By Maggie Fox, CNN

(CNN) — The US Centers for Disease Control and Prevention strengthened recommendations for booster doses of coronavirus vaccine Monday, saying all adults should get boosted six months after the second dose of Pfizer/BioNTech’s or Moderna’s vaccine or two months after the single dose Johnson & Johnson vaccine.

It’s a slight but significant tweak to the wording of guidance issued earlier this month when the CDC endorsed an expanded emergency use authorization for boosters from the US Food and Drug Administration.

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“Today, CDC is strengthening its recommendation on booster doses for individuals who are 18 years and older,” CDC Director Dr. Rochelle Walensky said in a statement.

“The recent emergence of the Omicron variant (B.1.1.529) further emphasizes the importance of vaccination, boosters, and prevention efforts needed to protect against COVID-19,” she added.

“Early data from South Africa suggest increased transmissibility of the Omicron variant, and scientists in the United States and around the world are urgently examining vaccine effectiveness related to this variant. I strongly encourage the 47 million adults who are not yet vaccinated to get vaccinated as soon as possible and to vaccinate the children and teens in their families as well because strong immunity will likely prevent serious illness.”

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Previously, the CDC said people should get a booster if they are 50 and older, or 18 and older and living in long term care. Otherwise, it advised that anyone 18 and older may get a booster. Now the word “should” applies to everyone 18 and older.

It will take a few weeks of testing to know for sure whether the Omicron variant is more transmissible than Delta, and whether it evades the protection offered by natural infection or vaccines. Scientists will also be looking to see if it causes more severe disease or evades the effects of treatments.

In the meantime, CDC will be watching for Omicron to appear in the US. That requires an extra step of testing as the tests used to diagnose Covid-19 won’t tell people which variant they are infected with. “I also want to encourage people to get a COVID-19 test if they are sick. Increased testing will help us identify Omicron quickly,” Walensky said.

“And finally, to stop the spread of COVID-19 we need to follow the prevention strategies we know work,” she added. These include vaccination, wearing masks, improving ventilation indoors and keeping a distance from others, especially if they are unvaccinated.

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