As of late December, about 62% of the US population has been fully vaccinated against COVID-19. And the country has administered more than 507 million doses of the shots. But research shows that vaccine efficacy can decrease over time — something that’s concerning health officials as the Omicron variant continues to spread across the country. Now, many are getting boosted for an added layer of protection. Here’s What To Know About Booster Shots… In the fall, the FDA and CDC gave the green light to Pfizer, Moderna, and Johnson & Johnson booster shots for...every fully vaxxed adult in the US. And has since expanded their guidance for younger Americans. Here are the nitty-gritty details to remember: Anyone 12 and up can get a Pfizer booster shot five months after their second dose. The CDC and FDA updated their guidance to shorten the interval from six months. The agencies also recommend a third dose for kids ages five to 11 that are immunocompromised. Any adult is eligible for a Moderna booster shot six months after their last shot. Those who got a J&J shot are eligible to get a booster two months after their initial dose. But in December, the CDC suggested people get Pfizer or Moderna shots over J&J shots. That’s because of rare but potentially deadly blood clots linked to the J&J vaccine. You can also mix and match your booster shot. Example: If you got a two-dose Pfizer regimen, you can get a Moderna booster. So far, more than 68 million Americans have gotten a booster shot. And the decision to expand eligibility for those ages 12 and up allows millions more to get an extra dose. Btw, if you’re planning to get a flu shot and a COVID-19 booster this year, the CDC says you can get both — even on the same day. But the US isn’t the only one hopping on the booster bandwagon: A number of countries, including Israel, Germany, and France have offered booster shots to certain groups of people. But the World Health Organization has said that people who are more vulnerable (think: immunocompromised) should get an additional dose, versus all adults. That’s because of vaccine inequity. Many living in poorer countries haven’t had a chance to get even one dose yet. Our World in Data shows that only 8.5% of people in low-income countries have gotten at least one shot. Delta and Omicron Variants’ Impact on Boosters Omicron is now the dominant variant in the US. And is extremely transmissible (potentially more than Delta). Pfizer’s and Moderna’s boosters may reduce cases of symptomatic disease by as much as 75%. Moderna also says that prelim data shows its current booster can increase antibodies by 37 times — and doubling that dosage can increase them by 83 times. Moderna and Pfizer are looking at potentially giving their boosters an upgrade to make them more effective against Omicron. But that could take months. And Dr. Anthony Fauci has said that the current vaccines available in the US don’t need to be altered to specifically target Omicron (at this point). Remember: We can still expect breakthrough infections. But every bit helps. That's why officials are still warning people to get vaxxed and boosted as Omicron sweeps through the country. We spoke to Dr. Fauci about the top three things to know about Omicron. Tap to hear what he had to say: Psst…We’re still learning about the Omicron variant. But here’s what we know so far. Meanwhile, the Delta variant is still circulating across the US. The CDC says that it’s more contagious than other strains — and that fully vaccinated people who experience breakthrough cases can spread the virus. But, the agency also noted that the vaccines authorized in the US are highly effective, including against this variant. Here’s What To Know About Breakthrough Cases… Breakthrough cases (aka when someone gets the virus after being fully vaxxed) are rare, but can still happen. The CDC is tracking how many breakthrough cases have led to hospitalizations, and how many have led to deaths. Reminder: Vaccines help prevent the spread of COVID-19 and prevent severe symptoms. But they are not a cure for the disease. And do not prevent illness 100% of the time. Other factors outside of variants can also cause breakthrough cases, including timing. It takes a couple of weeks for your body to build up immunity, so you could contract the virus right before or right after you get your shot. And if you are exposed to or test positive for COVID-19 after getting vaccinated, here’s what you need to know to keep yourself and others safe. theSkimm COVID-19 vaccines are essential in protecting people against the virus. But the virus’s variants are slowing those plans, causing new spikes and breakthrough cases around the world. Officials hope that booster shots will give tens of millions of Americans another layer of protection. Meanwhile, as the data rolls in, it’s important to stick to the basics — even if you’re fully vaxxed and boosted. Updated on Jan. 4 to include the latest guidelines on booster shots for those ages 5 and up. Updated on Dec. 21 to include the latest on the Omicron variant and the CDC's recommendation of Pfizer and Moderna vaccines over Johnson & Johnson.