Exploring environmental viral emissions after SARS-CoV-2 Human challenge

By Nidhi Saha, BDSDec 21 2022Reviewed by Aimee Molineux

A recent study on Preprints with The Lancet* captured the dynamics of pre-symptomatic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emissions and reported the extent, magnitude, and routes of viral transmission are heterogeneous, with nasal mucosa being an important source of viral transmission.

*Important notice: Preprints with The Lancet publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Background

SARS-CoV-2 can be transmitted through droplets, aerosols, or direct or indirect contact (through fomites) with saliva, respiratory secretions, or other body fluids.

Despite their effectiveness in mitigating disease severity, currently available vaccines against coronavirus disease 2019 (COVID-19) have been proven ineffective in preventing infection transmission. As part of future pandemic preparedness and COVID-19 management, developing instruments to reduce transmission is crucial. A significant amount of effort is now being placed into developing innovative vaccines and antiviral drugs to curtail infection transmission. This has mainly involved implementing nonpharmaceutical measures, such as social distancing and barrier protection.

Further, identifying individuals who are contagious and when is essential for the deployment of effective preventive strategies. Of note, measuring SARS-CoV-2 viral load (VL) from upper respiratory tract swabs is useful in determining contagiousness. Nevertheless, assessing viral emissions may be more precise in ascertaining the likelihood of onward transmission and identifying the potential transmission routes.

The study 

This phase 1, the open-label human experiment included 36 volunteers exposed to the wild-type strain of SARS-CoV-2 intranasally. A series of virological tests, like RT-qPCR, viral culture, plaque assay, and lateral flow antigen test (LFT), were performed on samples from an air sampler, hand and surface swabs, and facemasks. 

Findings

Although the infections were of moderate intensity, all infected volunteers emitted significant amounts of the virus for long periods. However, the extent, timing, and route of contamination varied. However, viral emission patterns varied in volume, timing, and contamination routes.

The results demonstrated that individuals with a relatively low nasal VL expelled substantial amounts of the virus, indicating that viral emissions were useful for determining individual infectivity.

Viable SARS-CoV-2 was recovered from expired air trapped in facemasks and surfaces that were frequently directly contacted and those that were not – suggesting possible surface contamination through direct contact and airborne droplets.

There was a significant association between viral emissions and VL in nasal swabs but not with VL in throat swabs. Two subjects emanated 86% of the total airborne virus; a larger part of this emission occurred over a few days. Reportedly, higher volumes were released during the symptomatic period, with nearly 10% being released even before the symptoms were noted.

Further, high symptom scores did not correspond to greater viral emissions, indicating that symptoms did not influence the amount or severity of viral emissions. 

Facemasks rendered most pre-symptomatic emissions, while Coriolis air samples did not, suggesting that short-range transmissions were more likely; however, sensitivity levels could vary considerably.

Only 7% of the emissions occurred before the first reported symptom, and 2% occurred before the first positive lateral flow antigen test.

In summary, after uniformly inoculating healthy, seronegative volunteers with the virus, a high-resolution analysis revealed widespread, although heterogeneous, viral contamination of air and the surroundings. Such contamination appears to be primarily caused by the nasal epithelium. 

Most viral emissions occurred after the appearance of the early symptoms and around the same time as LFT positivity detected infection. Therefore, awareness of early symptoms and self-testing can uncover a high proportion of viral transmissions.

Implications

that the results depicted viral loads in the upper respiratory tract do not correspond to the environmental emissions. This suggests a likely barrier that impedes viral discharge from the respiratory mucosa – indicating its important role in contagiousness. Hence, measuring viral emissions may better delineate contagiousness. 

The findings confirmed the nose as the most significant channel for virus transmission. This suggests significant implications for public health messages, such as regarding face mask usage and antiviral nasal spray targets. 

In addition to other critical efforts to stop SARS-CoV-2 airborne transmissions, recovery of live virus from surfaces and the viral loads of hands reestablished the role of hand hygiene and indoor surface cleaning in SARS-CoV-2 prevention. Early recognition of the symptoms and self-testing are key to detecting viral contagiousness. Self-testing with point-of-care tests, for example, lateral flow assay, should be encouraged in community settings.

Limitations

This study was conducted in a controlled environment, which may not precisely depict real-life settings. The study did not include infection-naive participants that would have provided insight into the disease's contagious nature. Further, this study was conducted on unvaccinated individuals with wild-type strain infections; thus, the outcomes projected here may differ from those of currently circulating SARS-CoV-2 variants of concern.

*Important notice: Preprints with The Lancet publishes 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.