Two hospitals in San Francisco reported hundreds of staff infected with coronavirus in July, with breakthrough infections comprising the majority of cases.

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As of August 2, 55 staff members out of over 7,000 staffers at Zuckerberg San Francisco General Hospital are COVID-19 positive, and none of the infected staff have required hospitalization, a hospital spokesperson confirmed to Fox News. ABC7 reported that up to 80% of those infected were fully vaccinated.

"Breakthrough cases were and still are expected," ZSFG hospital spokesperson Cristina Padilla wrote to Fox News. "We know vaccines won’t completely prevent infections, but they are very effective at making hospitalizations and death preventable."

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Dr. Lukejohn Day, chief medical officer of San Francisco General Hospital, told the outlet that more cases are cropping up among staff than before.

"More staff are getting COVID than we saw before, and it's mostly vaccinated staff. And that's just because of the easing of restrictions," Day said. "We are seeing it among physicians, nurses, ancillary staff, we sort of are seeing that across the board."

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He estimated that nearly all of the cases were linked back to community spread, with no staff-to-patient or patient-to-staff viral spread but investigations were still underway. Infected staff are in quarantine at home to prevent additional exposures, the hospital spokesperson confirmed.

Meanwhile, the University of California San Francisco (UCSF) Medical Center reported at least 183 infections among "employees or learners" out of 35,000 people, with 84% of cases in people who completed a full vaccination series and two vaccinated individuals infected with COVID-19 required hospitalization, the outlet reported.

UCSF Medical Center didn’t immediately return Fox News’ requests for comment.

Adler told ABC7 that breakthrough infections were expected, but the rate is "a little bit higher than we had originally predicted," based on predictions made with the original strain, not the highly transmissible delta variant. Area health officials are expected to announce new guidance to tamp down viral spread later on Monday.

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The hospitals reportedly spotted the cases through symptom screenings, and UCSF intends to implement a vaccine mandate on Sept. 1, the outlet reported. Patients admitted to an inpatient area undergo testing for COVID-19 and all visitors and staff are required to wear masks and undergo daily self-screening surveys.

State data indicates the area's hospitalizations are on the rise, with 95 COVID-19 patients, including four more patients from the day prior, as of July 31, the latest available data. San Francisco County has seen some 40,400 total cases and 562 reported deaths, and has administered over 1.23 million doses of vaccine.