WASHINGTON, Dec 3 (Reuters) - The Delta variant is still Americans" leading coronavirus health threat even as reports surface of the Omicron strain in a sixth U.S. state on Friday amid concern that winter holidays may further the spread, experts said. Nebraska on Friday reported six confirmed cases of the highly contagious Omicron strain, the state"s health department said. Only one of the six people was vaccinated and none have needed to be hospitalized with COVID-19, the department said.While Omicron has been grabbing headlines as the newest coronavirus variant, the predominant U.S. strain remains the Delta variant, said U.S. Centers for Disease Control and Prevention Director Rochelle Walensky at a White House briefing. Scientists are investigating the impact of Omicron, with some questioning whether this coronavirus strain may cause milder illness than its predecessors. Walensky and other health experts said the best way to fight the spread of COVID-19, regardless of the strain, remains to get vaccinated. Omicron is the newest challenge ahead of winter and the holiday season that will only further weigh on the nation"s hospitals, said Dr. Ashish Jha, dean of Brown University"s School of Public Health. More cases loom large for hospitals already stressed by high caseloads and fatigued staff, as well as the addition of influenza cases, Jha told MSNBC. "I"m very worried about our healthcare system over the next few weeks and few months. I don"t know how much more it can handle," he said, urging reinforcements from the National Guard and other contingency plans to bolster region"s doctors and nurses. Former FDA Commissioner Dr. Scott Gottlieb said states with higher vaccination rates would likely be better protected against Omicron, saying unvaccinated people who survive Delta could find their immunity is not protective against the new variant. "The risk for the future is that states that are relying on a lot of Delta infection to provide immunity to their populations ... they could be more susceptible to this new variant," Gottlieb, now a member of vaccine maker Pfizer"s board, told CNBC. There have been cases of Omicron detected in about 40 countries, including the United States, where it was found in California, New York, Colorado, Minnesota, Hawaii and Nebraska. In Nebraska, one case involved a traveler who returned from Nigeria on Nov. 23, while the other five cases were likely exposed through household contact with that same person, the department said. The CDC is working with local authorities to investigate suspected cases of the Omicron variant in states other than those where cases have already been reported.