As states lift restrictions and coronavirus variants spread, scientists and federal health officials have warned that a fourth surge of cases could arise in the United States even as the nation's vaccination campaign gathers speed. The seeds of such a surge may now be sprouting in the Upper Midwest and the Northeast.

Michigan is in tough shape. New cases and hospitalizations there have more than doubled in the past two weeks, and the six metro areas in the United States with the greatest number of new cases relative to their population are all in Michigan.

Several other states in the Upper Midwest, including Minnesota and Illinois, have also reported significant increases in new cases and hospitalizations. And in the Northeast, New York and New Jersey have continued to see elevated case counts.

Illinois is seeing a spike in cases as well. The daily average for new cases has jumped about 56% in the past two weeks, to about 2,832 a day. Hospitalizations have risen about 28% from two weeks ago. Wisconsin and North Dakota have seen their average case counts jump 50% or more in the past two weeks.

While new cases, hospitalizations and deaths nationwide have declined from January peaks, new infections have increased after plateauing.

Further progress in reducing new cases has stalled, hospitalizations have leveled off and deaths remain near an average of about 800 a day, according to a New York Times database. The average number of new cases had reached nearly 65,000 a day as of Tuesday, up 19% from two weeks ago.

Scientists are particularly concerned about the rising prevalence of variants, which they say could draw out the pandemic. On Wednesday, the director of the Centers for Disease Control and Prevention said that a highly infectious variant first identified in Britain had become the most common source of new infections in the U.S.

That variant, B.1.1.7, has been found to be most prevalent in California, Colorado, Florida, Massachusetts, Michigan and Minnesota, according to the CDC.

Until recently, the variant's rise was somewhat camouflaged by falling rates of infection overall, lulling Americans into a false sense of security and leading to prematurely relaxed restrictions, researchers say.

The CDC's efforts to track down variants have substantially improved in recent weeks and will continue to expand, in large part because of the $1.75 billion in funds for genomic sequencing in the stimulus package. By contrast, Britain, with a more centralized health care system, began a highly touted sequencing program last year that allowed it to track the spread of the B.1.1.7 variant.

Andy Slavitt, a senior health policy adviser to President Biden, said the administration had not ruled out sending extra vaccine doses to Michigan.

Other states, including Minnesota, could soon follow Michigan's path.

Minnesota is averaging 1,826 new cases a day, according to the Times database. It surpassed 2,000 new confirmed cases on April 1, a daily figure not seen since early January. Hospitalizations have also climbed about 41% from two weeks earlier. Minnesota's health department has attributed recent outbreaks in schools to the variant.

Said Dr. Ruth Lynfield, Minnesota state epidemiologist: "It's a race of vaccine against variants."