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 last 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 there has jumped about 56 percent in the past two weeks, to about 2,832 a day. Hospitalizations have risen about 28 percent from two weeks ago. Wisconsin and North Dakota have also seen their average case counts jump 50 percent or more in the last two weeks.

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