The action of UV-C against covid19

The research about the action of ultraviolet radiation against covid19 arrived at an interesting conclusion: the team, leaded by Istituto Nazionale di Astrofisica and Università Statale di Milano and with the collaboration of Istituto nazionale dei tumori and Irccs, has provided the first experimental data about the high germicidal power of ultraviolet rays against Sars-Cov-2.
Andrea Bianco, head of the astronomers' team, says to MediaINAF (Google Translate):

We have seen very distinct behaviors. At the lowest concentration, we had to wait six days for the virus to replicate and infect cells without being subjected to UV-C illumination. In the intermediate concentration, which we took as a reference, already with the lowest dose of irradiation, after 24 hours we had a reduction of a factor greater than 1000. Waiting longer, we saw that the virus no longer grows back, that is, it is no longer able to replicate itself: it is said that it has been completely inhibited. While in the highest concentration, at the lowest dose of irradiation, at the beginning there is a fairly significant decrease in the amount of active virus but after it goes back up, which means that the virus has been inactivated but the concentration is so high that it is able to restart (because it has not been completely inhibited). When we gave him the second dose - 16.7 mJ / cm² - it was seen that it is sufficient to completely inhibit him : even after 72 hours he is unable to "regain his strength". In this high concentration of the virus, therefore, this aspect emerged: at low doses I can have a significant decrease in the active virus but he is still able to infect the cells, so after a certain time he restarts with his activity.

But:

Going from Uv-A to Uv-C the danger for man increases, in the sense that Uv-C radiation causes mutations, therefore increases the probability of having skin cancers considerably. So much so that, by law, the maximum dose of Uv-C that one can take daily is very low. Uv-B and Uv-A are much less so, so much so that we tan, protecting ourselves with creams to avoid the risk of cancer and skin rashes. The risk is there but with much less probability.

Bianco, A., Biasin, M., Pareschi, G., Cavalleri, A., Cavatorta, C., Fenizia, C., ... & Saulle, I. (2020). UV-C irradiation is highly effective in inactivating and inhibiting SARS-CoV-2 replication. medRxiv. doi:10.1101/2020.06.05.20123463

The potential virucidal effects of UV-C irradiation on SARS-CoV-2 were experimentally evaluated for different illumination doses and virus concentrations (1000, 5, 0.05 MOI). Both virus inactivation and replication inhibition were investigated as a function of these parameters. At a virus density comparable to that observed in SARS-CoV-2 infection, an UV-C dose of just 3.7 mJ/cm2 was sufficient to achieve a 3-log inactivation, and complete inhibition of all viral concentrations was observed with 16.9 mJ/cm2. These results could explain the epidemiological trends of COVID-19 and are important for the development of novel sterilizing methods to contain SARS-CoV-2 infection.

Nicastro, F., Sironi, G., Antonello, E., Bianco, A., Biasin, M., Brucato, J. R., ... & Trabattoni, D. (2020). Modulation of COVID-19 Epidemiology by UV-B and-A Photons from the Sun. medRxiv. doi:10.1101/2020.06.03.20121392

It is well known that 200-290 nm ultraviolet photons (hereinafter UV-C radiation) photo-chemically interacts with DNA and RNA and are endowed with germicidal properties that are also effective on viruses. Fortunately, Solar UV-C photons of this wavelength are filtered out by the Ozone layer of the upper Atmosphere, at around 35 km. Softer UV photons from the Sun with wavelengths in the range 290-320 nm (UV-B) and 320-400 nm (UV-A), however, do reach the Earth s surface. The effect of these photons on Single- and Double-Stranded RNA/DNA viruses and the possible role they play on the seasonality of epidemics, are nevertheless little studied and highly debated in alternative or complementarity to other environmental causes. Notably though, the effects of both direct and indirect radiation from the Sun needs to be considered in order to completely explain the effects of UV radiations in life processes (with e.g. the UV virucidal effect enhanced in combination to the concomitant process of water droplets depletion because of Solar heat). Herein we present a number of concurring circumstantial evidence suggesting that the evolution and strength of the recent Severe Acute Respiratory Syndrome (SARS-Cov-2) pandemics, might be have been modulated by the intensity of UV-B and UV-A Solar radiation hitting different regions of Earth during the diffusion of the outbreak between January and May 2020. Out findings, if confirmed by more in depth data analysis and modeling of the epidemics, which includes Solar modulation, could help in designing the social behaviors to be adopted depending on season and environmental conditions.