As COVID-19 and other respiratory infections rise across the country, some major health systems are bringing back mask requirements to stop the spread of infections. 

This week, Mass General Brigham, the largest health system in Massachusetts, said it will require masking for health care staff who interact directly with patients in clinical care locations starting Jan. 2. 

Patients and visitors will be “strongly encouraged” to wear a facility-issued mask. Masks will not be required for staff in hallways and common areas. 

The health system in a statement said its policy is based on the percentage of patients presenting to emergency departments or outpatient clinics with symptoms of respiratory illness.

Once this figure exceeded 2.85 percent for two consecutive weeks, the masking requirement took effect. It will end once the level falls below 2.85 percent for a week.  

Most hospitals eased or eliminated their masking requirements last spring, after the federal government ended the COVID-19 public health emergency.

Another major Boston hospital, the Dana-Farber Cancer Institute, began requiring masks for patients and staff on Dec. 18. The hospital said the policy would be in place for the “foreseeable future.” 

The hospital said there is no single data point leaders use to gauge when the requirement will end. 

“We check a number of different data points, including rates of influenza-like illness, staff absenteeism, and emergency room visits and hospitalizations caused by respiratory viruses. We will lift the mask requirement when these data points remain consistently lower,” the hospital said on its website. 

Meanwhile, in D.C., the region’s largest rehabilitation hospital is requiring masks for all staff and patients who are admitted, but not for visitors or outpatient areas.

In a message to staff, Medstar National Rehabilitation Hospital said the move was meant to protect staff because there’s been an increase in positive COVID tests on admission. As a result, there’s been an increase in exposure from patients to staff members. 

And in Wisconsin, UW Health recently returned to requiring masks in medical clinics, outpatient care and waiting rooms.

The new requirements come as the JN.1 variant has become the most common strain of the virus spreading across the United States.  

According to the Centers for Disease Control and Prevention, the variant accounted for 44 percent of COVID-19 infections nationwide by mid-December, up from about 7 percent in late November.