**3. Inactivation of coronaviruses by gamma irradiation**

Gamma irradiation is commonly used for sterilization of plasticware (especially tissue culture flasks, bottles, pipette tips, and pipettes). For such applications, a high fluence (hundreds of kGy) may be used to kill any prokaryotic microbes and viruses [11]. When it comes to disinfecting surfaces, again, sufficiently high fluences may be employed to kill any microbes and viruses. For disinfection of frozen or liquid solutions, care must be taken to balance the need for adequate sterilization with maintenance of the expected performance of the solutions being irradiated [4]. Gamma radiation interacts with solutions in different manners, depending on a number of factors, including the temperature of the solution and the presence of radiation-scavenging compounds. At very low temperatures (< 60°C, for instance, and in the presence of radiation scavengers, such as concentrated proteins), the radiation impacts on the solution itself are limited, and the impacts on suspended microbes are more selectively targeted to vital macromolecules such as genomic material. These effects are termed "direct" radiation effects. At temperatures above freezing and in the absence of scavenging compounds, effects termed "indirect" are imparted to the solution. These are characterized as radiolysis products attributed to the interaction of photons with water, forming oxygen radicals that can damage not only suspended microbes but also any biological materials in solutions. As a result of the above, inactivation of viruses in solutions, such as animal serum or culture medium containing serum, is typically accomplished by irradiating the sample matrices frozen on dry ice [4, 8].

As mentioned already, gamma radiation is highly penetrating, therefore is ideal for pathogen reduction in deeply frozen containers of animal serum and other biological samples. The data pertaining to efficacy of gamma irradiation for inactivating coronaviruses [12–16] are displayed in **Table 1**. These data were collected using deeply frozen tissue culture medium containing small amounts of bovine serum (i.e., the harvest medium containing the virus that comprised the viral stocks tested). In each case, the sample temperature during irradiation was maintained through use of dry ice, so that primarily the direct effects of the radiation on the viral macromolecules were to be expected. As expected, based on the known mechanism of action of gamma radiation on the viruses, and the relatively large particle size (60–136 nm) of the coronaviruses, the inactivating efficacies of gamma irradiation on SARS-CoV, MERS-CoV, and SARS-CoV-2 were similar in the reported studies [12–16]. The consensus data indicate an efficacy of 0.5–0.9 log10 inactivation per kGy of gamma radiation. At the typical range of fluences administered to frozen animal serum (25–45 kGy), as an example, one would therefore expect >12 log10 inactivation of coronaviruses (i.e., 0.5 log10 inactivation per kGy 25 kGy). It may be predicted that


*a MERS-CoV, Middle East respiratory syndrome coronavirus; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; SARS-CoV, severe acute respiratory syndrome coronavirus. b ND, not determined. The highest fluence tested (0.15 kGy) failed to cause* ≥ *one log10 inactivation.*

#### **Table 1.**

*Efficacy of gamma irradiation for inactivating coronaviruses.*

#### *Physical Inactivation of SARS-CoV-2 and Other Coronaviruses: A Review DOI: http://dx.doi.org/10.5772/intechopen.103161*

members of the coronavirus family, including future emerging species, should each be highly susceptible to inactivation by gamma radiation.

Greater efficacy for inactivating coronaviruses may be expected when irradiating solutions at higher temperatures (especially above freezing) and in the absence of radiation scavengers. This is due to the additional contribution of the indirect effects of gamma radiation. Of course, under these conditions, the matrix being irradiated may be degraded to the point where it no longer is useful for the intended application.

As indicated in the above, gamma irradiation should be considered a very effective physical approach for inactivating coronaviruses, such as SARS-CoV-2. Reports have suggested, for instance, the suitability of gamma irradiation for inactivation of SARS-CoV-2, while preventing loss of antigenic content, for use in preparing vaccines [17]. In a practical sense, however, the requirement for a gamma radiation source such as cobalt<sup>60</sup> limit the general availability of this approach for routine use. Items or solutions to be gamma irradiated must be shipped to an irradiation facility to accomplish this.
