The CDC put half of Mass. at a ‘high’ community level for COVID. Here’s what that means.

The U.S. Centers for Disease Control and Prevention issued “high” COVID community levels for half of the counties in Massachusetts this week, as cases and hospitalizations rise.

Suffolk, Middlesex, Norfolk, Worcester, Franklin, and Berkshire counties are all in the red, or at a “high” community level, a CDC map updated on Thursday shows. Of the other seven counties in the state, only Bristol County was listed as having a “low” community level while the rest are at the “medium” level.

Jonathan Levy, the chair of the environmental health department at Boston University, said Massachusetts surpassed CDC thresholds for hospital admission rates this week, prompting the agency to elevate the county-wide levels.

“We’re certainly seeing cases continue to climb,” Levy told Boston.com. “Hospitalizations are growing and the wastewater data, after trending down a little bit, is ticking back up again, although it’s kind of wobbly in some places. So, we certainly are not yet on a downslope.”

Community levels are intended to help prevent strain on the health care system by providing communities and individuals contextualized virus risk to help them make decisions.

The CDC combines three metrics to determine the levels: new COVID hospital admissions per a 100,000 population in the past 7 days; the percent of staffed inpatient beds occupied by COVID patients; and new cases per 100,000 population in the past week, according to the agency.

“We’re seeing an appreciable, but somewhat quiet, wave happening right now in in Massachusetts, and so that means risk of exposure is quite high,” Levy said.

Indeed, the presence of COVID RNA is rising once again in wastewater sampled from around Boston and the surrounding suburbs by the Massachusetts Water Resources Authority.

Although the levels of the virus in the wastewater — which has helped forecast surges over the course of the pandemic — is not at the heights seen during the winter, there appears to be an increasing presence even after the levels rose and fell in April.

On Thursday, the state Department of Public Health reported 4,376 new COVID cases — the highest number of cases reported in a single day since Feb. 3.

The COVID test positivity rate in Boston, recorded on a seven-day moving average, hit 9.6 percent — well above the city’s 5 percent threshold, data from the Boston Public Health Commission showed.

The number of positive tests, measured on a seven-day average, was also above the threshold of 339.7 cases at 401.6 per day. Hospitalizations, however, were below the city-set threshold, with 88 percent of ICU beds occupied.

In keeping with CDC guidance for areas with a “high” community level, the BPHC on Friday advised residents to wear a mask in indoor public places, test for COVID-19, and get vaccinated and boosted.

According to the CDC, people living in “high” level areas should wear a “well-fitting” mask when indoors in public, regardless of vaccination status. The agency also advises people to “maintain improved ventilation throughout indoor spaces when possible.”

People who are immunocompromised or at high risk for severe disease should wear a mask or respirator that offers stronger protection and should consider avoiding non-essential indoor activities, among other guidance, the CDC says.

At the larger, community level, the CDC also recommends cities and towns issue “setting-specific” guidance to help prevent virus spread, as one of several steps officials could take.

“N95 masks work wonderfully and can reduce your exposure risk,” Levy said. “But if you’re in a poorly ventilated indoor setting, and those around you are not wearing masks at all, it may not be enough to fully protect you and certainly there are people, such as kids who can’t wear N95 masks all day. So, you know, we again … we have to think about everybody in our community and think about who can make individual risk assessments and act accordingly and who cannot.”

While the BA.2 omicron variant has now, for some time, been suspected as the driving factor behind new cases, a related variant known as BA.2.12.1 is even more transmissible than its quick-moving relative. Research has shown both variants are present in New England.

With the prevalence and availability of at-home COVID testing, Levy said many new cases are likely left off of official data reports. He estimated Massachusetts is probably experiencing “about three-and-a-half times the number of cases” than case totals being reported by officials.

Levy is particularly concerned how the latest trends will impact individuals who are most vulnerable to severe illness.

“Obviously, a lot of people are making their personal decisions about what they are and are not willing to do given high case rates,” Levy said. “I do think we have to keep a lens on those who don’t have that kind of choice or do not have the same types of protections.”

Levy pointed out that there remain many people who have not received vaccine booster shots, despite being eligible for the additional doses.

“Even among those who are over age 75, there’s a good fraction who have not even received a first booster,” he said. “It’s a little disconcerting that the population with the highest case rate growth in recent weeks has been those at 80-plus.”

Levy said it’s important the CDC recommendations are reinforced.

Public health and elected officials will have to make decisions about what comes next, he said.

“It’s a complex time,” Levy said. “People are tired of mandates, but presuming that everyone has the data and information and the ability to make complex individual risk assessments and have no constraints on their day-to-day activity is naive and is not really going to protect those who do not have all the layers of protection in place.”