Even as the coronavirus wave driven by the delta variant is receding in much of the United States, many counties across the country’s northernmost regions are experiencing rising cases as colder weather arrives.

The top five states in new daily cases per capita are led by Alaska, which is logging the highest daily average: 125 cases per 100,000 people, according to a New York Times database. The next four states, with at least 67 cases per 100,000 people, are Montana, Wyoming, North Dakota and Idaho.

Cases are at least trending downward or holding steady in those states. The five states with the fastest rising caseloads are Vermont, Colorado, New Hampshire, Michigan and Minnesota, and the two counties with the most cases per capita in Vermont and New Hampshire are on the Canadian border.

The virus followed a similar pattern last fall: Cases receded in the Southern regions after summer surges, while they steadily increased throughout the North as the weather became colder and people moved indoors.

The big difference this year is that COVID-19 vaccines are widely available, and most experts don’t expect another catastrophic winter wave, but they are warning Americans not to let their guard down as long as a large portion of the population remains unvaccinated.

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In Minnesota, the average reported cases have climbed by 12% in the past two weeks. Scott Smith, a spokesperson for Minnesota’s health department, said in an email that the department was more concerned about factors such as the reopening of schools and relaxed mitigation measures than wintry weather.

Dr. Rafael Meza, a professor of epidemiology at the University of Michigan, said increases were happening across Michigan but appeared to be higher in the center and in the Upper Peninsula. Meza said factors such as vaccination rates and school mask mandates could be part of the reason.

Cases have been high in Michigan’s school-aged children, especially in districts that do not have mask mandates, according to Dr. Natasha Bagdasarian, the state’s chief medical executive.

“The weather drives people indoors into poorly ventilated spaces, and when either academic activities or social activities occur without masks in indoor, poorly ventilated spaces, that’s when transmission occurs,” Bagdasarian said, adding that winter was “coming at a very bad time for us here in Michigan.”