LANSING, Mich. (AP) — Nearly half of Michigan residents live in counties where the federal government is urging everyone ages 2 and over, including the fully vaccinated, to wear masks in public indoor settings because transmission of the coronavirus is “high” or “substantial.”

The guidance affected 33 of 83 counties as of Monday — including large ones such as Oakland, Macomb, and Ingham — up from 10 mostly small, rural counties when the Centers for Disease Control and Prevention’s recommendations were revised nearly a week ago. The counties are home to more than 4.6 million residents or 46% of the state’s population.

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Michigan’s seven-day average of new daily COVID-19 cases was 678 on Saturday, an increase from 241 two weeks before. The case rate, 76.8 per 100,000 people, was lower than in all but four states, based on data compiled by Johns Hopkins University.

Gov. Gretchen Whitmer does not plan to reinstate a mask requirement, but she has urged people to get vaccinated and wear a face covering when inside and among groups. Nearly 57% of residents ages 12 and up are fully vaccinated, which is slightly under the national average of 58%. Michigan’s vaccine rate ranks around the middle of the pack among states.

Six counties have “high” transmission rates, which means they had at least 100 new infections per 100,000 residents over the past seven days, a test positivity rate of at least 10% or both. Twenty-seven counties have a “substantial” spread — a weekly case rate of between 50 and 99.9 per 100,000 people, a positivity rate between 8% to 9.99%, or both.

The CDC guidance has prompted criticism from Republican leaders nationally. But at Beaumont Health, the state’s largest system, a physician said individuals should probably return to universal masking when they are going to stores or gathering indoors in large groups.

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“If it helps prevent spread, then it helps prevent replication of the virus and would help prevent any further infections with delta variant or any future variant. That’s why we need to get on top of this right now,” said Dr. Joel Fishbain, medical director for infection prevention at Beaumont’s hospital in Grosse Pointe.

He expressed concern that vaccinated people can spread the coronavirus’ delta variant, but he said vaccination remains “very, very good.” More than 99.9% of the eight-hospital system’s recent COVID-19 patients were unvaccinated, he said.

“The vaccine works and prevents the vast majority of people from becoming severely ill, getting admitted and dying,” Fishbain said.

He warned of the potential for a “huge disaster,” an explosion of cases particularly among the unvaccinated, including those who are ineligible — kids younger than 12. While infected children tend not to get sick or as sick as adults, he said, they could spread the virus to vulnerable people.

The state health department announced Monday that it was investigating an outbreak associated with Muskegon Bike Time, a July 15-18 motorcycle event that included concerts. At least 16 cases had been identified among attendees and their contacts. Attendees were urged to get tested.

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