LANSING, Mich. (AP) — Michigan, which had one of the country’s lowest COVID-19 infection rates not long ago, is confronting an alarming spike that some experts worry could be a harbinger nationally.

In what public health authorities across the U.S. have been warning for months might happen around the country, the resurgence is being fueled by loosened restrictions, a more infectious variant, and pandemic fatigue.

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While vaccinations in Michigan help protect senior citizens and other vulnerable people, the upswing drives up hospitalizations among younger adults and threatening in-person instruction at schools.

“It’s a stark reminder that this virus is still authentic. It can come roaring back if we drop our guard,” said Democratic Gov. Gretchen Whitmer, who does not plan to tighten restrictions on indoor dining, sports, and other activities that were eased in recent months.

Michigan last year saw some of the nation’s fiercest resistance to lockdowns, including armed demonstrations at the state Capitol and an alleged right-wing plot to kidnap the governor.

Whitmer contended Thursday at a Michigan Chronicle event that this surge is different from last fall and spring in part because of the vaccine, which has been given to nearly two-thirds of residents 65 and older — those most at risk of dying. COVID-19 hospitalizations, though higher in recent weeks, remain below the peak from December.

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Still, the coronavirus’s spread is concerning when many adults have not yet gotten a dose.

Over the past two weeks, Michigan’s seven-day average of new cases per day has increased 122% — the largest change in the U.S. — rising to 3,753 from 1,687, the biggest jump in raw figures, too. The state’s seven-day per-capita case rate is third-highest, behind New York and New Jersey.

Nationwide, COVID-19 has killed more than 545,000 people. With the vaccine rollout hitting its stride, deaths have plummeted to fewer than 1,000 a day on average, down from a peak of more than 3,400 in mid-January. New cases have plunged as well but are running at a still-worrisome average of more than 57,000 per day, and the trend is reversing itself in some states.

Michigan will soon require regular testing of all teen athletes amid outbreaks that occurred after a months-long ban on contact sports was lifted. About 100 teams are sitting out the boys’ and girls’ state basketball tournaments.

And some schools are temporarily closing or delaying their long-anticipated reopenings.

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