An international team of researchers has made important discoveries about the sunlight inactivation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that suggest this process is not currently adequately understood.
SARS-CoV-2 is the agent responsible for the current coronavirus disease 2019 (COVID-19) pandemic that has now claimed the lives of more than 900,000 people globally.
The team says the widespread assumption that SARS-CoV-2 inactivation by sunlight is driven solely by ultraviolet light B (UVB) is not consistent with the evidence available.
The researchers suggest that ultraviolet light A (UVA) may play a significant role and that future experiments should test the effects of UVA and UVB on SARS-CoV-2 separately.
They also suggest that UVA should be included in models of SARS-CoV-2 sunlight inactivation.
Paolo Luzzatto-Fegiz (University of California) and colleagues say that if SARS-CoV-2 is found to be sensitive to UVA, this will imply that sunlight could help to curb outdoor transmission of the virus more than had previously been thought.
Furthermore, inexpensive sources of UVA could be used to improve air filtration systems at a relatively low risk to human health, particularly among high-risk settings such as public transport systems and hospitals.
A pre-print version of the paper is available on the server bioRxiv*, while the article undergoes peer review.
Studies have previously focused on UVB alone
Studies have recently shown that simulated sunlight can inactivate SARS-CoV-2, but the models used have so far only focused on mechanisms that involve the direct effects of UVB on viral RNA.
"There are still many unknowns, including the mechanism of action and which part of the light spectrum is principally responsible," writes Luzzatto-Fegiz and colleagues.
The researchers say that to the best of their knowledge, the effects of UVA-only exposure on SARS-CoV-2 have not yet been investigated.
One 2004 study did test the effects of exposing the related virus SARS-CoV-1 to either UVC or UVA light in Dulbecco's modified Eagle's medium (DMEM) over the course of 15 minutes
That experiment showed that UVC clearly inactivated the virus, whereas the effect that UVA light had was less certain. However, a related study found that exposure of SARS-CoV-1 to UVA in DMEM did have an inactivating effect after 30 minutes.
"Since the relative intensities of UVA and UVB vary greatly in sunlight, establishing UVA sensitivity of SARS-CoV-2 is necessary to obtain practically valuable predictions," say the authors.
Furthermore, given that UVA from sunlight is abundant over a broader range of dates and times than UVB, the vulnerability of SARS-CoV-2 to UVA would mean sunlight has much greater potential to inactivate the virus than was previously thought.
"This vulnerability could enable disinfection using inexpensive and efficient UVA light sources," write Luzzatto-Fegiz and colleagues.
Comparison of theories with data for SARS-CoV-2 on stainless steel surfaces. (A, B, C): simulated saliva, (D, E, F): gMEM (complete growth medium). (A, D): high simulated sunlight, (B, E): medium simulated sunlight, (C, F): low simulated sunlight. Purple dotted line: UVB-only theory (Equation 3) with D37,UVC = 3.0 J/m2 [6,7]. Green dot-dashed line: UVB-only theory with D37,UVC from a fit to all data. Solid line: present model, combining UVB model (with D37,UVC = 3.0 J/m2 ) and UVA model, with D37,UVA from fits using all data for each medium. Symbols: data of Ratnesar-Shumate et al. ; for clarity, averages at each time are plotted, with error bars showing standard deviation.
What did the researchers do?
Now, the team has tested the hypothesis that using an inactivation model that includes UVA may improve agreement across SARS-CoV-2 experiments and reveal UVA sensitivities that are comparable to those observed with other viruses.
The researchers found that a model based exclusively on a mechanism driven by the direct, endogenous effects of UVB was not enough to account for the rapid decay of SARS-CoV-2 on exposure to simulated sunlight. However, a model that considered indirect, exogenous inactivation by UVA improved agreement with the data currently available, irrespective of the experimental conditions.
Furthermore, the UVA sensitivities calculated for SARS-CoV-2 were similar to those established for SARS-CoV-1 and influenza once those viruses were exposed to UVA only.
The effects of UVA and UVB exposure need to be tested separately
The researchers say there is a need for targeted experiments that assess the effects of SARS-CoV-2 exposure to UVA and UVB separately and that future sunlight inactivation models may need to include the effect of UVA.
"If UVA sensitivity is confirmed, sunlight could mitigate outdoor transmission over a broader range of latitudes and daytimes than previously expected, since UVA is less strongly absorbed by atmospheric ozone than UVB," they write.
"Furthermore, inexpensive and energy-efficient UVA sources might be used to augment air filtration systems at relatively low risk for human health, especially in high-risk settings such as hospitals and public transportation," concludes the team.
bioRxiv 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.