Paramedics arrive with a patient with Covid-19 at the emergency department of Sharp Memorial Hospital in San Diego, California.Bing Guan | Bloomberg | Getty ImagesNew cases of Covid-19 are once again on the rise across more than half of the United States as officials race to vaccinate additional people before highly contagious variants become prevalent in the country.As of Sunday, the seven-day average of new cases rose by 5% or more in 27 states, according to a CNBC analysis of data compiled by Johns Hopkins University. Across the U.S., the nation logged an average of 54,308 new cases per day over the past week — a 1% rise from the prior week after months of rapidly declining case numbers, according to the data.Even as the U.S. picks up the pace of vaccinations, giving about 2.5 million shots every day, some health officials have warned the country remains in a precarious spot. The lifting of restrictions in many states and the spread of more contagious variants in the U.S. threaten to undo the nation’s progress, which has seen cases, hospitalizations and deaths all fall dramatically since the peak earlier this year.’Serious threat’Lifting restrictions is a “serious threat to the progress we have made,” Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, told reporters at a news briefing Monday. Walensky said she’s worried about an unavoidable surge in cases. “We are at a critical point in this pandemic, a fork in the road.”Daily new deaths continue to fall, likely helped by the prioritization for vaccination of the elderly and those with comorbid conditions who are most likely to die of Covid-19. About 68.8% of those 65 and older have received at least one shot of a vaccine as of Sunday, according to the CDC. In total, more than 124.4 million doses have been administered, but most of those are for two-dose vaccines.As optimism around the steady rise in vaccinations picks up, many states have begun to ease restrictions on businesses and gatherings, despite warnings from the CDC not to do so. Though some states, such as New Jersey, are beginning to consider holding off on further reopening as cases begin to rise.White House Chief Medical Advisor Dr. Anthony Fauci urged states last week not to declare victory prematurely.Fauci said he was concerned about a number of states and cities that were pulling back on public health measures and dropping mask mandates.”So it is unfortunate but not surprising to me that you are seeing increases in number of cases per day in areas — cities, states, or regions — even though vaccines are being distributed at a pretty good clip of 2 (million) to 3 million per day,” Fauci said Friday. “That could be overcome if certain areas pull back prematurely on the mitigation and public health measures that we all talk about.”B.1.1.7 in MichiganAdding to the urgency of the need to get people vaccinated quickly is the looming threat of new variants, which appear to have already caused severe surges across much of Europe and other parts of the world. The CDC has projected that the more contagious and potentially more deadly B.1.1.7 variant, which was discovered in the United Kingdom, could become the dominant strain in the U.S. by the end of the month.Some health officials have said that the B.1.1.7 variant could be what’s behind some particularly worrying surges seen in various states, including Michigan, where cases have risen dramatically in recent days. According to data from Johns Hopkins, Michigan is reporting an average of almost 3,000 new cases per day, up by about 50% from a week ago.The rise in cases there comes after the state allowed restaurants to reopen for indoor dining on Feb. 1, pushed for schools to offer in-person learning by March 1, and eased restrictions on restaurants and gatherings earlier this month. The recent surge there even prompted Fauci recently to plead with Democratic Gov. Gretchen Whitmer to “hold off for a bit” on reopening.Along with the easing of restrictions, Michigan also has the second-highest count of B.1.1.7 cases of any state in the country, behind only Florida. The state has 616 cases of the variant, according to the CDC, though there are likely many more that have not yet been confirmed, which means the strain could be driving the outbreak there.