Viral load of SARS-CoV-2 in droplets and bioaerosols directly captured during breathing, speaking and coughing

Tyler J Johnson,Yi-Chan James Lin,Kimberley A Watson,Jason S Olfert,David H Evans,John M Conly,Robert T Nishida,Ashlesha P Sonpar,Stephanie W Smith

doi:10.1038/s41598-022-07301-5

Abstract

Determining the viral load and infectivity of SARS-CoV-2 in macroscopic respiratory droplets, bioaerosols, and other bodily fluids and secretions is important for identifying transmission modes, assessing risks and informing public health guidelines. Here we show that viral load of SARS-CoV-2 Ribonucleic Acid (RNA) in participants’ naso-pharyngeal (NP) swabs positively correlated with RNA viral load they emitted in both droplets >10 upmu hbox {m} and bioaerosols <10 upmu hbox {m} directly captured during the combined expiratory activities of breathing, speaking and coughing using a standardized protocol, although the NP swabs had approx 10^3times more RNA on average. By identifying highly-infectious individuals (maximum of 18,000 PFU/mL in NP), we retrieved higher numbers of SARS-CoV-2 RNA gene copies in bioaerosol samples (maximum of 4.8{times }10^{5} gene copies/mL and minimum cycle threshold of 26.2) relative to other studies. However, all attempts to identify infectious virus in size-segregated droplets and bioaerosols were negative by plaque assay (0 of 58). This outcome is partly attributed to the insufficient amount of viral material in each sample (as indicated by SARS-CoV-2 gene copies) or may indicate no infectious virus was present in such samples, although other possible factors are identified.

Highlights

Determining the viral load and infectivity of SARS-CoV-2 in macroscopic respiratory droplets, bioaerosols, and other bodily fluids and secretions is important for identifying transmission modes, assessing risks and informing public health guidelines
SARS-CoV-2 Ribonucleic Acid (RNA) has been detected by nucleic acid-based tests (NATs) such as droplet digital polymerase chain reaction and quantitative reverse-transcription PCR (RT-qPCR) analyses of indoor air and surface samples in a range of s ettings[12], including in droplet nuclei suspended in a ir[15,16]
Results from RT-qPCR indicate SARS-CoV-2 RNA was present in samples of air emitted by participants, including in bioaerosols smaller than 10 μm in diameter

Summary

Introduction

Determining the viral load and infectivity of SARS-CoV-2 in macroscopic respiratory droplets, bioaerosols, and other bodily fluids and secretions is important for identifying transmission modes, assessing risks and informing public health guidelines. By common microbial contaminants, such as bacteria and fungi, and is not conclusive evidence of virus infectivity These previous studies did not determine the size of the pathogen-laden droplets/droplet nuclei or identify the exact source of the presumed pathogen-laden aerosol. Samples gathered 10 cm from each participant’s chin[39,40] or by sampling EBC41 were negative for the presence of SARS-CoV-2 RNA by RT-PCR It is unclear if non-aerosolized saliva (e.g. direct contact, spitting or drooling) may contaminate samples gathered directly from masks, sampling strips in masks, or EBC and if they adequately represent exhalatory activities such as breathing. Of studies to date which report direct sampling of air emitted by participants infected with COVID-19, attempts to culture the virus have not been reported for either the participants’ respiratory tract fluid (e.g. NP swab) or exhaled air samples

Methods

Results

Conclusion