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Sunlight Inactivates COVID Virus and Lowers New Case Rate
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Mark Mascolini
Researchers working separately in the United States and Spain found that sunlight can kill or control SARS-CoV-2, the virus that causes COVID-19 [1,2]. A US Department of Homeland Security team found evidence that strong sunlight swiftly inactivates SARS-CoV-2 on nonporous surfaces, in this analysis, stainless steel [1]. In a study of regions across the world, researchers at Barcelona's Hospital Clinic linked lower solar irradiance to bigger COVID-19 outbreaks [2].
Recent studies found that SARS-CoV-2 can persist for days indoors on nonporous surfaces [3,4]. But this work did not assess viral persistence at different temperatures, humidity, sunlight conditions, or the matrix in which the virus was suspended.
The US study used SARS-CoV-2 stocks at a titer of 1.5 x 10(7) +/- 7.5 x 10(6) median tissue culture infective dose (TCID50)/mL. To expose virus to simulated sunlight, the researchers created an environmentally controlled chamber with a quartz window. They produced light with a solar simulator consisting of a xenon arc lamp and a series of optical filters. Because light in the UVA portion of the spectrum (315 to 400 nm) did not damage SARS-CoV-2 in a previous study, the researchers quantified the impact of exposure in the UVB portion of the spectrum (280 to 315 nm). They adjusted light to one of three intensities with neutral density filters and by adjusting power supply to the lamp. The researchers set temperature at 20 +/- 4°C and relative humidity at 19 +/- 5%.
For each day of experiments, the US team thawed frozen batches of concentrated SARS-CoV-2 and diluted virus in a 1-to-10 ratio in either growth medium or simulated saliva (a brew mimicking the tonicity, pH, and protein content of saliva). They dribbled 5-μL droplets of viral suspension onto stainless steel bars, then let the droplets dry for 30 minutes outside the test chamber. Once the droplets dried, they put the steel bars into the chamber and exposed them to simulated sunlight for 2 to 18 minutes or to no simulated sunlight (resulting in a dark chamber) for up to 60 minutes. Finally, they recovered virus from the steel and measured concentrations of infectious virus by microtitration assay.
Viral inactivation rates proved significantly greater for any simulated sun exposure (0.3, 0.7, or 1.6 Watt (W)/m22) than for darkness in simulated saliva and at exposures of 0.7 or 1.6 W/m2 versus darkness in growth medium. Inactivation rates for 0.3 W/m2 were significantly lower than at 1.6 W/m2 for both saliva and growth medium.
In simulated saliva viral inactivation rates ranged from near 0 in darkness to 0.15 log10 TCID50 loss/minute at maximum simulated sun exposure. The researchers calculated that 90% of infectious virus would be lost every 6.8, 8.0 and 12.8 minutes for integrated UVB irradiances of 1.6, 0.7, and 0.3 W/m2. An irradiance of 1.6 W/m2 represents sunlight at midday on the summer solstice at 40° north latitude, which runs through Portugal, Spain, Italy, Turkey, Armenia, Azerbaijan, four Central Asian countries, China, North Korea, Japan, and the United States.
In growth medium viral inactivation rates ranged from near 0 in darkness to 0.7 log10 TCID50 loss/minute at maximum simulated sun UVB. The investigators figured that 90% of infectious virus would be lost every 14.3 minutes at an integrated UVB irradiance of 1.6 W/m2 and every 17.6 minutes at an irradiance of 0.7 W/m2. At all simulated sun irradiances, inactivation proved about 2-fold greater in simulated saliva than in growth medium.
Previous research showed that UVC light-not part of the natural sunlight spectrum-inactivates coronaviruses. This is the first study to show that simulated-sunlight UVB rapidly inactivates SARS-CoV-2 dried on stainless steel [1]. The US researchers believe their findings "suggest that the potential for fomite transmission [of SARS-CoV-2] may be significantly reduced in outdoor environments exposed to direct sunlight relative to indoor environments." Also, they propose that "natural sunlight may be effective as a disinfectant for contaminated non-porous materials." They caution that time of year and local weather conditions, especially clouds, would affect the sun's antiviral impact.
A worldwide demographic and climatologic study in Barcelona also yielded evidence of the sun's potential antiviral prowess [2]. Whether the COVID-19 pandemic continues to wane in northern hemisphere countries with warmer temperatures and higher humidity common in summer remains uncertain. In the southern hemisphere Sao Paulo, Brazil, for example, still enjoys highs in 70s (F) and ample sunshine as its autumn proceeds, but the pandemic continues to expand rapidly there (https://www.worldometers.info/coronavirus/country/brazil).
