SARS‐CoV‐2 and its variants of concern including Omicron: A never ending pandemic

Abstract The ongoing COVID‐19 pandemic caused by SARS‐CoV‐2 is associated with high morbidity and mortality. This zoonotic virus has emerged in Wuhan of China in December 2019 from bats and pangolins probably and continuing the human‐to‐human transmission globally since last two years. As there is no efficient approved treatment, a number of vaccines were developed at an unprecedented speed to counter the pandemic. Moreover, vaccine hesitancy is observed that may be another possible reason for this never ending pandemic. In the meantime, several variants and mutations were identified and causing multiple waves globally. Now the safety and efficacy of these vaccines are debatable and recommended to determine whether vaccines are able to interrupt transmission of SARS‐CoV‐2 variant of concern (VOC). Moreover, the VOCs continue to emerge that appear more transmissible and less sensitive to virus‐specific immune responses. In this overview, we have highlighted various drugs and vaccines used to counter this pandemic along with their reported side effects. Moreover, the preliminary data for the novel VOC “Omicron” are discussed with the existing animal models.


| INTRODUCTION
The unwanted COVID-19 outbreak after its possible emergence from Wuhan Seafood market of China, spread to the whole world, and has taken millions of people under its cover with successive hits Mohapatra et al., 2021aMohapatra et al., , 2021bMohapatra & Rahman, 2021). As reported, the causative virus (SARS-CoV-2) emergence from bats and pangolins. After its possible emergence on December 31, 2019, the disease is with us since last two years and putting the healthcare establishments under tremendous pressure (Mohapatra et al., 2021c;Pal et al., 2022). The virus and its variants normally affect the respiratory system with common symptoms breathing difficulties, cough, fatigue, and fever. It may also relate to neurological complications including loss of smell, taste, and also cerebrovascular disorders. However, it is still unclear that whether the neurological complications are due to the viral infections or the consequence of immune reactions (Alonso-Bellido et al., 2021). COVID-19 is causing severe complications in immunocompromised persons with diabetes, cardiovascular disorders, obesity, psychiatric disorder, or organ transplant history (Arumugam et al., 2020). As reported, it may also affect kidneys, heart, liver, gut, nervous system, and finally progresses to multiple organ damage . Furthermore, the pandemic has devastated the stock and financial markets, and the global economy dramatically (Lenzen et al., 2020;Nicola et al., 2020).
The emergence of large numbers of variants and mutations of SARS-CoV-2 is not only responsible for life loss but also affect various educational, cultural, and socioeconomic activities (Mohapatra et al., 2021dMohapatra, Pintilie, et al., 2020). The variants, such as Alpha (B.1.1.7), Beta (B.1.351), Delta (B.1.617.2), Gamma (P.1), Epsilon (B.1.427/B.1.1429), Eta (B.1.525), Zeta (P.2), Iota (B.1.526), Theta (P3), and Kappa (B.1.617.1), were identified previously in multiple waves of this pandemic (Choudhary et al., 2021;Mohapatra, Sarangi, et al., 2022). Now, the recent variant of concern (VOC) B.1.1.529 (Omicron) was identified from South Africa and Botswana (Choudhary et al., 2021;Mohapatra, Sarangi, et al., 2022). This new variant has large number of mutations, which are concerning. As there is no efficient approved treatment, the world is going to observe another wave of this outbreak soon if not we are wrong. The Omicron wave is observed very differently in most countries as compared to the prior waves of this pandemic (Arnaout & Arnaout, 2022). As per another recent study, initially the Omicron wave has an exponential-like growth rate and will show small resurgences in the month of April and June in Northern and Southern Hemisphere countries, respectively (Huang et al., 2022). Moreover, the safety and efficacy of the current vaccines against the VOCs of SARS-CoV-2 is debatable Uddin et al., 2021). The COVID-19 vaccination program must be supported worldwide, especially in countries with low vaccination rate, which will be an effective strategy for preventing the virus circulation as well as the emergence of its novel VOCs (Islam et al., 2022;Mattiuzzi & Lippi, 2022). The COVID-19 case fatality rate is not fixed but it changes with population, time, and other socioeconomic factors along with the mitigatory efforts of the countries (Ghayda et al., 2022). In some regions, vaccine hesitancy is observed which may be another possible reason for this never ending pandemic. In this overview, we have highlighted various drugs and vaccines used to counter this pandemic. The vaccine safety, hesitancy, and vaccinerelated side effects are also discussed herewith to make a complete understanding. Furthermore, we have also discussed the preliminary data for the novel VOC Omicron and the existing animal models for easy understanding.

