HELENA, Mont. (AP) — Montana health officials reported another 121 confirmed COVID-19 cases from Saturday through Monday, marking one of the state's highest three-day totals as infections surge.

Health officials announced 39 additional cases on Saturday, 45 on Sunday and 37 on Monday, pushing the statewide total so far to almost 1,250 cases.

More than a third of the newly confirmed cases over the weekend were in Yellowstone County, which includes Billings.

The number of infections is thought to be far higher because many people have not been tested, and studies suggest people can be infected without feeling sick.

Montana's rate of infections still trails most of the nation. But the number of cases has risen sharply over the last month as restrictions on commerce and social gatherings were eased. Some local health agencies have begun adopting measures such as mandatory face coverings, which state officials have so far resisted.

The Missoula City-County Board of Health will vote Thursday on whether to mandate masks in all public spaces in the county to reduce the virus' spread.

“Voluntary mask use has occurred but not at the widespread level that we need to really reduce or slow spread,” health officer Ellen Leahy, told the Missoulian on Monday.

The proposal would apply to all indoor settings including businesses, nonprofits and government offices.

Yellowstone County has 149 known active cases, the most in the state, followed by 83 in Gallatin County, which includes Bozeman, and 74 in Missoula County.

There are now almost 550 active cases that have been confirmed statewide.

In Teton County, health officials asked anyone who attended an unsanctioned prom organized by students last month to self-quarantine after a girl who attended later tested positive for the virus, the Great Falls Tribune reported. About 40 students attended the June 27 indoor event at the Choteau Country Club.

That's fewer than the 50-person limit under state guidelines and Teton County Health Director Melissa Moyer said they followed the governor's directive on large gatherings.

“This is unfortunate,” she said. “We don’t feel the kids did anything wrong.”

For most people, the coronavirus causes mild or moderate symptoms, such as fever and cough. For some, especially older adults and people with existing health problems, it can cause more severe illness, including pneumonia and death. The vast majority of people recover.

No additional deaths were reported in Montana over the weekend, leaving the toll at 23 people killed by COVID-19. Twenty people remained hospitalized with the virus as of Monday.