An overview on inactivated and live‐attenuated SARS‐CoV‐2 vaccines

Abstract After about 2 years since severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2), first infections were detected in Wuhan city of China in December 2019, which was followed by a worldwide pandemic with a record of 5.41 million deaths. Due to urgent need for the development of a safe and effective vaccine for coronavirus disease 2019 (COVID‐19), attempts for producing efficient vaccines are inexhaustibly continuing. According to a report by the World Health Organization (WHO) on COVID‐19 vaccine tracker and landscape, there are 149 vaccine candidates all over the world. Inactivated SARS‐CoV‐2 vaccines as a conventional vaccine platform consist of whole virus particles grown in cell culture and inactivated by chemicals. Because of benefits such as antigenic similarity to real virion inducing humoral and cellular immune responses and ease for transport and storage, these vaccines, including the vaccines produced by Bharat Biotech, Sinopharm, and Sinovac, are in use at large scales. In this study, we have a review on inactivated SARS‐CoV‐2 vaccines that are passing their phase 3 and 4 clinical trials, population which was included in the trials, vaccine producers, the efficiency, adverse effects, and components of vaccines, and other vaccine features.


| INTRODUC TI ON
The world community has been battling a global epidemic for about 2 years. Coronavirus disease 2019  is caused by the severe acute respiratory syndrome corona virus 2 (SARS-CoV-2). 1 Recent studies have reported cellular and humoral immune responses to COVID-19 in infected patients ( Figure 1A,B). Due to the high-rate transmission of the Middle East respiratory coronavirus compared with SARS-CoV-2, the need for an immediate vaccine design for this virus is exceedingly felt. 2 Vaccines are used for the prevention and treatment of COVID-19. 3 DNA-based vaccines, RNA-based vaccines, recombinant subunit vaccines, adenovirus-based vectors, and inactivated viruses are various types of SARS-CoV-2 vaccines that have lately been developed. 4 In many countries of the world, vaccination has been or is being carried out on a large scale, especially among frontline workers. 5 According to a document prepared by the US Food and Drug Administration, COVID-19 vaccines must have key attributes, for example, clinical data, toxicity, and description of immune responses in the animal model, to be licensed. 6 Inactivated vaccines are produced through the growth of SARS-CoV-2 in the cell culture and subsequent inactivation of the virus. 7 There are several methods of inactivation, such as the use of formaldehyde, glutaraldehyde, ultraviolet, and gamma rays. 8 Therefore, a biosafety level 3 is required to produce inactivated vaccines. Different countries, including China, Kazakhstan, and India, have developed this type of vaccine. Inactivated vaccines are given intramuscularly. Immune responses are induced against the spike proteins, matrix, envelope, and nucleoprotein. 7 The level of antibodies decreases over time, indicating the need for a long-term study of the protective effect of inactivated vaccines. 9 Ease of use is one of the benefits of using inactivated vaccines. So far, SARS-CoV-2 adjuvants have been evaluated in humans to be used for the improvement of immunogenicity. 10 Another merit of inactivated vaccines is their high speed of development, which makes them a viable option for developing COVID-19 vaccines. 2 Moreover, inactivated vaccines can be stored at 2-8°C, making them suitable for countries with limited cold storage capacity. 11 However, there are disadvantages such as the need for high levels of contagious virus. 10 Virus antigens and epitopes may be destroyed during the inactivation process, leading to a weakened immune response. 8 Inactivated vaccines have hitherto been developed for two viral diseases, influenza virus and poliovirus. 12 As of June 21, 2021, 1049 doses of SARS-CoV-2 inactivated vaccine had been vaccinated in China. 13 Due to genetic changes in the SARS-CoV-2 genome, the emergence of new strains of the virus is often observed.
The effect of vaccines on these new strains has not yet been determined. 14 With this in mind, this study was undertaken to evaluate the efficacy and immunogenicity of various inactivated vaccines produced against SARS-CoV-2. such as COVI-VAC deliver a robust immune response and are associated with the long-lasting cellular immunity. 15 The results of a preclinical study performed to assess the immunogenicity and safety of COVI-VAC in Syrian golden hamsters (Mesocricetus auratus) have demonstrated that this vaccine is safe and effective in small animal models at a single dose. Also, lower tissue viral loads and milder lung pathology were observed in Syrian golden hamsters vaccinated with COVI-VAC compared with those inoculated with wild-type viruses. In addition, the vaccine appeared to be resistant to reversal and could grow to a large extent. 16 In another study, the efficiency of this vaccine was evaluated against challenge with the Beta (B.1.351) variant in the same hamsters.
Twenty-seven days' postvaccination by COVI-VAC, animals were challenged intranasally with wild-type SARS-CoV-2 Beta variant. According to this report, COVI-VAC stimulates serum and mucosal antibody immune responses. 19 NCT05233826 is an ongoing phase 1 clinical trial study evaluating the safety and immunization of COVI-VAC as a booster dose in 30 healthy adults previously vaccinated with authorized mRNA or adenovirus vectors vaccine against COVID-19. There is still no available report on its results. 20 ISRCTN15779782 is an ongoing large, international, randomized controlled phase 3 clinical trial designed to provide adequate evidence of the safety and efficacy of this vaccine and is supported by the World Health Organization (WHO). The volunteers, healthy adults (aged ≥ 16 years) were randomly allocated either to placebo or vaccine group. 21 No results from this phase have been published so far.

