Coronavirus vaccines for children younger than 5 could be available far sooner than expected — perhaps by the end of February — under a plan that would lead to the potential authorization of a two-shot regimen in the coming weeks, people briefed on the situation said Monday.

Pfizer and its partner, BioNTech, the manufacturers of the vaccine, are expected to submit to the Food and Drug Administration as early as Tuesday a request for emergency use authorization for the vaccine for children six months to 5 years old. Older children already can receive the shot.

The FDA urged the companies to submit the application so that regulators could begin reviewing the two-shot data, according to the knowledgeable individuals, who spoke on the condition of anonymity because they were not authorized to speak publicly.

The companies in the last few months have been testing a third dose, following disappointing results for the two-shot regimen showing that while the vaccine is safe, two doses did not provide a strong enough immune response in all age groups. But data on a third shot will not be available until at least late March. Once that information is submitted, regulators can decide whether to authorize a third dose.

"The idea is, let's go ahead and start the review of two doses," said one of the people familiar with the situation. "If the data holds up in the submission, you could start kids on their primary baseline months earlier than if you don't do anything until the third-dose data comes in."

A Biden administration official said there is consensus among health officials in "seeing this move forward," referring to the plan by Pfizer-BioNTech to submit an application. Last Friday, Pfizer presented updated trial data to federal health officials about the shots, according to an administration official who spoke on the condition of anonymity to discuss a sensitive topic.

The participants in the briefing included Anthony Fauci, chief medical adviser for the White House coronavirus response; David Kessler, chief science officer for the response; a representative from the Centers for Disease Control and Prevention; and other officials from the Department of Health and Human Services.

The session included a "robust conversation" that three doses were likely to be much better than two shots, the administration official said. "But to get to three, you have to get two shots first. . . . There's interest in seeing this move forward."

The FDA's outside advisers are expected to meet on the two-dose application in mid-February. The CDC's outside experts, the Advisory Committee on Immunization Practices, also are expected to convene to consider the issue.

In December, Pfizer and BioNTech announced that two doses of the vaccine in 2-, 3- and 4-year-olds did not trigger an immune response comparable to what was generated in teens and adults. But the two-shot regimen did create a protective immune response in children 6 months to 2 years old. That's when drugmakers added a third shot to the trial to improve the immune response, a crushing blow to many pediatricians and parents who would now have to wait several months to protect children.

People familiar with the updated two-dose data say the vaccine has a good safety profile in young children and showed an ability to prevent a significant number of covid-19 cases.

But outside experts were eager to see any new data showing how effective the vaccine was. And some worried that any effort to speed access to the shots could backfire by increasing skepticism about vaccines.

In the trial, children between 6 months to 5 years old received two doses of 3 microgram shots, a tenth of the dose given to adults, three weeks apart.

The trial was designed to measure immune responses in younger children after immunization, to see if their immune responses were comparable to what was reported to be protective in teens and young adults. This approach is called "immunobridging." It is often used to show vaccines work and are safe in other age groups — and typically takes far less time than efficacy trials, which enroll more study subjects and take more time because they wait to see if people who are vaccinated are less likely to fall sick than people who receive a placebo.

Pfizer and BioNTech are expected to provide updated data when they submit their request for authorization to the FDA. There were so many cases of covid-19 during the omicron surge when the trial was ongoing, that the companies have some data showing how well the vaccine worked to prevent illness.

Outside experts who were not involved in the trial said they would be interested to see what new data Pfizer submitted, but expressed skepticism about the idea that the two-dose data would be enough, if it depends on the likelihood that a third dose would raise the immune response.

"I would say it's certainly fine to release an under 5 [year old] covid vaccine on the basis of immunobridging data, but there has to be sufficiently robust immune response," said Peter Hotez, dean for the National School of Tropical Medicine at Baylor College of Medicine.

Additional data will become available when the company files with the FDA for emergency use authorization, according to people familiar with the situation.

"At this time, we have not filed a submission and we're continuing to collect and analyze data from both two and three doses in our younger age cohort," Pfizer spokeswoman Jerica Pitts said in an email. "As part of our ongoing commitment, we will share new updates as they become available."

A segment of parents have been trying to push the FDA to move faster on children's vaccines. A grass-roots group of physicians, parents and advocates called "Protect Their Future" has collected more than 5,700 signatures on a petition asking for vaccines to become available to younger age groups. That includes through off-label use of the higher-dose vaccine authorized for 5- to 11-year-olds, or by authorizing the vaccine in the youngest children, in whom it triggered a protective immune response.

The American Academy of Pediatrics said parents with children under 5 are facing "an especially challenging moment in this pandemic."

"We understand the urgent need for a safe and effective vaccine for that age group," American Academy of Pediatrics chief executive Mark Del Monte said in a statement. "We are eager to see the data and will continue to follow the science."

But some doctors said they worry that an effort to accelerate pediatric vaccines could hurt the effort to increase uptake of boosters in adults and vaccinations in older children. Kavita Patel, a primary care physician who worked in the Obama administration, said Monday the government should avoid any steps that could undermine faith in the shots, considering the extensive skepticism that has dogged inoculation efforts.

While those demanding the vaccine have been vocal, vaccine uptake among other children already eligible has been slow - and some pediatricians fear that acceptance will be lower in younger age groups. In areas where shots have been available to 5- to 11-year-olds since early November, more than 70% of eligible children have not gotten a single shot, according to the Kaiser Family Foundation.

Much of the conversation at the meeting involving Pfizer and the government officials Friday was about whether the manufacturers could ramp up production within the next four weeks to produce enough doses for this population, the administration official said. Pfizer said it could, the official said.