| MATERIALSANDMETHOD
The data were collected from authentic academic databases, such as Scopus, PubMed, Web of Science, and Google Scholar, along with some public and government health organization websites including the World Health Organization (WHO), and Centre for Disease Control and Prevention (CDC) resources. After literature search covering 2020-2022, the obtained data were carefully examined, and only the relevant studies/reports were considered for critical discussion. As per the initial search, 284 articles/ data found, out of which 150 articles/data were selected for further analysis based on the specific inclusion and exclusion criteria. The most relevant and critical literature was given preference, and the obtained data were used to develop this specific area review.

| COVID-19drugs
The COVID-19 pandemic is playing its game since last two years and taking millions of lives and livelihood. There is no efficient approved treatment strategy (drugs), and emergence of multiple variants along with large number of mutations is a matter of concern. A number of repurposing drugs and steroids are commonly used to treat the disease. However, some serious side effects are observed as listed in Table 1 There were 9 deaths,out of which 2deaths are from 5-day remdesivir group,3 from 10-day remdesivir group and 4 from SOC group (Spinner et al., 2020) Hypokalemia, Headache, Nausea (Spinner et al., 2020) Tocilizumab Interleukin−6 receptor inhibitor Tocilizumab can bind to both mIL−6R and sIL−6R. And inhibit classical and transsignals. Also inhibit interleukin and thus decrease the cytokine storm  two new drugs to treat COVID-19 patients (WHO, 2022). An oral drug, namely, baricitinib, has been strongly recommended for the severe and critical COVID-19 patients. It is a class of JAK inhibitors and recommended to use with corticosteroids and suppress the overstimulation of immune system. The second drug sotrovimab (monoclonal antibody drug) has also recommended by WHO for moderate COVID-19 patients having high risk of hospitalization (older, immunocompromised, and unvaccinated). Vaccines and antivirals protect the people against severe COVID-19 disease manifestations and complications. In this context, the emergency use of two antivirals, namely, molnupiravir and nirmatrelvir, has been approved, which may reduce disease progression by 30% and 89%, respectively, with 5 days use (Soriano et al., 2022). The commonly used drugs, their mechanism of action, and clinical trial results are also reported in Table 1.

| COVID-19vaccines
The COVID-19 caused by the novel SARS-CoV-2 has been in existence in the last two years after its discovery in December 2019, in Wuhan, China. The COVID-19 is continuing to cause severe morbidity and mortality throughout the world. The WHO has been evaluating the results of the safety and efficacy of vaccines that are currently under different stages of clinical development. More than 10 COVID-19 vaccines have been approved by the WHO for emergency use application (EUA). However, there is widespread hesitancy regarding the vaccine safety, and uncertainty on the efficacy of the available vaccines (Table  2). Moreover, the vaccines that have been currently approved have not completed the phase III clinical trials. Most countries have been plagued by the vaccine hesitancy problem, wherein people are hesitating to take the vaccines considering fears over both short-term and longterm safety concerns. The long-term safety of the vaccines would certainly require further extensive observational studies that could contribute to improved acceptance of the vaccines. Among the vaccines that are currently available, more than 70% efficacy was noted except for the Ad26. COV2·S vaccine developed by the Janssen pharmaceutical, USA, that revealed 66.9% efficacy as evidenced by the results of phase III clinical trials (Sadoff et al., 2021). More than 90% vaccine efficacy was noted among the Sputnik V, mRNA-1273/Moderna, and BNT162b2/Pfizer/BioNTech vaccines (Baden et al., 2021;Logunov et al., 2021;Polack et al., 2020). The vaccine adverse events (AE) were noted in all the vaccines and were mild, self-limited, and required no hospitalization. Common AE's notes were injection site pain, fatigue, myalgia, chills, and nausea. More serious AE's notes with the administration of BNT162b2/ Pfizer/BioNTech vaccine included lymphadenopathy, herpes zoster, appendicitis, myocarditis, Facial palsy, among others (Barda et al., 2021).
