Loosened federal recommendations that allow COVID-infected students to return to class after five days of isolation won’t apply to many Philadelphia children because school conditions make relaxing the guidelines unsafe, the city’s top health official said Wednesday.

“The return after five days is unduly risky for many schools,” Cheryl Bettigole, the Philadelphia health commissioner, said in a morning news conference. She said a lack of school funding “means that many of the layers of mitigation recommended by the CDC cannot be attained at all schools.”

In the same news conference, Bettigole also said the Philadelphia Department of Public Health would no longer require a school to close if more than 10% of its students test positive for the virus.

“This guidance is our best current attempt to keep children learning in-person as much as possible while keeping children, teachers, and other staff members as safe as possible,” Bettigole said.

The recommendations from the city health department follow the latest CDC guidelines in reducing the number of days infected students and staff must isolate — although schools can’t adopt the shorter time frame unless they implement other mitigation measures that both the department and the Philadelphia School District said many schools would not be able to meet.

The announcement — in particular, the removal of recommendations that schools close if they reach a certain threshold of COVID-19 cases — drew pushback from the Philadelphia Federation of Teachers. The union’s president, Jerry Jordan, said it was “particularly alarming to learn that again, [the health department] has realigned their guidance not based on science, but to keep school buildings open with no concern for school communities.”

The surge in cases has caused major disruption in Philadelphia schools since winter break, with 40% of the district’s 238 schools temporarily closing amid staffing shortages. Most opened in person this week, though 15 schools were offering only remote learning as of Wednesday.

In an effort to help schools stay open, the Children’s Hospital of Philadelphia PolicyLab research group and hospital leadership recently advocated that schools adopt the CDC’s shorter COVID-19 isolation periods, along with other measures. The CDC says students who test positive can return to school after isolating for five days after the onset of symptoms, as long as their symptoms are resolving and they strictly mask for the five days in school.

Many schools in the city, though, don’t have the ability to safely implement that approach, Bettigole said. Required, she said, is the ability to ensure good ventilation, contact tracing for high-risk exposures, screening testing, and strict masking. (The health department says schools should “open windows and/or doors on opposite sides of the building and consider using fans to blow outside air through the building,” or “optimize ventilation” provided by HVAC systems through a number of steps, including providing at least six air exchanges per hour or the maximum possible and installing air filters that meet certain standards.)

The department also recommends setting up an area where students, teachers, and staff who have been recently infected can eat without masks from six to 10 days after testing positive.

Schools that meet all recommended layers of mitigation can stick to the CDC guidance — five days of isolation for students, followed by five days of strict masking back at school with the approval of the health department. If a school can’t meet the strategies, Bettigole said, it has to maintain a 10-day isolation period for infected students.

Monica Lewis, a spokesperson for the School District, said that “we agree with Dr. Bettigole that few schools will likely be able to meet all of the mitigation requirements for [the health department] to approve the 5-day isolation/quarantine period due to staffing, space and other constraints.”

She said the district “will continue our extensive health and safety protocols currently in place,” including “mandatory masking, hand sanitizing stations and encouraging frequent hand washing, air purifiers in classrooms and common spaces, mandatory COVID testing for staff and students who present COVID symptoms, and vaccine requirements for staff and student-athletes.”

While students may have to isolate for 10 days, teachers and staff who had the virus can return to work if rapid tests are consecutively negative five and six days after testing positive, Bettigole said. That option isn’t available for students, she said, due to the volume of tests that would be needed.

But schools should be conducting weekly screening, Bettigole said.

The department’s guidance calls for a random sample of 10% of a school’s unvaccinated population, including staff and students, to be tested weekly — and says that schools unable to fully meet the recommendation “are ineligible to follow the updated quarantine/isolation guidance.”

Philadelphia has been testing all district staff weekly.

The testing guidance diverges from the CHOP PolicyLab, which recently advised schools to no longer require testing for asymptomatic individuals amid the current surge in cases. The research group said it was more feasible for schools to require masking rather than conducting widespread testing.

Matt Rankin, a spokesperson for the health department, said the testing requirement and other mitigation measures were added because the “CDC themselves noted that more than 30% of people would still be infectious after five days. These additional mitigation layers are intended to help cut the risk.”

Because COVID-19 is so widespread, Bettigole said, case counts will not be used by the health department to require school closures. Those decisions fall to school administrators and likely will be implemented if staff absences due to COVID-19 make it impossible to run the school.

The health department earlier this month had recommended that if 10% of students and staff in a given school tested positive for the virus, the school should close. That was a shift from the department’s earlier threshold, which was 3% of a school’s population.

Jordan, the teachers’ union president, called the removal of the 10% threshold “another shifting goalpost that leaves our students and staff vulnerable.” The union has called for a pause on in-person learning to implement a safety plan it supports, better-quality masks and adequate supplies, and a testing program that includes asymptomatic students, among other requests.

Rankin said Wednesday that case investigations “continue to identify that most students who test positive for COVID identify the exposure as being at home or in their community, not in school. So by pausing an entire school due to case counts, we could be putting children at a greater risk of exposure.”

The department “may need to recalibrate” in the future, he said, “but for now, school is the safest place for children.”