Michigan’s top public health officials worry that rising numbers of the Delta coronavirus variant may spark another surge of COVID-19 cases.

The variant, which has triggered waves of new cases in India, the United Kingdom and southeast Asia, has increased in Michigan for the past three weeks, with cases detected in 15 counties, up from six counties two weeks ago.

In a report issued Wednesday, a team led by the state’s top epidemiologist, Dr. Sarah Lyon-Callo, warned that “there is potential for another surge in Michigan” as the variant spreads because it is 50 percent more contagious than the B.1.1.7 variant, now known as Alpha.

The Delta variant has prompted Los Angeles County in California to recommend — but not mandate — the use of masks indoors, even for those who are fully vaccinated.

In addition, the World Health Organization now suggests everyone wear masks and follow social distancing and hygiene guidelines. The U.S. Centers for Disease Control and Prevention has not changed its recommendations that those who are fully vaccinated, in most cases, don’t need masks.

The variant is blamed for a fivefold increase in the U.K. since June 1, but it’s too early for Michigan to add new restrictions, argued Dr. Arnold Monto, an infectious disease expert at the University of Michigan.

“I don’t think it’s time to change anything,” he said.

Gov. Gretchen Whitmer lifted the last major restrictions in Michigan just last week, and the state has a similar vaccination rate as the U.K. and Israel, where hospitalizations have remained stable despite case spikes.

Roughly 60 percent of Israelis are fully vaccinated, compared to nearly 50 percent in the U.K. and 47 percent in Michigan (where nearly 55 percent of those 12 and older are fully vaccinated).

Michigan epidemiologists estimate that if the Delta variant had been the dominant strain during the winter-spring surge, cases would have topped 13,000 per day — nearly double the 7,000 daily cases the state reached when Michigan had the highest rates in the country.

The good news is public health officials say those who have had both doses of the Pfizer-BioNTech or Moderna vaccines or the Johnson & Johnson single-dose vaccine are protected from the Delta variant.

But just one dose of the two-dose vaccines only offers limited protection against the virus, according to reports.

And the new monoclonal antibody treatments being used to treat the disease have proven less effective in COVID-19 cases caused by the Delta variant.

Statewide, nearly 5 million people — 61.7 percent of those 16 and older — have at least one dose of the vaccine and 52 percent of everyone 12 and over has completed their vaccinations.

But there are parts of the state where rates are half that high; neighborhood data released in May showed some parts of Detroit, Flint, Saginaw and some rural areas were 20 to 30 percentage points below the statewide vaccination rates.

Monto said he fears what will happen when the Delta variant — or any variant — gets into areas where fewer people are vaccinated.

“The virus seems to have an uncanny ability to find pockets” of people who have neither vaccine immunity or natural immunity from a prior COVID-19 infection, Monto said. “I’m concerned about the pockets.”

The rising fears about the Delta variant come as new coronavirus cases in Michigan are at their lowest levels since just weeks after the pandemic began, the result of mass vaccinations and warmer weather.

Dr. Matthew Sims, who oversees infectious disease research at Beaumont Health in metro Detroit, said a rise in cases from the Delta variant is inevitable. But it likely won’t create a surge that ripped through Michigan from February through April, he said.

“I think there is a limit (to the number of new cases),” Sims said. “I don’t think we’re going to hit where we were in the last surge.”

Still, because the variant is so transmissible, he expects there will be pressure to recommend indoor mask usage and social distancing. He said  many state residents, who just discarded their masks, won’t be happy.

“I think there will be a lot of pushback against (masks),” he said.

Despite the overall reduction in cases and hospitalizations, hospital officials are keeping an eye on new cases and the Delta variant, said Ruthanne Sudderth, a spokesperson for the Michigan Health and Hospital Association.

“Our concern continues to be that its increased transmissibility will disproportionately impact communities across Michigan with lower vaccination rates,” Sudderth wrote in an email to Bridge Michigan. “Our hospitals are urging their communities to get fully vaccinated as quickly as possible to protect both themselves and their children who aren’t old enough to be vaccinated yet.”

That’s the same message from the state, Monto and others.

“The key is to keep encouraging people to get vaccinations,” Monto said.