SARS-CoV-2 lurks in small airborne particles

Investigations into the role of small aerosol particles in COVID-19 transmission suggest that SARS-CoV-2 may be an airborne hazard. A recent medRxiv preprint study from Singapore adds to this body of evidence.

In the study, fine respiratory aerosols (≤5 μm) generated by COVID-19 patients when talking and singing contained greater viral loads than coarse aerosols (>5 μm). This, according to the investigators, shows that SARS-CoV-2 can be “aerosolized” without coughing or sneezing and even in the absence of aerosol-generating medical procedures.

“Coarse aerosols are inhalable and deposit in the upper airways, whereas fine aerosols are respirable and deposit in the lower airways. The amount of infectious virus these size fractions carry and their relative importance to SARS-CoV-2 transmission and infection is not well-understood,” they noted.

Establishing whether the virus can be emitted via expiratory activities, such as talking and singing, is critical to minimizing its spread, given that a significant proportion of SARS-CoV-2 transmission is estimated to be from asymptomatic individuals and that multiple superspreading events suggest that aerosols is a key driver of the COVID-19 pandemic, the investigators stressed. [JAMA Netw Open 2021;4:e2035057; Indoor Air 2021;31:314-323; Morb Mortal Wkly Rep 2021;69;606-610; J Infect 2021;83:207-216]

The present study included 22 patients (median age 38 years, 86 percent male) positive for COVID-19 via reverse transcription-quantitative polymerase chain reaction. Of the patients, five (23 percent) never developed symptoms and 16 (73 percent) were infected with a SARS-CoV-2 variant of concern or variant of interest (Alpha n=4, Beta n=8, Kappa n=3, and Delta n=1).

A G-II exhaled breath collector facilitated expiratory sample collection. Seated facing a truncated cone-shaped inlet, the patients were asked to perform three separate expiratory activities on the same day: 30 minutes of tidal breathing, 15 minutes of talking, and 15 minutes of singing.

Thirteen (59 percent) patients emitted detectable levels of SARS-CoV-2 RNA in respiratory aerosols, including three asymptomatic and one presymptomatic patients. Viral loads ranged from 63 to 5,821 N gene copies, with patients earlier in illness being more likely to emit detectable RNA (median day of illness, 3 vs 5; p=0.025). [medRxiv 2021;doi:10.1101/2021.07.15.21260561]

Fine aerosols constituted 85 percent of the viral load detected overall. The highest number of SARS-CoV-2 RNA copies was emitted by singing (53 percent), followed by talking (41 percent) and breathing (6 percent). For all three activities, viral loads were consistently greater in fine vs coarse aerosols.

“Our results … [are] in agreement with other expert views suggesting that SARS-CoV-2 transmission events are driven by the airborne route and could explain the difficulty in containing the virus,” the investigators said. [Lancet 2021;397:1603-1605]

To minimize exposure to fine respiratory aerosols, the investigators pointed out that nonpharmaceutical interventions, such as universal masking, physical distancing, and increased room ventilation, might help. In indoor spaces, portable high efficiency particulate air cleaners, upper-room ultraviolet air disinfection, and the use of fans to control airflow patterns can also reduce exposure. [Morb Mortal Wkly Rep 2021;70:972-976]

“[A] multilayered approach of control measures is most effective at decreasing the risk of airborne SARS-CoV-2 transmission,” they said.

The present study was limited by the inability to collect respiratory swabs on the day of aerosol sampling for comparison of culturability. Additionally, aerosol shedding patterns related to SARS-CoV-2 variant type could not be established due to the small number of patients having the previous strains.

“While COVID-19 patients shed detectable levels of SARS-CoV-2 RNA in respiratory aerosols, culturing SARS-CoV-2 from patient aerosol samples remains challenging,” the investigators said, adding that more studies are needed to determine whether isolating viable virus in respiratory aerosols can be more easily accomplished from sampling patients infected with emerging SARS-CoV-2 variants.