Although more than 80% of the infections were asymptomatic, mild, and self-limiting, geriatric age patients and people with underlying chronic diseases either succumbed or needed ICU treatment and extended hospitalizations. Due to the novelty of the virus, there were no antiviral agents, and most initial therapeutic interventions were taking the help of repurposed drugs, convalescent sera, and monoclonal antibodies to manage serious cases of COVID-19. After the discovery, manufacture, approval, and availability of vaccines, people throughout the world are being vaccinated. However, the vaccination by the novel mRNA carrying the spike protein using a viral vector had caused uncertainty over the efficacy and safety of vaccination that resulted in widespread vaccine hesitancy. This is because most vaccine candidates that were approved for emergency use by the WHO, USFDA, and local regulatory authorities were still under phase III of clinical trials and not much data on the vaccine candidate's safety, efficacy, and long-term consequences were available in the public domain.

| COVID-19vaccinesideeffects
The vaccine side effects normally play a crucial role in public confidence on vaccine and its uptake process. Riad, Pokorná, et al. (2021), have carried out a cross-sectional survey to understand the COVID-19 vaccine side effects following the Pfizer-BioNTech vaccine among the Czech Republic healthcare workers. The study reported injection site pain (89.8%) and fatigue (62.2%) along with headache (45.6%), muscle pain (37.1%), and chills (33.9%) as most common side effects. These side effects were observed among 43-year-old group people for one day (45.1%) or three days (35.8%) after vaccination. Antihistamine is the commonly used drug for side effects, and recommended for further investigation. The people having two doses of vaccine were generally associated with higher frequency of side effects.
Bangladesh started its COVID-19 vaccine administration in early 2021. Jahan et al. (2021) have evaluated the side effects shown by the Bangladeshi residents after receiving Oxford-AstraZeneca's Covishield vaccine first dose. This study was conducted on 474 vaccine recipients with both online and offline questionnaires from March to April 2021 and the data were analyzed using SPSS. Headache, fever, pain at the injection site, myalgia, and fatigue were the commonly reported symptoms. The overall side effects were found among the younger populations significantly. Moreover, a higher number of female individuals suffered post-vaccination side effects than the males. The study also concluded that the Covishield vaccine is a well-tolerated vaccine among the people of different age groups. Menni et al. (2021) have investigated the safety and effectiveness of Pfizer-BioNTech (BNT162b2) and the Oxford-AstraZeneca (ChAdOx1 nCoV-19) COVID-19 vaccines. Systemic and local side effects were reported by individuals after both the doses of the above vaccines. The systemic side effects were found to be more common among the previous COVID-19 infected individuals than among those without known past infection. However, both the vaccines decreased the risk of COVID-19 infection pandemic. In quick time, a number of COVID-19 vaccines were developed and authorized. Hatmal et al. (2021) have assessed the side effects and perceptions following COVID-19 vaccination in Jordan through a crosssectional study with total of 2213 participants. The data were statistically analyzed, and certain machine learning tools have been used to predict the severity of the side effects. As per the study, 10% severe, 39% moderate, and 21% mild side effects were reported in individuals. The postvaccination side effects (fever, headache, fatigue, chills, joint pain, dizziness, and myalgia) were common and non-life-threatening. Saeed et al. (2021) have reported a cross-sectional survey on Sinopharm COVID-19 vaccine side effects in the United Arab Emirates. Normal injection site pain, fatigue, lethargy, and headache were more commonly reported after first vaccine dose. The postvaccination side effects were more common in females than the males. The reported post-vaccination side effects were mild and no need to hospitalization (CDC, 2021). So the study will be helpful to reduce vaccine hesitancy among people. Elnaem et al. (2021) have investigated the attitudes, perceptions, and experiences of the COVID-19 vaccine side effects in Malaysia through a cross-sectional survey conducted between May and July 2021. A total of 428 respondents registered in this survey and 76.8% had experienced vaccine-related side effects. Nearly 40% respondents registered side effects with the second dose of Pfizer-BioNTech vaccine. Injection site pain (61.1%) and tiredness (48.8%) were the most common side effects that reported. The respondents received Sinovac vaccine was experiencing lower side effects than others with Pfizer-BioNTech and Oxford-AstraZeneca. Moreover, Xu et al. (2021) have also reported the delayed hypersensitivity reactions to Pfizer/Moderna SARS-CoV-2 mRNA vaccines with second-dose administration. The overall attitudes toward the COVID-19 vaccination drive were still positive. However, independent studies are strongly recommended to strengthen public confidence on the COVID-19 vaccine to counter the consecutive waves of this pandemic.