To address questions of climate and demographics on COVID-19 spread, the Barcelona team correlated the number of cases in the first weeks of the epidemic in several countries and regions with climatologic data-including sunshine-and other measures. They collected COVID-19 data from the Johns Hopkins 2019-nCoV Data Repository and additional datasets in Italy and Spain. The analysis focused on 359 countries and regions in 6 geographical areas: Asia, Oceania, Europe, Africa, North America, and South America.
From the day confirmed COVID-19 cases numbered at least 10 in each country or region, the researchers defined the study period as running from day -7 to day +30 (or to March 23, 2020, the day of the analysis). For this period they calculated cumulative incidence of COVID-19 as cases per 100,000 population for the study period.
Population density significantly correlated positively with higher COVID-19 incidence: the higher the density, the greater the incidence (rho = 0.32). All measures of solar irradiance significantly correlated negatively with COVID-19 incidence: the lower the irradiance, the greater the incidence (rho = -0.41 for global horizontal solar irradiance, rho = -0.37 for diffuse horizontal solar irradiance, and rho = -0.25 for maximum ultraviolet index). Other factors that correlated negatively with COVID-19 incidence were average (rho = -0.28) and maximum (rho = -0.30) wind speed, average sea level atmospheric pressure (rho = -0.21), and accumulated precipitation (rho = -0.29). A multivariable linear regression model identified two independent predictors of cumulative COVID-19 incidence: population density (P = 0.019) and global horizontal solar irradiance (P = 0.027).
The investigators believe their findings suggest that ultraviolet radiation may be more important than other climatologic factors in determining the spread of COVID-19. They propose that UBV radiation "is the principal environmentally effective virucide, several orders of magnitude more relevant than other primary physical factors, such as temperature and relative humidity." The authors predict that, "even if not sufficient on its own, increasing sunlight exposure [in the northern hemisphere] in the upcoming weeks may help flatten the epidemic."
References
1. Ratnesar-Shumate S, Williams G, Green B, et al. Simulated sunlight rapidly inactivates SARS-CoV-2 on surfaces. J Infect Dis. 2020 May 20:jiaa274. doi: 10.1093/infdis/jiaa274. https://academic.oup.com/jid/advance-article/doi/10.1093/infdis/jiaa274/5841129
2. Guasp M, Laredo C, Urra x. Higher solar irradiance is associated with a lower incidence of COVID-19. Clin Infect Dis. 2020 May 19:ciaa575. doi: 10.1093/cid/ciaa575. https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa575/5840498
3. van Doremalen N, Bushmaker T, Morris DH, et al. Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1. N Engl J. Med. 2020;382:1564-1567.
4. Chin AWH, Shu JTS, Mahen RP, et al. Stability of SARS-CoV-2 in different environmental conditions. Lancet Microbe. April 02, 2020. DOI: https://doi.org/10.1016/S2666-5247(20)30003-3
Ref#1. ABSTRACT: Previous studies have demonstrated that SARS-CoV-2 is stable on surfaces for extended periods under indoor conditions. In the present study, simulated sunlight rapidly inactivated SARS-CoV-2 suspended in either simulated saliva or culture media and dried on stainless steel coupons. Ninety percent of infectious virus was inactivated every 6.8 minutes in simulated saliva and every 14.3 minutes in culture media when exposed to simulated sunlight representative of the summer solstice at 40oN latitude at sea level on a clear day. Significant inactivation also occurred, albeit at a slower rate, under lower simulated sunlight levels. The present study provides the first evidence that sunlight may rapidly inactivate SARS-CoV-2 on surfaces, suggesting that persistence, and subsequently exposure risk, may vary significantly between indoor and outdoor environments. Additionally, these data indicate that natural sunlight may be effective as a disinfectant for contaminated non-porous materials.
Ref#2. These results highlight the sterilizing properties of ultraviolet radiation and suggest that, even if not sufficient on its own, increasing sunlight exposure in the upcoming weeks may help flatten the epidemic. Further studies on the potential protective effect of sunlight over COVID-19 will be needed to study the effect of changes in weather conditions in the next months on the spread of COVID-19, and to ascertain whether UV radiation may have a meaningful protective effect on the disease.....suggesting that prolonged exposure to UV radiation may be needed for its protective effect. In addition to the potential virucidal effect of UV radiation, it also modulates host immunity against viral infections, both directly activating the innate immune response and controlling vitamin D production, which in turn harbor positive immune effects.
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