A year ago scientists everywhere were scrambling to get their minds around the
SARS-CoV-2, a novel coronavirus that caused the pandemic from which we are
only now beginning to emerge. The world clung to every new development, every
bit of science that could provide clues to managing life in the presence of
this mysterious killer.
Many science-backed COVID-19 management concepts remain unchanged to this
day: handwashing with soap and warm water disrupts the virus' lipid
membrane. Social distancing can attenuate the virus's spread, ideally
keeping it out of a host until it degrades. Other notions, such as droplet
contact being the primary mode of transmission, were modified when emerging
evidence showed that under certain conditions, the virus could remain
suspended in air for extended periods of time.
In a letter in the Journal of Infectious Diseases, a team of researchers
from UC Santa Barbara, Oregon State University, University of Manchester and
ETH Zurich examines another of SARS-CoV-2's well known characteristics --
its vulnerability to sunlight. Their conclusion? It might take more than
UV-B rays to explain sunlight inactivation of SARS-CoV-2.
The idea that an additional mechanism might be in play came when the team
compared data from a July 2020 study(link is external) that reported rapid
sunlight inactivation of SARS-CoV-2 in a lab setting, with a theory(link is
external) of coronavirus inactivation by solar radiation that was published
just a month earlier.
"The theory assumes that inactivation works by having UV-B hit the RNA of
the virus, damaging it," said UC Santa Barbara mechanical engineering
professor and lead author Paolo Luzzatto-Fegiz(link is external). Judging
from the discrepancies between the experimental results and the predictions
of the theoretical model, however, the research team felt that RNA
inactivation by UV-B "might not be the whole story."
According to the letter, the experiments demonstrated virus inactivation
times of about 10-20 minutes -- much faster than predicted by the theory.
"The theory predicts that inactivation should happen an order of magnitude
slower," Luzzatto-Fegiz said. In the experiments, viruses in simulated
saliva and exposed to UV-B lamps were inactivated more than eight times
faster than would have been predicted by the theory, while those cultured in
a complete growth medium before exposure to UV-B were inactivated more than
three times faster than expected. To make the math of the theory fit the
data, according to the letter, SARS-CoV-2 would have to exceed the highest
UV-B sensitivity of any currently known virus.
Or, Luzzato-Fegiz and colleagues reasoned, there could be another mechanism
at play aside from RNA inactivation by UV-B rays. For instance, UV-A,
another, less energetic component of sunlight might be playing a more active
role than previously thought.
"People think of UV-A as not having much of an effect, but it might be
interacting with some of the molecules in the medium," he said. Those
reactive intermediate molecules in turn could be interacting with the virus,
hastening inactivation. It's a concept familiar to those who work in
wastewater treatment and other environmental science fields.
"So, scientists don't yet know what's going on," Luzzatto-Fegiz said; "Our
analysis points to the need for additional experiments to separately test
the effects of specific light wavelengths and medium composition."
Results of such experiments might provide clues into new ways of managing
the virus with widely available and accessible UV-A and UV-B radiation.
While UV-C radiation is proved effective against SARS-CoV-2, this wavelength
does not reach the earth's surface and must be manufactured. Although UV-C
is presently used in air filtration and in other settings, its short
wavelengths and high energy also makes UV-C the most damaging form of UV
radiation, limiting its practical application and raising other safety
concerns.
"UV-C is great for hospitals," said co-author Julie McMurry. "But in other
environments -- for instance kitchens or subways -- UV-C would interact with
the particulates to produce harmful ozone." While no single intervention
will eliminate risk, this research would provide one further tool to reduce
exposure, thus slowing transmission and improving health outcomes.
Co-author and UCSB mechanical engineering professor Yangying Zhu(link is
external) added that UV-A turning out to be capable of inactivating the
virus could be very advantageous: there are now widely available inexpensive
LED bulbs that are many times stronger than natural sunlight, which could
accelerate inactivation times. UV-A could potentially be used far more
broadly to augment air filtration systems at relatively low risk for human
health, especially in high-risk settings such as hospitals and public
transportation, but the specifics of each setting warrant consideration,
said co-author Fernando Temprano-Coleto.
Research in this paper was conducted also by François J. Peaudecerf at ETH
Zurich and Julien Landel at University of Manchester.
Reference:
Paolo Luzzatto-Fegiz, Fernando Temprano-Coleto, François J Peaudecerf,
Julien R Landel, Yangying Zhu, Julie A McMurry. UVB Radiation Alone May Not
Explain Sunlight Inactivation of SARS-CoV-2. The Journal of Infectious
Diseases, 2021; DOI:
10.1093/infdis/jiab070