| CoronaVac VACCINE
CoronaVac, also known as the Sinovac COVID-19 vaccine, is a two-dose β-propiolactone (BPL)-inactivated aluminum hydroxideadjuvanted SARS-CoV-2 vaccine produced by Sinovac Research and Development Co., Ltd. CoronaVac is one of the eight emergency use listing vaccines approved by WHO in 54 countries all over the world. 22 A double-blind, placebo-controlled, phase 1/2 clinical trial (NCT04383574) of this vaccine was performed in 422 healthy adults aged 60 years and older in China. In phase 1, participants (n = 72) received a 3μg or 6μg vaccine or placebo. In phase 2, participants (n = 350) were given either CoronaVac at 1.5, 3, or 6 µg per dose or placebo. In the safety populations from both phases, any adverse reaction within 28 days after injection occurred in 20% of participants in the 1.5μg group, 20% in the 3μg group, 22% in the 6μg group, and 21% in the placebo group. All adverse reactions were mild or moderate in severity, and injection site pain was the most frequently reported event. In phase 1, seroconversion after the second dose was observed in 100% of participants in the 3μg group and 95.7% in the 6μg group. In phase 2, seroconversion was identified in 90.7% of participants in the 1.5μg group, 98% in the 3μg group, and 99% in the 6 μg group. There were no detectable antibody responses in the placebo group. 23 To assess the immune persistence of a two-dose schedule of CoronaVac, and the immunogenicity and safety of its third dose in healthy adults aged 18 years and older, a double-blind, randomized, placebo-controlled phase 2 clinical trial was conducted. There were two vaccination schedule cohorts: days 0 and 14 (cohort 1) and days 0 and 28 (cohort 2) vaccination cohorts. Half of the subjects in each cohort were selected to receive an additional dose 28 days after the second dose, and the other half of the subjects were chosen to receive the third dose 6 months after the second dose. The results of these trials showed that a two-dose schedule of CoronaVac could generate favorable immune memory. Although neutralizing antibody titers decreased to near or below the lower limit of seropositivity 6 months after the second dose, the third dose given 8 months after the second dose was highly effective at recalling a SARS-CoV-2specific immune response. 24 It consists of BPL inactivated the whole virus with high spike protein density, combined with two adjuvants, alum and Dynavax's CpG

