By Catherine Ho

San Francisco health officials have identified more than 200 latent cases of tuberculosis associated with an outbreak that was first reported at Archbishop Riordan High School in November — up from 50 cases identified as of late January.

Latent TB is not symptomatic or contagious, though it can develop into an infectious active case if it goes untreated. Health officials believe they have identified all cases at Riordan that could potentially transmit to others and are well on their way to resolving the outbreak, the San Francisco Department of Public Health said Friday.

The first active case of TB was reported in November, and contact tracing found two additional active cases in January. Health officials said they then tested nearly all of the school’s students and staff, which found that 219 of the 1,261 members of the school population — about 17% — had latent TB as of Wednesday.

The testing also found three additional suspected active cases of TB. In those cases, individuals had a cough or other symptoms, tested positive on a blood test, and had abnormal chest X-ray results — but were still awaiting final lab results for confirmation.

The figures represent test results from 90% of students and staff at the private Catholic school in San Francisco’s Ingleside neighborhood. The remaining 10% of test results were due Friday. Officials have not said whether the active cases were students or staff.

In latent TB cases, individuals have been exposed to the tuberculosis bacteria, but their immune system has walled it off and kept it dormant. However, the body cannot always keep the bacteria dormant permanently, and latent TB can turn into active TB months or years later. This happens in roughly 5% to 10% of untreated latent cases, according to the U.S. Centers for Disease Control and Prevention. The CDC estimates that up to 13 million Americans have latent TB.

Active TB is contagious.

People at Riordan with latent cases were “strongly recommended” to start treatment to prevent the infection from becoming active, San Francisco Health Officer Dr. Susan Philip said. This typically consists of taking an antibiotic for four months, which reduces the risk of active infection in the future by about 90%, she said. It was unclear how many Riordan students and staffers were on the antibiotic regimen.

The suspected active cases are being treated as active cases, which entails taking a combination of anti-TB medications for six months or longer. People with an active infection who are on these medications are not contagious.

“The most important thing is they are on treatment and not infectious, and not in a position where they’re potentially going to transmit to anyone else,” Philip said.

The people with the three original active infections are on treatment and no longer infectious.

Riordan canceled in-person classes the first week of February to conduct testing. The school has been mostly in-person for the past two weeks, with the exception of a small percentage of students who got a Zoom option while they completed the full testing process, said school President Tim Reardon.

“Riordan has embarked on an exhaustive testing regimen in close partnership with the San Francisco Department of Public Health,” Reardon said. “As a result, approximately 99% of our student body has completed TB testing. The health and wellbeing of our students, faculty and staff remains our top priority, and we continue to follow guidance from medical experts. We are grateful for the patience and understanding of our community as we navigate this situation and remain focused on steady progress moving forward.”

Full in-person classes were expected to resume Monday, the health department said.

“We’ll continue to support families until the complete resolution of this outbreak,” Philip said. “We’re well on our way.”

In 2024, San Francisco reported 91 active TB cases in residents citywide. The outbreak associated with Riordan is “not anything that’s outside the realm of what we know at times we might encounter in public health work around TB,” Philip said.