SARS-CoV-2 RNA in air samples collected at a nurses station at a Boston hospital were identified in all particle sizes and were genetically identical to human samples from a healthcare-associated outbreak, according to a new study in JAMA Network Open.
Air sampling during hospital outbreak
For the study, researchers at Harvard, the Veterans Affairs Boston Healthcare System (VABHS), Boston University, and Brigham and Women's Hospital in Boston collected air samples to detect SARS-CoV-2 RNA at a VABHS hospital and long-term care center from Nov 16, 2020, to Mar 11, 2021.
They used a microenvironmental cascade impactor that collects airborne particles in three size ranges: larger than 10.0 micrometer (μm,) 2.5 to 10.0 μm, and smaller than 2.5 μm. They collected samples about every week, with a break from Dec 10, 2020, to Jan 4, 2021
The team also conducted an investigation into a COVID-19 outbreak that eventually involved 103 patients and healthcare personnel (HCP) from Dec 27, 2020, to Jan 8, 2021. It began on a medical ward not dedicated to COVID-19 patients shortly after vaccines had become available to HCP but not patients. It preceded both the Delta and Omicron variants, which are known to spread faster.
The outbreak began on "ward A" when a nurse developed COVID symptoms 4 days after receiving the first dose of Moderna vaccine, then tested positive. The investigators presumed this nurse to be the index case.
Contract tracing over the next 6 days based on polymerase chain reaction (PCR) and antigen testing identified 8 more infected nurses and 8 infected patients from ward A, as well as 2 infected nurses on a second ward (ward B). Patients diagnosed as having COVID-19 were transferred to a COVID unit (ward C), where nurses wore surgical masks but not more protective respirators. Patients wore masks only when outside their room.
Four infected nurses and 7 infected patients were on ward A during 3 days of air sampling at the nurses station, including 3 nurses and 3 patients with cycle threshold (Ct) values less than 24, an indicator of high viral load.
One patient, who had a Ct value of 17—in general, the lower the value the higher level of viruses—often wandered or sat unmasked in front of the nurses station within 15 feet from the sampler. In addition, the researchers report that nurses at the station would at times lower their masks to drink.
All 3 particle sizes test positive
The researchers found evidence of SARS-CoV-2 in samples of all three particle sizes at the ward A nurses station.
Fragments of SARS-CoV-2 RNA in the smallest aerosols (under 2.5 μm) in ward A showed 100% sequence identity with the samples from people. Samples from the other size particles in ward A had greater genetic relationship with human samples (2.5 to 10 μm, 99.91%; larger than 10 μm, 99.97%) than did samples collected over the same dates on ward C. Those ranged from 99.36% to 99.86% from a nurses station and break room.
Surveillance of HCP on selected units across the medical center demonstrated that active surveillance and isolation of infected HCP were associated with less SARS-CoV-2 RNA in air samples from those units.
The researchers detected fragments of SARS-CoV-2 RNA by PCR in 24 of 300 samples (8.0%) in units across the medical center where HCP were not under surveillance and 7 of 210 (3.3%) in units where they were. About half of all positive samples (20 of 38 [52.6%]) came from medium-sized particles—those ranging from 2.5 to 10 μm.
Reducing airborne transmission
The study authors write, "[Hospital-based] transmission of SARS-CoV-2 occurred on a medical unit during coincidental collection of air samples, and several observations were consistent with aerosol transmission."
They add, "At least 6 nurses and patients who were present during air sample collection had nasopharyngeal samples with a Ct less than 25, a range associated with shedding of replication-competent virus, and most were early in their illness, when detection of viral RNA in exhaled aerosols is most frequent. Viral sequences from 3 infected persons were nearly identical, suggesting nosocomial transmission from a common source."
The researchers also note that surveillance and isolation of infected HCP appeared to reduce aerosol spread of viruses.
The authors conclude, "Improvements in air filtration, ventilation, and masking in shared hospital spaces may further decrease transmission of SARS-CoV-2 and other airborne respiratory viruses."