| COVID-19vaccinehesitancy
The effectiveness of COVID The study showed high acceptability of COVID-19 vaccination among children, but substantial hesitancy among adults may be due to uncertainty in vaccine safety and effectiveness. Vaccine hesitancy is the major region for the spread of COVID-19 pandemic. Qunaibi et al. (2021) have conducted an online survey among Arab-speaking subjects from January 14, 2021, to January 29, 2021. This large-scale multinational study showed significant rate of COVID-19 vaccine hesitancy (>80%) among Arabs in and outside the Arab region. The reported side effects and healthcare policies are the possible reasons for COVID-19 vaccine hesitancy. It is recommended that the health authorities have to transparently address these concerns to improve vaccine acceptance. Lucia et al. (2021) have evaluated the COVID-19 vaccine hesitancy among US medical students as they are the frontline healthcare providers and exposed more. As per this study, all the participants had positive attitudes toward COVID-19 vaccines, but only 53% agreed to participate in a vaccine trial, while 23% were unwilling to take the vaccine immediately upon FDA approval. To promote the COVID-19 vaccine drive, the study also highlighted the need of an educational curriculum with vaccine safety and effectiveness. Holeva et al. (2022) have reported a web based survey conducted in Greece and asking the respondents about their attitude toward the COVID-19 vaccine. The data are based on age, gender, marital status, professional status, educational level, and residential area. As per the present study, females and less educated participants were more hesitant toward COVID-19 vaccines. Hudson et al. (2021) have conducted a literature review on several factors associated with vaccine hesitancy and the public health responses toward COVID-19. The study suggested pursuing strategies to enhance public confidence on available COVID-19 vaccines. Kelekar et al. (2021) have conducted an online survey to assess vaccine hesitancy among the dental and medical students as they are exposed to COVID-19 patients. As per the survey, 45% dental and 23% medical students were hesitant to receive COVID-19 vaccines. The authors have also highlighted the need of specific curricula design to increase the knowledge of students about the COVID-19 vaccines. Ehde et al. (2021) have conducted a survey in the United States on early vaccine hesitancy, factors and reasons associated with it, and whether vaccine willingness changed among the participants. 90% of the undecided group wanted additional information about the vaccine before deciding. The COVID-19 vaccine hesitancy decreased during the pandemic over the time. Freeman et al. (2021) have discussed on various written information or statements about COVID-19 vaccination, efficacy, and safety to increase vaccine acceptance. In this UK-based study registered with ISRCTN, ISRCTN37254291, nearly 10% population was strongly hesitant about the vaccines. Gender and ethnicity have not affected the outcomes appreciably. Jain et al. (2021) have assessed vaccine hesitancy and the related factors among Indian medical students through an online questionnaire filled from February 2 to March 7, 2021. This study also reported vaccine hesitancy among 10.6% participants. Lack of awareness along with safety and efficacy are the major reason for COVID-19 vaccine hesitancy. Among two available vaccines, the medical students preferred Covishield. Troiano et al. (2021) have discussed the vaccine acceptance during this COVID-19 pandemic. The percentage of COVID-19 vaccine acceptance for students (86.1%), general population (77.6%), and for influenza vaccine (69%) was recorded. Several factors influenced the vaccine hesitancy (gender, age, religiosity, ethnicity, working status, education, income, politics, etc.). The common reasons for vaccine hesitancy are concerns about safety, lack of trust, and doubts on efficiency of vaccines. Hence, efforts should be done to provide correct information to the people about vaccines. Duong et al. (2021) have examined the COVID-19 vaccine hesitancy on 387 school principals across Taiwan through an online survey. As per the survey, male principals with higher health literacy showed lower vaccine hesitancy.
