WASHINGTON, Dec 17 (Reuters) - The U.S. Centers for Disease Control and Prevention (CDC) released a new strategy called "test-to-stay" that allows unvaccinated children to stay in school even if they have been exposed to the coronavirus, agency Director Rochelle Walensky said on Friday. "If exposed children meet a certain criteria and continue to test negative, they can stay at school instead of quarantining at home," Walensky said during a press briefing. Some states are already advising their schools to use "test-to-stay" strategies in order to keep more children in class. Schools must test their students twice a week to implement the test-to-stay strategy, Walensky said, adding that many schools already meet that standard. A general view of the U.S. Centers for Disease Control and Prevention (CDC) headquarters in Atlanta, Georgia September 30, 2014. REUTERS/Tami Chappell/Files Children arrive at school, amid the coronavirus disease (COVID-19) pandemic in Brooklyn, New York, U.S. October 4, 2021. REUTERS/Brendan McDermid/Files A general view of the U.S. Centers for Disease Control and Prevention (CDC) headquarters in Atlanta, Georgia September 30, 2014. REUTERS/Tami Chappell/Files 1/2 Children arrive at school, amid the coronavirus disease (COVID-19) pandemic in Brooklyn, New York, U.S. October 4, 2021. REUTERS/Brendan McDermid/Files The new guidance comes as the Omicron variant continues to spread in the United States. More than 39 states and 75 countries have reported cases of the new variant, which is highly contagious and infects vaccinated people at elevated rates. "We expect it to become the dominant strain in the United States, as it has in other countries, in the coming weeks, Walensky said. Some data has suggested that cases of Omicron are less severe than past variants, but top U.S. infectious disease expert Anthony Fauci said the severity of the new variant is "still up in the air." Fauci said that third booster shots of currently authorized COVID-19 vaccines provide increased protection against Omicron. He said there has been no decision yet on whether to encourage people to get boosted sooner than six months after their initial inoculations.