But the Delta variant is already fueling another increase in Covid-19 hospitalizations in some states as the weather gets colder and people spend more time indoors.

Despite several unknowns about the Omicron strain, doctors say booster shots are the best weapon against Omicron, Delta and even more variants down the road.

Anthony Fauci said there's plenty of reason to get vaccinated and boosted as soon possible.

"We know from experience that even with variants that are not specifically directed at by the vaccine, such as the Delta variant, if you get the level of antibody high enough, the protection spills over to those other variants," said Fauci, director of the National Institute of Allergy and Infectious Diseases.

The 20 nations that have confirmed the Omicron variant in their Covid-19 cases includes Brazil, which on Tuesday became the first Latin American country to have citizens affected by that variant.

That's the reason for the concern," Collins said.

But here's the good news: "All of the previous variants, which have also had differences in the spike protein, have responded to vaccines -- and especially boosters," Collins said.

The Delta variant keeps spreading this holiday season

The battle against the Delta variant is far from over as some states grapple with increasing hospitalizations.

At least 20 states have reported more patients hospitalized with Covid-19 this past week than the previous week, according to data Tuesday from the Department of Health and Human Services.

Many of those states are in colder parts of the country, such as the Northeast and Midwest.

Rochelle Walensky said the increased surveillance will take place in Atlanta, New York, San Francisco and Newark, New Jersey.

Vaccines (and boosters) help prevent more variants

Many of the people weary of hearing about new variants -- or the possibility of new Covid-19 restrictions -- can help stop the cycle by getting vaccinated and boosted, experts say.

"The virus mutates when people get infected.

Get your booster, CDC urges

While the CDC doubled down on its recommendation for eligible Americans to get booster shots, more Americans may be eligible for boosters soon.

"Everyone ages 18 and older should get a booster shot either when they are six months after their initial Pfizer or Moderna series or two months after their initial J&J vaccine," Walensky said Monday.

"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," Walensky said in a written statement.