That leads some authorities to pin much of the increase on another culprit: the B.1.1.7 variant of the virus, first identified in Britain, which has proved to be more contagious and potentially deadlier than the original version that spread in the United States.
Federal health officials have expressed concerned about the spread of variants, as the United States remains behind in its attempts to track them. Britain, with a more centralized health care system, began a highly touted genetic sequencing program last year that allowed it to track the spread of the B.1.1.7 variant.
But the Centers for Disease Control and Prevention’s efforts to locate the variants have substantially improved in recent weeks and will continue to grow, in large part because of $1.75 billion in funds for genomic sequencing in the stimulus package that President Biden signed into law this month. Dr. Rochelle Walensky, the agency’s director, told lawmakers last week that between 10,000 and 14,000 test samples were being sequenced each week to locate variants, and that the C.D.C. was aiming for about 25,000.
Officials in Michigan said this week the B.1.1.7 variant has turned up in about one-eighth of the thousands of positive test samples that the state has genetically analyzed so far this year. In mid-March, state health officials said Michigan accounted for 15 percent of all known cases of the variant in the U.S., second only to Florida.
Dr. Lyon-Callo, the state’s chief epidemiologist, said this week that although the B.1.1.7 variant is certainly a player in the increasing rate of new cases, more than half of the 908 confirmed variant cases in Michigan stem from a January outbreak at state prisons. That suggests that the variant is less widespread in the general population than the numbers might indicate.
Dr. Jennifer Morse, the medical director for district health departments in 19 counties in Michigan’s lower peninsula, said the nature of outbreaks in the region suggests that the variant has spread well beyond the limited number of documented cases there.
“We’ve had several outbreaks, and some have been quite rapid and large,” she said. “I do feel it’s very prevalent in our area, looking at how our numbers are going and how it’s spreading.”