Pfizer and its German partner, BioNTech, said Monday that an early analysis showed their three-dose coronavirus vaccine regimen triggered a strong immune response in young children, proving 80% effective at preventing symptomatic infections in children 6 months to 4 years old. 

The results, along with other recent developments, signal that the long and frustrating wait for a vaccine for the youngest children, the last group to lack access, could be over within weeks.

A few hours after Pfizer and BioNTech issued a news release announcing the data, which has not been peer reviewed, the Food and Drug Administration said its outside experts will meet June 14 and 15 to discuss the Moderna and Pfizer-BioNTech pediatric vaccines. Pfizer and BioNTech said they plan to finish filing data with the FDA this week - and warned that the efficacy number was fluid because results are still arriving.

If the FDA advisory panel looks favorably on the vaccines, the agency could authorize them as soon as June 16 or 17.

Vaccine advisers to the Centers for Disease Control and Prevention then would consider who should get the vaccines, with a final recommendation coming from agency director Rochelle Walensky shortly afterward. The vaccines would probably be available immediately.

"This is incredibly exciting data!" Kawsar Talaat, a pediatrician and vaccine expert at Johns Hopkins Bloomberg School of Public Health, wrote in an email. "I also think that it reinforces what we've seen in adults as well. For the omicron variant, a third dose is necessary for optimal protection."

Federal officials are already reviewing the pediatric vaccine from biotechnology company Moderna, a two-shot regimen that was 51% effective in preventing illness in children between 6 months and 2 years old, and 37% effective in children 2 to 5 years old.

Regulators previously had set aside three dates for the FDA's outside experts to review the vaccines for young children, beginning with a session on June 8. Those meetings are now canceled.

Under the revised schedule, the FDA and its outside experts will discuss the Moderna vaccine for children and adolescents from 6 to 17 years old on June 14. The following day, they will review vaccines for the youngest children, with advisers evaluating the Moderna vaccine for children 6 months through 5 years old and the Pfizer-BioNTech vaccine for children ages 6 months through 4 years old.

"The overall data are encouraging such that it is really hard to look at one vaccine apart from the other," according to an official familiar with the process who spoke on the condition of anonymity because that person was not authorized to speak publicly. The official suggested the two vaccines would probably be reviewed side by side.

A CDC planning document notes that vaccines are expected to be shipped immediately after being authorized by the FDA. Preordering for doses could begin in late May or early June, but an exact date will be contingent on when the FDA's external advisers meet.

The Pfizer-BioNTech vaccine for children younger than 5 is a three-shot regimen tested in nearly 1,700 children. Each dose is one-tenth of the adult dose. The third shot was added in December after it became clear that two shots failed to muster an immune response equivalent to what was generated in young adults in early coronavirus vaccine trials. It is given two months after the second shot.

Although that setback was hugely disappointing to parents, the addition of a third shot was seen by many experts as necessary because the omicron variant of the coronavirus had fundamentally changed the pandemic. The two shots that provided robust protection against infection and severe illness early on were markedly less protective against the omicron variant.

"Omicron has really thrown a curveball on us - it seems that two doses are not sufficient for adequate efficacy against infection with omicron, with any vaccine, at any age," Flor Munoz, a pediatric infectious-disease specialist at Baylor College of Medicine, said in an email before the new data was released.

While the adult trials recruited tens of thousands of volunteers and waited to see if vaccinated people were better protected, the children's vaccine trials were primarily designed to measure immune responses using blood tests.

The criteria for success was whether a vaccine provoked a comparable immune response to what was seen among young adults in trials conducted before the widespread emergence of variants. Both the Pfizer-BioNTech and Moderna pediatric vaccines succeeded on that measure, although the significance of that benchmark has shifted with the emergence of the omicron variant.

The companies also measured cases of symptomatic illness in the population, and Pfizer and BioNTech said the 80% efficacy finding was preliminary and based on 10 cases of COVID-19 in the study population as of the end of April. Once 21 cases have occurred, the companies will conduct a more formal analysis of efficacy.

David Benkeser, a biostatistician at Emory University's Rollins School of Public Health, said the updated data would probably be ready before a decision would need to be made, and that he wouldn't be surprised if the efficacy number declined somewhat as more cases occur.

"Even still, it appears the data are so far pointing towards a safe and effective vaccine for young children," Benkeser wrote in an email.

Moderna's two-shot vaccine regimen was about 51% effective in children 6 months to 2 years old, and 37% effective in children 2 to 5 years old.

If Pfizer's efficacy data holds up, it could pose a conundrum for public health officials, physicians and parents. If both vaccines are cleared by the FDA, the CDC advisory committee could weigh whether one vaccine should be recommended over the other.

Moderna is studying a booster given six months after the last shot.

In either case, the hope is that children will be fully vaccinated in advance of a potential surge in the fall.

On Sunday, before the Pfizer announcement, White House coronavirus response coordinator Ashish Jha predicted on ABC News's "This Week" that children could have access to a shot "in the next few weeks" and that action would be taken on the Moderna vaccine as soon as regulators were finished with their review.