WASHINGTON, Dec 21 (Reuters) - U.S. President Joe Biden announced on Tuesday the opening of more federal vaccination and testing sites to tackle a surge in COVID-19 cases sparked by the Omicron variant, and said some 500 million at-home rapid tests will be available to Americans for free starting in January. Biden offered both a warning to the unvaccinated, who he said have "good reason to be concerned," and reassurance that those who are inoculated can gather for the holidays despite the new variant sweeping the country. "No this is not March of 2020," Biden told reporters at the White House. "Two hundred million people are fully vaccinated, we"re prepared, we know more." Striking a more dire tone about the risks to the one-in-four American adults who are not fully vaccinated, Biden said those who still have not been vaccinated "have a significantly higher risk of ending up in the hospital or even dying." The measures announced on Tuesday include activating some 1,000 military medical personnel to support hospitals already being overwhelmed by COVID patients in some areas. Biden"s COVID-19 response has been criticized for focusing on vaccines at the detriment of testing and masking, and for underestimating the impact of the politically motivated anti-vaccine movement in the United States. The new federal measures will not be fully in place ahead of Christmas on Saturday, leaving many Americans scrambling to find available tests ahead of holiday gatherings and travel - and confused about whether it is safe to press ahead with their plans. The Omicron variant, first detected last month, is causing infections to double in 1.5 to 3 days, according to the World Health Organization. It is not yet known whether it causes more serious illness than the Delta variant. Omicron now accounts for 73% of all new cases, according to the latest U.S. data, up from less than 1% at the beginning of the month. read more The rapid rise of infections is once again disrupting life across the country, canceling events from Broadway shows to professional sports. On Monday, the National Hockey League and its players" union agreed to postpone all games until after Christmas following Tuesday"s contests, one of which was postponed earlier on Tuesday due an outbreak among the Washington Capitals. read more In New York, Washington and other U.S. cities there were long lines for COVID-19 tests as people clamored to find out if they were infected before celebrating the holidays. "If I don’t I could take a chance of sending it home to my family, and I’m not trying to do that," said Ronald Tives as he lined up to get tested in Washington"s Farragut Square on Tuesday. The rollout of at-home rapid tests adds to Biden"s plan announced earlier this month that called on health insurers to provide test kits at no cost to individuals with coverage, also expected to begin in January. U.S. President Joe Biden speaks about his administration"s plan to fight the coronavirus disease (COVID-19) with the emergence of the Omicron variant, during his visit to the National Institutes of Health, in Bethesda, Maryland, U.S., December 2, 2021. REUTERS/Kevin Lamarque A woman gets a COVID-19 test during the coronavirus disease pandemic in the Manhattan borough of New York City, New York, U.S., December 17, 2021. REUTERS/Carlo Allegri U.S. President Joe Biden speaks about his administration"s plan to fight the coronavirus disease (COVID-19) with the emergence of the Omicron variant, during his visit to the National Institutes of Health, in Bethesda, Maryland, U.S., December 2, 2021. REUTERS/Kevin Lamarque 1/2 A woman gets a COVID-19 test during the coronavirus disease pandemic in the Manhattan borough of New York City, New York, U.S., December 17, 2021. REUTERS/Carlo Allegri The administration will open multiple federal testing centers, starting in New York City ahead of Christmas, Biden said. More federal vaccination and testing sites will be opened across the country in areas of high need and when requested by local and state officials. Top White House medical adviser Dr. Anthony Fauci said on Tuesday the government also is mulling reducing the recommended 10-day quarantine time for people who get infected, to help asymptomatic people return to work or school. read more $100 FOR A BOOSTER IN NYC With new infections spiking, local officials are also taking steps to encourage vaccinations, including boosters, and increase testing and mitigation measures. New York City Mayor Bill de Blasio announced on Tuesday a $100 incentive for residents who get a COVID-19 vaccine booster at city-run sites by the end of the year. Biden noted that the Omicron variant is so contagious that it will infect vaccinated Americans but that they will be far less likely to become seriously ill. These breakthrough infections are rising among the 61% of the country"s fully vaccinated population, including the 30% who have gotten booster shots. Still, Biden will tell Americans that those who are vaccinated and follow guidance around using masks, especially while traveling, should feel comfortable celebrating the holidays as planned. New COVID-19 cases rose 19% in the United States in the past week and are up 72% since the start of December, according to a Reuters tally. Nearly 283,000 new infections were reported on Monday, the most new cases in a single day since Sept. 3, although some states are reporting multiple days of test results. The number of hospitalized COVID-19 patients has increased 27% this month, with hospitals in some areas already strained by the Delta variant that emerged earlier this year. There have been more than 51 million infections and 810,000 coronavirus-related deaths reported in the United States since the pandemic began, the most of any country.