Nosocomial spread of COVID-19 throughout a medical unit likely occurred through aerosol transmission from a symptomatic nurse, researchers said.

An outbreak in a Boston hospital among 103 healthcare professionals and patients arose on a non-COVID ward after a nurse became symptomatic 4 days after receiving a COVID vaccine in late December 2020, reported Eric Garshick, MD, of Harvard Medical School and Brigham and Women’s Hospital, and colleagues.

Air sampling of the nurses’ station revealed “genetically concordant SARS-CoV-2 RNA fragments” linked to at least six nurses and patients who were present and had nasopharyngeal samples consistent with shedding infectious virus, the authors wrote in JAMA Network Open.

They noted that SARS-CoV-2 RNA has been detected in air samples from hospital rooms or wards with unmasked COVID patients, and that positive samples have been most often collected in areas of the hospital where masking is less consistent, such as nurses’ stations.

Garshick’s group hypothesized that nosocomial transmission of COVID-19 might be linked with aerosolization of SARS-CoV-2 within shared hospital spaces.

For this cohort study, they examined data from an outbreak that took place from Dec. 27, 2020 to Jan. 8, 2021, in which a case was defined as a positive RT-PCR test from January 2 to 8 in those with no history of COVID infection in the prior 90 days. Patient rooms were not under negative pressure, and patients were unmasked inside their rooms, while nurses wore surgical masks, the authors noted.

Following a positive test in a nurse, contact tracing over the next 6 days revealed eight nurses and eight patients who tested positive on the ward, in addition to two nurses from another ward. The nurses from the initial ward worked shifts on nonconsecutive days, and all nine infected nurses had exposure to at least one other nurse or patient, while all eight patients were exposed to either an infected nurse or another infected patient.

Garshick’s team pointed out that one patient “often wandered or sat unmasked in front of the nurses station” and nurses there “would occasionally lower their masks to drink,” while other patients were confined to their rooms except when testing was required in other locations of the hospital.

Interestingly, the outbreak occurred within 2 weeks of mRNA vaccine availability — among 11 infected nurses, five had received their first shot of Moderna vaccine less than 14 days prior to the outbreak and five were unvaccinated, the authors noted, and no patients were vaccinated at the time.

“Several observations were consistent with aerosol transmission,” based on a coincidental collection of air samples, Garshick and colleagues said.

“The temporal sequence of transmission suggested that the infection was introduced by a symptomatic nurse and spread among nurses and patients,” they wrote, adding that at least six nurses and patients had nasopharyngeal samples with a Ct value less than 25 during air sample collection, and most were early in their illness.

Moreover, viral sequences from three infected individuals were identical, “suggesting nosocomial transmission from a common source,” they added.

Limitations to the study included a lack of sampling from Dec. 10, 2020 to Jan. 4, 2021, which may have missed a number of positive samples. In addition, the PCR test may have “underestimated the frequency of positive samples,” and sequencing and Ct values were not available for all infected healthcare personnel and patients.

This study suggests “aerosol transmission across long and short distances may have contributed to hospital transmission,” Garshick’s team concluded.

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