It's been only three weeks since omicron hit the news, but the variant is already the country's dominant Covid strain, responsible for 73% of U.S. cases — including more than 250,000 cases just over the last day, according to Johns Hopkins University.

But there's a way to help protect yourself against symptomatic and severe illness: booster shots.

Most recently, Moderna announced on Monday that its Covid vaccine boosters showed a significant increase in antibody levels against omicron in preliminary lab trials. Pfizer announced similar results earlier this month, offering additional hope ahead of the winter holidays that the mRNA boosters provide protection against the omicron variant.

Emerging real world data has also shown that boosters can protect against symptomatic and severe disease. But while 204 million Americans are fully vaccinated — 62% of the U.S. population, according to the Centers for Disease Control and Prevention — only 30% of those fully vaccinated people have gotten a booster shot, as of Tuesday.

If you got an mRNA vaccine from Pfizer or Moderna, you're eligible for a booster six months after your second dose. Recipients of Johnson & Johnson's one-shot vaccine are booster-eligible after just two months.

So exactly how much protection do boosters provide? How long does it take for them to kick in, and how long does that protection last?

Here's what you need to know about boosters and omicron right now:

How much do boosters protect against infection?

While Moderna and Pfizer's lab trial numbers are very promising, it's unclear how those results will translate to fighting omicron in the real world. But recent studies based on emerging real-world data show that Pfizer's boosters provide roughly 75% protection against symptomatic omicron infection.

In other studies based on real-world data, that number rises to 85% protection against severe disease and hospitalization, according to world-renowned virologist and Columbia University professor Dr. David Ho.

In other words: Even if you test positive, you're far less likely to experience Covid symptoms — particularly those severe enough to hospitalize or kill you — if you're boosted. "The real protection of the vaccine [booster] is largely against disease, and not against acquisition of infection," Ho says.

Omicron is about twice as contagious as delta, according to models and epidemiologists from the U.K., Ho says. And a recent study Ho conducted on the variant shows that it is "markedly resistant" to the antibodies found in even fully vaccinated people who aren't yet boosted.

"[Omicron] is going to rip right through the population," says Ho. "Those who are vaccinated and boosted are largely going to do OK, even if infected. Those who are vaccinated and not boosted probably will have it slightly worse. But I really fear for those who are not vaccinated."

Which booster should you get, when will protection kick in and how long will it last?

Go with an mRNA vaccine, says Ho. Moderna's booster is a higher dosage than Pfizer's, but the results seem to be "quite similar," he says.

With mRNA vaccines, you'll reach peak protection roughly two or three weeks post-booster. The antibodies you'll gain from the booster will eventually wane over time, and scientists are still working to learn how long your peak protection will last.

Ho, one of the scientists studying that question, says early results show that your booster's protection could wane at a similar rate to your second vaccine dose.

"The waning that's been seen so far [after the original two shot regimen], you would lose half your level by two months, so a half life of two months," he says.

At some point, Ho says, you'll probably need another booster shot — echoing Pfizer CEO Albert Bourla, who told CNBC earlier this month that omicron's spread could accelerate the need for a fourth shot. But, Ho stresses, it's far too early to know what that extra dose's timing will look like.

What omicron symptoms should prompt you to get tested?

Covid has a wide range of symptoms, according to the CDC — and if you're experiencing any of them, you should get tested. Omicron's symptoms may differ slightly, though, more resembling a harsh cold: runny nose, sneezing, sore throat and/or body aches.

Especially given omicron's rampant transmissibility, it's better to be safe than sorry. Even if you think you just have a cold, consider getting a Covid test.

Lines at many testing clinics are lengthy right now, and some pharmacies are limiting the number of at-home tests you can buy at a time due to recent surges in demand — so plan accordingly.

If you come into close contact with someone who tests positive, you should also get tested — even if you're vaccinated or boosted, the CDC says. Dr. Ho points out that vaccinated people may not test positive for Covid for at least three days post-exposure, and the CDC says it can take as long as five to seven days.

If you're unsure if you should get tested, the CDC's interactive testing tool can help you find an answer.

What should you do if you test positive for Covid?

If you test positive for Covid, you have to isolate — no matter your symptoms.

Even if you're asymptomatic, you still need to isolate for 10 days, starting the count a day after your first positive test. "An infected person, even if boosted, could transmit the virus to others," says Ho.

And if you have symptoms, the 10-day count is a minimum, rather than a strict figure. Once you pass that benchmark, make sure your "symptoms have improved, and no fever is present for at least 24 hours without use of fever-reducing medications," the CDC says.

You can always help keep yourself and the people around you safe by getting vaccinated and boosted, wearing a mask in public, washing your hands and reconsidering travel and large events — especially where others' vaccine status may not be known.

For additional information, check CDC guidelines.

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