Here’s how low COVID-19 case stats should go before people stop wearing masks indoors, according to the CDC

Free N95 masks provided by the U.S. Department of Health and Human Services on display at a Mississippi grocery storeRogelio V. Solis/Associated Press

Massachusetts COVID-19 case counts are declining as the Omicron surge eases, and some cities and towns are dropping mask mandates. But the US Centers for Disease Control and Prevention continues to recommend that people wear masks in indoor public spaces.

Here’s a look at how far cases would have to fall before CDC guidelines would recommend that people doff their masks indoors.

The CDC recommends that in areas of “substantial” and “high” coronavirus transmission, even if people are “up to date” with their vaccines – meaning fully vaccinated and boosted, if eligible – people should wear a mask in indoor public spaces “to maximize protection and prevent possibly spreading COVID-19 to others.”

(If you’re unvaccinated, or not up to date with your vaccines, or immunocompromised, you should wear a mask in indoor public spaces no matter what, the CDC recommends.)

How does the CDC define substantial and high transmission?

One measure it uses is new cases per 100,000 persons in the past 7 days. Substantial and high transmission are defined as 50 to 100, and 100 and higher, cases per 100,000 residents, respectively.

Massachusetts counties have a long way to go. The counties with the lowest cases per 100,000 residents in the past seven days are Nantucket and Dukes counties, with 307 and 317 cases per 100,000 in the past 7 days.

Franklin and Berkshire counties are doing the worst in the state, with 738 and 748 cases per 100,000 in the past 7 days.

Massachusetts counties also fall short when looking at another metric the CDC uses to determine transmission levels, percentage of positive tests over the past 7 days. (The CDC uses whichever metric suggests levels are higher.)

Massachusetts counties have plenty of company in having substantial and high transmission levels. The CDC says 99.26 percent of US counties have high levels of community transmission and another 0.25 percent have substantial levels.

The Omicron surge may be waning, but more than 99 percent of US counties remain at high levels of community transmission of COVID-19, according to the CDC.CDC

Julia Raifman, an assistant professor of health law, policy, and management at the Boston University School of Public Health, said, “We all wish the pandemic was behind us. It’s here, and it’s still causing harm.”

“Right now, we still see transmission is high. It makes sense to keep [masks] on longer now,” said Raifman, who is a proponent of “data-driven” mask policies that turn mask requirements on and off based on pandemic metrics. The tactic has been tried in Nevada using the CDC transmission level numbers.

Massachusetts did not mandate masking in indoor public spaces this winter, but did recommend, as Omicron surged, that “all residents, regardless of vaccination status, wear a mask or face covering when indoors (and not in your own home).”

Masking in schools has been in the headlines in recent days as some governors have dropped statewide school mask requirements due to improvements in the pandemic. The CDC continues to recommend universal masking in schools, but White House spokeswoman Jen Psaki said in a briefing Monday, “It’s always up to local school districts to determine how they implement.”

Debate is growing in Massachusetts about whether the state’s mandatory school mask rules should be revised or removed. The policy, in effect through Feb. 28, calls for the requirement to be lifted if at at least 80 percent of all students and staff in a school building are vaccinated.

Massachusetts cases have fallen precipitously statewide since reaching an unprecedented peak early this year, but they have not yet reached the lows the state saw last summer.