1. Introduction

In December 2019, a novel Betacoronavirus causing coronavirus disease 2019 (COVID-19), provisionally named 2019 novel coronavirus (2019-nCoV) and subsequently officially renamed as SARS-CoV-2 by the International Committee on Taxonomy of Viruses, was associated with a cluster of respiratory tract infections in Wuhan, Hubei Province, China and has rapidly spread across continents (Yang et al., 2020; Lupia et al., 2019).

COVID-19 causes clusters of severe respiratory illness (with serious infection of the lower respiratory tract followed by bronchitis, pneumonia and fibrosis (Conti et al., 2020)) similar to SARS-CoV and is associated with intensive care unit admission and high mortality (Huang et al., 2020).

The CoV virion contains at least four structural proteins: the spike (S), the envelope (E), the membrane (M) and the nucleocapsid (N) (Tortorici and Veesler, 2019); coronavirus entry into host cells is mediated by the transmembrane spike (S) glycoprotein that forms homotrimers protruding from the viral surface. S comprises two functional subunits responsible for binding to the host cell receptor (S₁ subunit) and fusion of the viral and cellular membranes (S₂ subunit) (Walls et al., 2020): S1 contains the receptor-binding domain, which directly binds to the peptidase domain of angiotensin-converting enzyme 2 (ACE2), whereas S2 is responsible for membrane fusion. When S1 binds to the host receptor ACE2, another cleavage site on S2 is exposed and cleaved by host proteases, a process that is critical for viral infection. The first step in viral entry is the binding of the viral trimeric spike protein to the human receptor ACE2, which is a type I membrane protein expressed in the lungs (Walls et al., 2020; Yan et al., 2020).

The complete clinical manifestation of COVID-19 is not clear yet, as the reported symptoms range from mild to severe, with some cases even resulting in death. The most commonly reported symptoms are fever, cough, myalgia or fatigue, pneumonia and complicated dyspnea, whereas the less common reported symptoms include headache, diarrhoea, haemoptysis, runny nose and cough.

As SARS-CoV-2 is a newly emerged virus, there is no established therapy for it; several therapeutic options have been considered, either as supportive therapies (i.e. steroids, heparin) or as antiviral or immunomodulatory treatment (i.e. remdesivir, hydroxychloroquine, tocilizumab, hyperimmune plasma). There has been an expanding number of studies published online and in academic journals on this topic, but some of these articles may be of limited quality and are pre-published without sufficient peer review. A critical appraisal of existing studies is needed to determine whether existing evidence is sufficient to support currently proposed management strategies (Bhimraj et al., 2020).

Amongst different possible therapies, ozone therapy seems to have an immunological role because of the modulation of cytokines and interferons, including the induction of gamma interferon. Some data suggest the possible role of ozone therapy in SARS, either as a monotherapy or, more realistically, as an adjunct to standard treatment regimens; therefore, there is increasing interest in the role of ozone therapy in COVID-19 treatment (Elvis and Ekta, 2011). The aim of this narrative review is to summarise current evidence on the antiviral activity of ozone in patients with COVID-19 and its possible therapeutic applicability in clinical practice.

2. Material and methods

A literature search was performed using the PubMed and Scopus database and the Italian Scientific Society of Oxygen Ozone Therapy (SIOOT) website (ZZZZZ, 2020). The search was limited to articles focused on ozone therapy published from January 2011 to July 2020. Search terms included “ozone” and “coronavirus”. The MeSH terms were “ozone” [All Fields] AND “coronavirus” [All Fields]. Articles were selected and narratively reviewed. All authors reviewed the title of the articles found by the initial search. Titles suggestive of ozone therapy in viral diseases were selected for abstract review. Articles not focused on the medical application of ozone (i.e. ambient and air pollution, sanitisation, disinfection and sterilisation of surfaces and water) and articles focused on ozone therapy uses other than pulmonary diseases or viral diseases were excluded.

3. Results

A total of 280 articles met the search criteria applied (Fig. 1 ). Of these, 33 titles appeared to be relevant to ozone therapy use in viral diseases; 220 titles were excluded because they were not focused on the medical application of ozone, and 27 were removed because they were focused on uses of ozone therapy other than pulmonary diseases or viral diseases. Nineteen articles were then excluded after the abstract review. Of the remaining 14 articles, one was a specific case report, and it was not included in the qualitative analysis.

3.1.1. Pharmacological properties

Ozone is triatomic oxygen (O₃). It is the most powerful oxidant found in nature and is produced by lightning and solar ultraviolet radiation. The pharmacological properties of ozone are attributed to its molecular structure ( Fig. 2 ). It reacts with organic compounds containing double bonds (i.e. polyunsaturated fatty acids), and it adds the three oxygen atoms to the unsaturated bond, forming ozonides. In blood, ozonides are immediately transformed into stable hydroperoxides, which have the ability to release oxygen when the pH increases as it occurs in degenerative processes and/or ischemic conditions (Di Mauro et al., 2019; Smith et al., 2017). Ozone is a molecule that could perform antiviral action by interfering with the virus replication phase; this feature is linked to the ability of ozone to oxidise cysteine residues through the formation of disulphide bridges present in the structures of the virus itself in high quantities. Coronaviruses, including SARS-CoV-2, are rich in cysteine, and these residues must be intact for viral activity (Rowen, 2019; Rowen and Robins, 2020). Ozone’s action consists of oxidation and inactivation of the specific viral receptors used for the creation of the bond structure of the cell membrane, therefore inhibiting the level of its first phase: cellular penetration. ACE2 receptors’ activity can be regulated and blocked through the control of Nrf2, an important nuclear message transductor. Ozone acts directly on Nrf2, and it could be an important physiological mechanism to block endogenous COVID-19 replication by preventing contact with SARS-CoV-2 receptors. Moreover, the activation of Nrf2 leads to a reduction of iron overload and the subsequent oxidative stress induced by elevated ferritine; thus, this ozone activity protects from apoptosis induced by oxidative stress (Smith et al., 2017; Sagai and Bocci, 2011). NLRP3 inflammasome, a cytosolic complex responsible for the production of IL-1β and IL-18, plays a crucial role at the beginning of and during inflammation in various diseases, including viral infections such as COVID-19. Ozone shows its anti-inflammatory activity through the modulation of the NLRP3 inflammasome. Therefore, it could protect from acute coronary syndromes and ischaemia reperfusion damage that occurs in the lungs of patients affected by COVID-19, attenuating NLRP3-mediated inflammation, enhancing the antioxidant activity of Nrf2 and inhibiting apoptosis (Wang et al., 2018).