31
Preclinical studies have suggested that combination of adjuvantinduced high titers of neutralizing antibodies is associated with a shift in the cellular immune response toward Th1. 32  The hCoV-19/Turkey/ERAGEM-001/2020 strain was employed to test the safety of ERUCOV-VAC. In BALB/c mice, the vaccine was found to be highly immunogenic and induced a significant immune response. In K18-hACE2 mice, the ERUCOV-VAC vaccine displayed 100% protection against a lethal SARS-CoV-2 challenge. In the ferret models, viral clearance rates were similar to the safety assessment of the vaccine in upper respiratory tracts. The most prevalent minor adverse effect was discomfort at the place of the injection. 44 ERUCOV-VAC is presently in phase 3 clinical trial (NCT04942405).
Its name was shifted to TURKOVAC in this phase. protection against the SARS-CoV-2 variant of concern, B.1.617.2 (Delta). There was no report of anaphylaxis or deaths in relation to the vaccine. BBV152 was well-tolerated with an incidence of AVs over a median of 146 days, which was less than that detected in other COVID-19 vaccines. 51 The safety and immunogenicity of BBV152, adjuvanted with aluminum hydroxide gel (Algel) or a novel TLR7/8 agonist chemisorbed Algel, were examined in another study. In this regard, a strain of SAS-CoV-2 and a Vero cell platform were selected to induce a highly purified inacti-  (Table 1). 69 In another recent survey, Jingwen Ai et al. In addition, whole microbial vaccines provide faster immunity than vaccines based on nucleic acid, protein, and viral vector. The remarkable point is that nucleic acid-based vaccines are slower to develop than other types of vaccines, but they are safe and effective. 71

| INAC TIVATED VACCINE S IN PATIENTS WITH NE W VARIANTS
One of the most important issues in relation to vaccines is the emergence of new strains of SARS-CoV-2. Natural selection is responsible for choosing mutations to maintain the survival, proliferation, and fitness of organisms. 75 Therefore, the immunogenicity of vaccines against new strains requires constant evaluation.
Research has emphasized that the Omicron variant declines the neutralization ability of vaccines by evading neutralizing antibodies, which reduces immune responses. Decreased immune responses were found to be even less than in Mu and Beta strains. 76 Studies have pointed out that after injecting two doses of inactivated vaccine, a booster is needed. Booster vaccines can be a variety of mRNAs, protein subunits, and inactivated vaccines. 70,77,78 Immunoglobulin immune responses to Omicron variant occur only after booster injection. 79 In one study, the protective effect of BBIBP-CorV against new variants, B.1.1.7 in the UK (Alpha) and B.1.351 in South Africa (Beta), and Wuhan-1 reference strain (wild-type) was assessed.
The results showed that BBIBP-CorV was more effective against B.1.1.7 than B.1.351. 80 The efficacy of BBV152/COVAXIN against the Alpha strain and COVI-VAC vaccine against Beta strain has also been highlighted. 17,81 The results of an investigation on the protective effect of the CoronaVac against seven variants of The efficiency of available vaccines against new strains is a subject that needs further investigation.

CO N FLI C T O F I NTE R E S T S
The authors declare that they have no conflict of interest.

AUTH O R CO NTR I B UTI O N S
SK, MA, SA, MK, HG, NS, and MH contributed to comprehensive research and wrote the paper. AS participated in editing the manuscript. Notably, all authors have read and approved the manuscript.

DATA AVA I L A B I L I T Y S TAT E M E N T
The authors confirm that the data supporting the findings of this study are available within the article.

Type of adjuvant Efficacy Side effects Reference
No Adjuvant COVI-VAC stimulates serum and mucosal antibody immune responses based on phase 1 trial Well-tolerated, with no significant adverse events reported across the 48 patients enrolled in phase 1 16,19 Aluminum hydroxide 50.7% against symptomatic COVID−19 and 100% against hospitalization based on phase3 trial in Brazil, 65.9% based on study in Chile and 83% based on phase 3 in Turkey Mild/moderate, and most of the common adverse events were pain at the injection site, headache, fatigue, and myalgia [26][27][28] Alum and CpG 1018 Good humoral and cellular immune responses, based on phase 1/2 trial. Superiority against ChAdOx1-S in terms of geometric mean titer for neutralization antibodies based on phase 3.