It's open season for Covid boosters: All adults are now eligible to receive a booster shot, from any of the country's three authorized vaccines.

On Friday, the U.S. Food and Drug Administration and Centers for Disease Control and Prevention both approved Covid vaccine booster shots for all Pfizer and Moderna recipients, ages 18 or older. The agencies also approved boosters for all Johnson & Johnson recipients last month.

If you got Pfizer or Moderna, you're eligible for a booster six months after your second dose. For J&J's one-shot vaccine, you're eligible after just two months. So far, 35 million U.S. adults have gotten a booster, out of the 183 million adults who are considered fully vaccinated, according to CDC data.

CDC Director Dr. Rochelle Walensky called booster shots "an important public health tool to strengthen our defenses against the virus as we enter the winter holidays," in a release Friday.

In September, an FDA advisory group actually voted against authorizing boosters for people ages 16 and up. At the time, experts said, two doses of the mRNA Covid vaccines still provided sufficient protection against severe hospitalization and death for young, healthy Americans.

But now, an influx of new data points to benefits of widespread boosters, Christopher Mores, a global health professor at the George Washington University Milken Institute School of Public Health, tells CNBC Make It.

White House chief medical advisor Dr. Anthony Fauci agrees. "There's no lack of clarity," he said on Sunday during an interview with ABC's "This Week."

Here's what that data means, and why you should get the booster once you're eligible.

Why Covid vaccine booster shots are more important now than they were in September

The initial rationale for distributing booster shots was simple: Some people needed additional protection against Covid to prevent hospitalizations and deaths, but most didn't. For the majority of fully vaccinated people, getting Covid meant extremely mild symptoms, or no symptoms at all.

That calculus has changed, says Dr. Colleen Kelley, an associate professor of medicine in the division of infectious diseases at Emory University School of Medicine. The virus that causes Covid continues to spread, due largely to millions of people who remain unvaccinated, and vaccine protection is waning more significantly over time than experts expected two months ago.

That means the odds of breakthrough infections in fully vaccinated people — with potentially serious symptoms — are increasing. Booster shots, Kelley says, can increase antibody levels enough to help prevent those infections and return any breakthrough symptoms to a mild or nonexistent state.

Last month, an Israeli study on 4.8 million vaccinated adults compared the rates of breakthrough infections in people four to six months after becoming fully vaccinated. At six months, the rate nearly doubled to 3.3 cases per 1,000 people — up from just 1.7 at four months.

"While the Delta variant surely has played a role in the resurgence of Covid-19 in recent months, these findings suggest that waning immunity also is an important factor," National Institutes of Health director Dr. Francis Collins wrote in a blog post on Tuesday about the Israeli study.

Similarly, a large study on Covid vaccines in U.S. veterans published earlier this month found that for all three of the country's approved options, protection waned significantly in people who'd been fully vaccinated for six months — down to 48.1%, from an initial 87.9%. All three vaccines, the study noted, remain very effective at preventing death.

"Boosters have a role there to pick that up, and not allow so much breakthrough to occur," Mores says.

Why you should consider getting a booster as soon as you're eligible

Everyone's individual health and risk profile is different, so consider consulting with your doctor before proceeding — but generally speaking, most people should get a booster shot once eligible. It's a low-risk, high-reward choice, experts say.

"There's no indication that there's something inherently risky about obtaining a booster of this vaccine," Mores says. "There is certainly something inherently risky about becoming infected with Covid."

You should especially plan for a booster shot if you want to be indoors with other people during the winter months, or travel over the holidays, says Dr. Sadiya Khan, assistant professor of medicine at Northwestern University Feinberg School of Medicine.

The bigger question may be which booster to get. Anyone who's eligible can get a booster dose of any of the country's three authorized vaccines, regardless of which one they initially received.

Adult women younger than 50 should consider an mRNA booster from either Pfizer or Moderna, says Dr. William Schaffner, Vanderbilt University professor of medicine in the division of infectious diseases. That's because J&J's vaccine can, in rare instances, lead to blood-clotting disorders for that demographic.

Similarly, both mRNA vaccines have become associated with increased rates of myocarditis, or inflammation of the heart, particularly among younger men — but Kelley says such cases are very rare, extremely mild and fairly easy to manage.

"It's really important to think about the booster as a way to more safely return to normal life," Khan says. "That's really the goal here: not to keep living in a pandemic world, but to try to move past that."

Update: This story has been updated to include the CDC's endorsement of booster shots for all adults 18 and older.

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