Millions of Americans as young as 12 could soon be able to get a booster shot of COVID-19 vaccine, after the Food and Drug Administration announced Monday that it has authorized third doses of Pfizer-BioNTech's shot for that age group. The FDA also said it would allow for children as young as 5 with compromised immune systems to get an additional shot. And the agency shortened the amount of time between a person's second dose of the Pfizer vaccine and being eligible for a booster to five months instead of six months for all age groups. "Based on the FDA's assessment of currently available data, a booster dose of the currently authorized vaccines may help provide better protection," especially against the Omicron variant, said Dr. Peter Marks, the FDA's top vaccines official. "With this in mind, the FDA has extended the range of individuals eligible to receive a booster, shortened the length of time between the completion of the Pfizer primary series for individuals to receive a booster and is authorizing a third protective vaccine dose for some of our youngest and most vulnerable individuals." The Centers for Disease Control and Prevention must also adopt formal recommendations before the third shots can be rolled out to younger recipients. That decision could come later this week, following a meeting of the agency's outside vaccine experts. The FDA said it based the decision on promising data from Israel, which rolled out booster shots for residents as young as 12 there in August, for those who had received their second dose at least five months earlier. The FDA also cited data from Pfizer, which had said earlier this month it was evaluating a potential three-dose series for children as young as 6 months old. If the CDC also signs off of the expanded eligibility this week, the agency's vaccination data suggests around 30% of children 12 to 15 years old could soon be eligible. That works out to around 5 million more adolescents who have completed their first two doses five months ago. Currently, a little more than half of Americans in this age group are fully vaccinated. Booster shots of the Pfizer vaccine were authorized in December for ages 16 and up. Boosters from Moderna and Johnson & Johnson are available to adults 18 and older. The CDC could call on its Advisory Committee on Immunization Practices to vote over updated recommendations before issuing updated guidance allowing children to receive the third dose. CDC Director Dr. Rochelle Walensky has occasionally skipped or disagreed with the vote of the panel, which met for a record 13 times last year. That and similar moves by the FDA to skip its own outside group of vaccine experts has provoked some criticism, including from former top-ranking federal scientists. On the other hand, some governors have complained that eligibility was not expanding quickly enough to respond to the virus. In the past, the committee has grappled with weighing the risk of myocarditis, a rare but sometimes serious heart inflammation side effect linked to the Pfizer and Moderna vaccines in mostly younger males after the second shot, against generally lower rates of severe disease among children compared to adults. Against the Delta variant, the CDC published early findings last week from a study of adolescents in Arizona through early December estimating that the vaccine's first two shots were 92% effective at curbing infections. Data collected by the CDC showed rates of cases beginning to climb in November among vaccinated children, though remaining still a fraction of the spread among unvaccinated children. Health authorities in the United Kingdom reported on Friday that children there appeared to face a lower risk of hospitalization from Omicron than Delta, but cautioned that the reduction does "not necessarily imply reduced hospital burden" given the sheer number of cases being caused by the fast-spreading variant. Nationwide, new hospital admissions of children with COVID-19 soared to record highs in recent weeks. The spike now threatens to overrun more than a thousand hospitals now facing staffing shortages, despite growing evidence that Omicron poses a smaller individual risk of severe disease compared to the Delta variant. However, health officials have cautioned that the figure could be overcounting the true number of children hospitalized because of the disease. "Many children and adults get into the hospital for another reason: an accident, a heart attack, an ulcer, diabetes. And yet, when they get tested in the hospital, they find out that they have COVID," Dr. Anthony Fauci, the president's chief medical adviser, told CBSN last week. The booster shots could also come at a key time for local officials around the country now weighing the return to in-person schooling in the new year. Some districts have announced plans to step up other measures, like frequent testing of students, in hopes of limiting the spread of the virus in classrooms. "I know we've had an Omicron surge, but I still believe very firmly and very passionately not only as an educator, but as a parent, that our students belong in the classroom, and we can do it safely. We have better tools than we had in the past to get it done. We know what works," Education Secretary Miguel Cardona told "Face the Nation" on Sunday.