SARS-CoV-2 in the exhaled breath of COVID-19 patients might be mainly transmitted via respiratory droplets

In a recent study posted to Preprints with The Lancet*, researchers found evidence that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) particles are released directly into the air through the breathing of coronavirus disease 2019 (COVID-19) patients resulting in respiratory transmission of the virus and contamination of the surrounding environmental surfaces.

Study: SARS-CoV-2 is Mainly Distributed in Respiratory Droplets in the Exhaled Breath of COVID-19 Patients. Image Credit: Kleber Cordeiro/ShutterstockStudy: SARS-CoV-2 is Mainly Distributed in Respiratory Droplets in the Exhaled Breath of COVID-19 Patients. Image Credit: Kleber Cordeiro/Shutterstock

Understanding the detailed characteristics of the exhaled breath of COVID-19 patients is essential for devising appropriate intervention policies to prevent SARS-CoV-2 transmission. Previous studies have not provided concrete evidence that SARS-CoV-2 transmits mainly via virus-containing droplets (>5 µm) or aerosols (≤5 µm) from the exhaled breath of COVID-19 patients.

As the size distribution characteristics of SARS-CoV-2 particles in exhaled breath are still unknown, determining that would prove quite beneficial in preventing respiratory transmission of SARS-CoV-2 by taking adequate measures.

About the study

In the present study, researchers analyzed the exhaled breath of COVID-19 patients and their surrounding environment surfaces possibly contaminated by SARS-CoV-2.

The researchers collected test samples for the current study from only those COVID-19 patients who showed mild and moderate symptoms during hospitalization. The serial nasopharyngeal swabs and exhaled breath samples were taken at 1, 3, 5, 7, 9, 11, and 13 days post-hospitalization (dph) using a viral sampling tube and an air sampler. The exhaled breath was also collected onto gelatin filters for 30 minutes to detect the SARS-CoV-2 particle size distribution.

The sampling tube contained 2 ml phosphate buffer saline (PBS) containing 1% penicillin to detect its viral load. The air sampler was an Anderson six-stage air sampler that separated airborne particles into six ranges - 0.65-1.1, 1.1-2.1, 2.1-3.3, 3.3-4.7, 4.7-7, and >7 µm.

Additionally, the researchers gathered samples of the patient’s hands and surrounding environments, such as pillowcases, masks, bed rails, bedside tables, and mobile phones at 1, 3, 5, 7, 9, 11, and 13 dph. These sodden sterile swabs were also stored in 1 ml PBS containing 1% penicillin. All the test specimens were stored at -80°C until extraction.

The researchers employed real-time quantitative reverse-transcription polymerase chain reaction (qRT–PCR) for SARS-CoV-2 RNA detection that measured nucleocapsid (N) gene amplification. The qRT–PCR analysis was replicated once for each positive sample. Ct values measure the numbers of viral RNA copies in the samples, and a cutoff cycle threshold (Ct) of 40 shows a negative test.

Study findings

The researchers found evidence of the presence of SARS-CoV-2 in the exhaled breath of COVID-19 patients, which is transmitted via respiratory droplets, and the environmental surface contamination was also associated with the exhaled breath. These findings reinstate the importance and effectiveness of wearing a mask to minimize the respiratory transmission of SARS-CoV-2, as masks reduce transmission of large viral particles (>5 μm).

During earlier stages of illness, the COVID-19 patients exhaled 10 million SARS-CoV-2 particles per hour (107 viral RNA copies). The viral loads gradually decreased in both paired nasopharyngeal swabs and the exhaled breath specimens. However, the nasopharyngeal swab had a higher viral load than the paired exhaled breath specimen, except for patient 2 at 11 dph.

In the exhaled breath, SARS-CoV-2 was present in two sizes - 2.1-4.7 and >4.7 μm. The larger >4.7 μm particles came from respiratory droplets, accounting for 87.13% of the total virus, and the smaller 2.1-4.7 μm particles came from aerosols <5.0 μm, accounting for only 10.79% of the total virus.

The average positive rate of environmental contamination by SARS-CoV-2 was 44%, and patients’ hands were the most contaminated (50.48%) surface. The contamination rate of bedside tables, bed rails, mobile phones, and pillowcases was 44.76%, 42.38%, 41.43%, and 40.95%, respectively. The surface contamination attained 60% at 3dph and decreased gradually to 36% at 11dph and 0% at 13dph. 

Intriguingly, at 13dph, SARS-CoV-2 was not detected in both the exhaled breath and environment surface specimens. Further analysis also showed a strong positive correlation between the viral load of the environmental surface and exhaled breath, as also observed between viral loads of paired nasopharyngeal swabs and exhaled breath specimens.

Conclusions 

According to the authors of the present study, to date, there was no concrete evidence to prove the respiratory droplet transmission of SARS-CoV-2. They provided evidence of the presence of SARS-CoV-2 in droplets of the exhaled breath of COVID-19 patients, suggesting that respiratory droplets are a major SARS-CoV-2 transmission route.

SARS-CoV-2 is distributed as aerosols in the air but mainly transmits as droplets in the exhaled breath. Therefore, there was no significant difference in air droplets of the surrounding environment and the exhaled breath of a COVID-19 patient, although exhaled breath contained more droplet particles than air.

Consistent with the findings of previous studies, the study results showed that the positive rate of environmental surface contamination by SARS-CoV-2 was more than 40% and was higher in the first week of hospitalization. Thus, suggesting that direct contact with contaminated surfaces, especially during the first seven days of illness, increases the risk for SARS-CoV-2 infection. Also, the SARS-CoV-2 load in the environmental surface showed a positive correlation with the viral load in the exhaled breath, thus suggesting that COVID-19 patients should be isolated to prevent more transmissions. 

*Important notice


Preprints with The Lancet publish preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Neha Mathur

Written by

Neha Mathur

Neha is a digital marketing professional based in Gurugram, India. She has a Master’s degree from the University of Rajasthan with a specialization in Biotechnology in 2008. She has experience in pre-clinical research as part of her research project in The Department of Toxicology at the prestigious Central Drug Research Institute (CDRI), Lucknow, India. She also holds a certification in C++ programming.

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