• Dr. Rochelle Walensky told CNBC's "The News with Shepard Smith" that "just being vaccinated with two doses may not be enough."
• Walensky said the CDC was "examining" its definition of fully vaccinated.
• Omicron has become the dominant variant in the U.S., making up 73% of all reported cases.

CDC Director Dr. Rochelle Walensky said the omicron variant of the coronavirus has "over 50 mutations and because of those mutations, just being vaccinated with two doses may not be enough" ahead of the holidays.

Walensky told CNBC's "The News with Shepard Smith" on Monday that the Centers for Disease Control and Prevention was "examining" its definition of fully vaccinated. Right now, the agency defines full vaccination as two weeks after the second dose of one of the two-shot vaccines from Moderna or Pfizer or two weeks after the one-dose vaccine from Johnson & Johnson.

Even so, Walensky said that vaccines "may not prevent infection" according to data so far. She said this means people need "to continue to wear their masks to prevent the infections overall."

Omicron has become the dominant variant in the U.S. making up 73% of all reported cases. Walensky said based on omicron's growth rate she "anticipated this number would be about this high at around this time."

The CDC director said there have been reported deaths from omicron, "although there have been a minority of them, fewer than we've seen for other variants so far."

Walensky advised vaccination and booster shots as the best prevention against death saying those who are boosted are "20 times less likely to die than our unvaccinated" population. On Monday, Moderna said its booster shot is effective against omicron, but it still plans to create a new variant-specific shot.

Ahead of the holidays Walensky recommended for families that are gathering to get tested, "to make sure … that people are not gathering when they have a symptomatic infection."