COVID-19 vaccine hesitancy will pose substantial risks for the people and also making difficulty to achieve herd immunity against COVID-19. The decision-makers should be careful to develop targeted strategies to reduce vaccine hesitancy among general public as mass vaccination is the only pathway to counter this pandemic . The attitude of healthcare professionals is crucial in creating public trust on vaccines. Keeping in mind, Riad, Abdulqader, et al. (2021) have evaluated the attitude of dental students toward COVID-19 vaccines through a cross-sectional study using an online questionnaire. Among 6639 students from 22 countries, 22.5% students were hesitant, and 13.9% students rejected COVID-19 vaccines. The participants from low-and lower-middle income economies showed greater vaccine hesitancy.
Vaccine hesitancy is also an important obstacle for pediatric vaccination. Bagateli et al. (2021) have reported 2.8% vaccine hesitancy among the parents of children and adolescents living in Brazil on the basis of a validated questionnaire. As per this survey, vaccine hesitancy is very low among caregivers living in Brazil; however, they are willing to vaccinate their offspring against COVID-19. A recent study reported the willingness to receive the booster dose of COVID-19 vaccine in Poland (Rzymski et al., 2021). Women, elder individuals, people with obesity, chronic diseases, and people taken influenza vaccine previously were willing to receive a booster COVID-19 dose. This study also reported some vaccine hesitancy in the studied group.

| SARS-CoV-2anditsvariants
Continuous changes occur in the genetic code (genetic mutations) of SARS-CoV-2 during replication of the genome. A variant is a viral genome (genetic code) and has one or more mutations which differentiate it from the other variants of the virus. However, mutation is a single change in the virus's genome (genetic code). A large number of variants of SARS-CoV-2 are documented worldwide throughout this pandemic since last two years. Moreover, a group of variants having similar genetic changes (lineage or group of lineages) is designated as VBM, VOI, VOC, and VOHC as listed below (Tables 3 and 4) and are routinely monitored (CDC, 2021c;ECDC, 2022). Vaccine inequity, hesitancy, and the presence of large number of immunocompromised persons in a particular region are the possible combined reasons for the continuous emergence of multiple SARS-CoV-2 variants (Burki, 2022;Dhawan et al., 2022).

| Omicron, the highly transmitted VOC
Omicron has become the dominant variant in many countries worldwide. The B.1.1.529 variant (Omicron, a new variant with large number of mutations) was first reported as VOC by WHO on 26 November 2021 and was identified from the specimens collected from South Africa and Botswana (GISAID, 2021b;WHO, 2021). As per preliminary evidences, this novel variant has increased risk of reinfection as compared with the other SARS-CoV-2 VOCs. It may be associated with enhanced transmissibility and reduced vaccine-induced immunity. Sore throat, body ache, fever, and weakness are the reported earliest common symptoms of Omicron. This new variant has started a fresh wave like other variants of concern (alpha, beta, and delta) across the entire world (Fontanet et al., 2021). The COVID-19 cases are increasing rapidly worldwide, and the UK and the USA have reported highest cases (GISAID, 2021b;Karim & Karim, 2021;Mohapatra, Sarangi, et al., 2022). In this context, to counter the spread of Omicron several countries has announced that masks are again compulsory on public transport and in schools and shops.
The current vaccines are less effective against Omicron, but still providing some protection. Omicron contains more than 50 mutations, out of which at least 30 in its spike protein (Callaway, 2021;Torjesen, 2021). Most of the COVID-19 vaccines are based on the S-protein RBD. The Delta variant has two mutations and Beta variant has three mutations; however, Omicron has between 10 and 15 mutations in the said S-protein RBD region. The used COVID-19 vaccines were less effective against Delta and Beta variants, which may be due to the mutations that helped partially evade immune responses. However, for this new Omicron variant, many functions are still unknown. The initial laboratory data suggested that Omicron variant likely to weaken the COVID-19 vaccine protection (Callaway, 2021;Cross, 2021;Mohapatra et al., 2022b;Torjesen, 2021). As the Omicron S-protein RBD has large number of mutations, which is the primary target for the monoclonal antibody-based therapy. So, the FDA-approved monoclonal antibodies may be less effective against the B.1.1.529 (Omicron) variant (Takashita et al., 2022). Recently, Pajon et al. (2022) have reported that the neutralization of Omicron (B.1.1.529) variant has increased substantially after mRNA-1273 vaccine booster dose. Researchers are still trying to answer whether antibodies produced from vaccination can neutralize Omicron, still it is debatable as very few data's are available on it. As per the reported studies, the risk of hospitalization has been found to be lower for Omicron as compared with Delta. Moreover, the vaccine vendors have also planned to make new versions of their vaccines T A B L E 4 The currently existing variant of concern (VOC) (CDC, 2021c; ECDC, 2022) to counter such emerging variants of SARS-CoV-2. In between this, the World Health Organization has suggested the individuals to take measures (maintaining physical distancing, wearing well-fitted masks, following hand hygiene, avoiding crowded spaces, receiving up-to-date vaccines, etc.) to reduce the risk of COVID-19 infection from such emerging variants. Furthermore, it is also essential to enhance the surveillance and sequencing efforts to understand circulating such VOCs.

