Covid booster shots are here for children ages 12 and up — just in time for kids to go back to school after the holidays, as omicron surges across the country.

On Wednesday evening, the Centers for Disease Control and Prevention authorized booster doses of Pfizer's Covid vaccine for kids ages 12 to 15, following a similar authorization from the U.S. Food and Drug Administration on Monday. Both agencies also authorized a booster dose for immunocompromised children ages 5 to 11 this week.

Kids are now eligible for those booster shots five months after their second Pfizer dose. Data reviewed by the agencies included safety statistics on more than 6,300 kids in Israel ages 12 to 15, who received a booster dose of Pfizer's vaccine at least five months following a two-dose vaccination series.

The authorizations may be well-timed: Plenty of school systems across the country are holding in-person classes after the holidays, despite the nationwide rise of Covid's highly infectious omicron variant. On Monday, the U.S. reported a record single-day number of Covid cases — more than 1 million — according to data compiled by Johns Hopkins University.

"The added protection against omicron comes at a critical moment" for children, Dr. Allison Messina, chief of the Pediatric Infectious Diseases Division at Johns Hopkins All Children's Hospital, tells CNBC Make It.

Why Covid booster shots matter for kids

Boosters significantly bolster your immune system's protection against omicron, in terms of both how likely you are to get sick and how severe your illness could be. Initial research from the United Kingdom last month found that Covid boosters in adults can provide up to 86% protection against severe disease from omicron, and roughly 97% protection against Covid's delta variant.

And each day's tally of new Covid cases includes tens of thousands of pediatric diagnoses, says Dr. Matthew Harris, medical director for Northwell Health's vaccine program and a pediatric emergency doctor at Cohen Children's Medical Center in Queens, New York.

While most of those kids won't require hospitalization, Messina says, the ones who do "can get really sick." During the week of Dec. 21-27, an average of 334 children 17 and under per day were admitted to U.S. hospitals with Covid, according to a CDC report — a record high and 58% increase from the week before.

Almost every teenager Messina has seen in the hospital with Covid has been unvaccinated, she says. "It may be rare, but when you see it all the time, it seems not so rare," she says. "I think we just need to keep our eyes on the prize. We have the tools to keep kids and adults safe."

Harris' message for parents is that there are few — if any — downsides to getting your child vaccinated and boosted against Covid. He says his own seven-year-old twins are already vaccinated, and his three-month-old son will follow suit once eligible.

"We will not escape out of this pandemic until we get those who are unvaccinated or under-vaccinated appropriately vaccinated," he says. "We know that the booster doses protect you from omicron, and we know they reduce hospitalization."

Perhaps equally importantly, Harris says, vaccinations and booster shots can help prevent children from inadvertently spreading the disease to siblings or friends with immunocompromising conditions, like leukemia.

"90 to 95% of our ICU kids with Covid are immunocompromised," he says. "And they're getting it from home, from their sibling and from their schoolmates."

Why kids need better masks in schools, too

In addition to vaccines and boosters, Dr. Sonja Rasmussen, a pediatrics and epidemiology professor at the University of Florida College of Medicine and College of Public Health, says parents need to get their children better masks.

"[T]hey need N95s, K95s or a surgical mask," Rasmussen says. "Something a step up from those cloth masks that we've been using."

For months, health experts have urged people to swap their cloth masks for well-fitting surgical masks, to better filter out tiny virus particles. That's particularly crucial against omicron, which is roughly twice as contagious as Covid's delta variant.

The CDC currently recommends "universal indoor masking" for all students, teachers, staff and visitors in K-12 schools, regardless of vaccination status. Masks are required in some public schools across states like California and New Jersey, while states like Texas and Florida have banned schools from mandating mask-wearing.

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