4. Discussion

Ozone seems to have low antiviral activity by inhibiting viral replication and inactivating viruses; it is also an antiviral drug enhancer. Ozone is not an alternative to antiviral drugs, but antiviral activity is decreased with the combination of ozone therapy and antiviral drug treatment.

Combined ozone therapy and antiviral drugs demonstrated a reduction in inflammation and lung damage, helping to increase host immunity against infection. It can activate the cellular and humoural immune systems, and it can reduce inflammatory/apoptotic processes (Smith et al., 2017; Sagai and Bocci, 2011; Wang et al., 2018; Galiè et al., 2018). Several data describe the role of ozone in treating hypoxia by improving oxygen saturation and increasing oxygen supply (Elvis and Ekta, 2011; Rowen, 2019; Rowen and Robins, 2020).

The efficacy of ozone therapy is shown in the early stage of viral diseases before invasive ventilation is necessary; it is unsuccessful when used in cases of serious or critical conditions.

DIV is the easier route of administration, and its use has been hypothesized in the treatment of several pulmonary diseases (Bocci et al., 2015 Hernández Rosales et al., 2005). This route of administration requires minimal medical waste and very inexpensive (it only requires a butterfly syringe once an ozone generator is procured). EBOO results in rapid improvement, but it is expensive and must be performed by technicians who specialise in extravascular blood circulation. For the treatment of pulmonary diseases, MAH is preferred because it does not produce adverse lung reactions. A specific device is necessary to create ozone by medical oxygen when MAH is applied.

Ozone therapy is very inexpensive and safe, and resistance is not developed, so it may safely exploit the critical vulnerability in many viruses, including SARS-CoV-2 (Rowen, 2019; Rowen and Robins, 2020).

5. Conclusion

Systemic ozone therapy has several positive effects, such as control of inflammation, stimulation of immunity, low antiviral activity and protection from acute coronary syndromes and ischaemia reperfusion damage. This therapy could be a new method of immune therapy, so its use in combination with other antiviral drugs in COVID-19-positive patients may be justified, helpful and synergic. Ozone therapy carries no known adverse or toxic effects when performed properly. DIV is the easier route of administration, and its use has been hypothesized in the treatment of several pulmonary diseases, including theoretically SARS-CoV-2 infections.

Funding

Not applicable.

Authors’ contribution

The search on databases and the selection of articles were performed by Susanna Giordano and Cecilia Bertiond. The first draft of the manuscript was written by Susanna Giordano and Cecilia Bertiond. Silvia Corcione, Tommaso Lupia and Matilde Scaldaferri revised the manuscript and all authors commented on subsequent versions of the manuscript. All authors read and approved the final manuscript.

Author statement

The submitting author affirms that all individuals listed as authors agree that they have met the criteria of authorship and agree to the conclusions of the study. In order to meet the requirements of authorship, each author must have contributed to at least one aspect of each of the four criteria, as listed below. Please note that for Criteria 1 and 2, authors only to meet one of the two items listed. These criteria are not to be used as a means to disqualify colleagues from authorship who otherwise meet authorship criteria by denying them the opportunity to meet criteria 2 or 3. Therefore, all individuals who meet the first criterion should have the opportunity to participate in the drafting, review, and final approval of the manuscript. Any individuals not meeting the criteria may be mentioned in the Acknowledgements section of the manuscript.

Per the criteria defined by the International Committee for Medical Journal Editors (ICJME), please note the contribution made by each author listed in the manuscript. Please check all boxes that apply.

Cattel F.: contributed to conception and design critically revised manuscript gave final approval agrees to be accountable for all aspects of work ensuring integrity and accuracy.

Giordano S.: contributed to conception and design drafted manuscript gave final approval agrees to be accountable for all aspects of work ensuring integrity and accuracy.

Bertiond C.: contributed to conception and design drafted manuscript gave final approval agrees to be accountable for all aspects of work ensuring integrity and accuracy.

Corcione S.: contributed to design critically revised manuscript gave final approval agrees to be accountable for all aspects of work ensuring integrity and accuracy.

Lupia T. contributed to design critically revised manuscript gave final approval agrees to be accountable for all aspects of work ensuring integrity and accuracy.Scaldaferri M.: contributed to design critically revised manuscript gave final approval agrees to be accountable for all aspects of work ensuring integrity and accuracy.

Angelone L.: contributed to design critically revised manuscript gave final approval agrees to be accountable for all aspects of work ensuring integrity and accuracy.

De Rosa F: contributed to conception and design critically revised manuscript gave final approval agrees to be accountable for all aspects of work ensuring integrity and accuracy.

Declaration of Competing Interest

The authors report no declarations of interest.