman and Dr. Raehannah Jamshidi contributed to this report.