When Michigan's Saint Clair County held a walk-in COVID-19 vaccination clinic last week with 1,600 shots at the ready, only about 800 people signed up in advance. Walk-in traffic was slow. A quarter of the available shots went unused, Jennifer Michaluk, a county health department official, was quoted as saying by Reuters on Thursday.

Saint Clair County, which sits on the Canadian border just northeast of Detroit, is a COVID-19 hotbed. Earlier in the month, 30 percent of virus tests were coming back positive. Michigan has seen the biggest case spike of any state in recent weeks.

Despite the surge, health officials for more than 20 counties in Michigan said that eligible residents are not showing up for their shots, particularly in rural areas that supported Republican Donald Trump in November's election, increasingly inhibiting their ability to contain the outbreak.

In the early stages of the vaccination campaign, officials said, people were knocking down the doors. But this month, even as the state opened up the shots to all adults, counties across Michigan like Saint Clair County noticed the once-crushing demand was beginning to lessen.

"For a 500-appointment clinic, we are getting 50 to 60 percent of the appointments filled. A month ago, we could fill a clinic in 15 minutes flat," said Danielle Persky, deputy health officer for Cass and Van Buren counties, which sit near Lake Michigan in the southwestern part of the state and strongly supported Trump in November.

While there are areas such as Oakland County outside of Detroit where demand still outstrips supply, other regions in the state say that just about everyone who wants a shot can get one now. Experts worry that momentum is slowing, both in Michigan and elsewhere in the United States.

As of April Wednesday, 40.9 percent of the U.S. population is now vaccinated against at least one COVID-19 dose, while the number of new cases continues to rise with an average of 62,595 in the past week, according to the U.S. Centers for Disease Control and Prevention.

More than 135 million Americans have received at least one shot. However, vaccinations are lagging in parts of the Midwest with a week-over-week decline.

Meanwhile, the Northeast and Midwest were once again hardest hit by the new wave of COVID-19 outbreak, with more than 30,678 people infected in Pennsylvania last week, the most since January.

The slowing rollouts of coronavirus vaccines in the U.S. have been fueling concerns that vaccine supply will soon exceed demand. The seven-day moving average of vaccination doses has dropped below three million per day, data from the CDC indicate.

(With input from Reuters and AP)