CoV-2VOCs
During the pandemic, the virus was noted to frequently undergo mutations and evolve into different variants that were named as variants of concern, variants of interest, and variants under observation depending on the type of mutations Mohapatra et al., 2022b). The initial Wuhan strain underwent mutation (D614G), probably the first, that was identified in March 2020, and later the virus further developed into the Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), and Delta (B.1.617.2). All of these variants along with the most recent Omicron variants (B.1.1.529) were categorized as VOCs due to the increased mutations at the spike protein, N-terminal domain (NTD), and other regions including the RBDs. Among the SARS-CoV-2 VOCs, the Delta variant and the Omicron variants showed more significant mutations, and therefore, the current vaccines and their efficacy in protecting against infection were carefully considered (Mohapatra et al., 2022c). A single dose of ChAdOx1 (Oxford/AstraZeneca) COVID-19 vaccine was not sufficient for the development of neutralizing antibodies, especially against the Delta variant. The concentrations of serum neutralizing antibodies measured after vaccination revealed that although the antibody concentrations were sufficient to neutralize the Alpha variant, there was no protection against the Delta variant. However, twodose vaccination/complete vaccination gave 95% protection against the Alpha variant with three to fivefold lower protection against the Delta variant (Planas et al., 2021). Al-Tawfiq et al. (2022) have reported a significant reduction in neutralization rates against Delta (B.1.617.2) variants for vaccinated and convalescent patients with prior COVID-19 history. However, lower rate of infection due to Delta variant was found after the second dose of Oxford-AstraZeneca, Pfizer-BioNTech, and Moderna vaccines. Two-dose ChAdOx1 (Oxford/AstraZeneca) vaccines and an equal number of people who were given 2 doses of BNT162b2 (Pfizer-BioNTech) vaccine were analyzed for neutralization antibody responses . This study noted that the 2 doses of vaccine as compared to a single dose produced higher concentrations of neutralizing antibodies. Despite effective neutralization of the original Wuhan strain, the antibody titers were reduced by 11.30-fold against the Delta variant. The BNT162b2 (Pfizer-BioNTech) vaccination elicited higher titers (mean titer = 11473) of antibodies against full vaccination with ChAdOx1 (Oxford/AstraZeneca) vaccine (mean titer = 1325.6). The serum neutralizing antibody titers were 6-fold less sensitive to the Delta variant among the convalescent patients. The serum neutralizing antibodies against Delta variant were reduced by 8-fold after complete vaccination with ChAdOx1 (Oxford/AstraZeneca), and BNT162b2 (Pfizer-BioNTech) vaccine with ChAdOx1 (serum geometric mean titer-654) being less effective (pvalue-0.0006) as against the BNT162b2 vaccine (serum geometric mean titer-3372) (Mlcochova et al., 2021). After complete vaccination with BNT162b2 (Pfizer-BioNTech) vaccine, the concentration of neutralization antibodies was found to be reduced by more than 10-fold against the Delta variant that could have been responsible for breakthrough infections (Mlcochova et al., 2021). The neutralization antibody titers were reduced by 4-fold against the Delta variant and other VOCs after complete vaccination against most mRNA vaccines. Also, it was noted that there is a possibility of a 2-to 10-fold increase in neutralizing antibody titer after a single-dose vaccination among previously infected people (protection of 75% (95% CI 53-89) after full vaccination against 95% (CI: 85-98) protection for a previously infected person with single-dose vaccination) . A booster with mRNA-1273, and Coronavac (Sinovac) delivered after 6 months after the second dose demonstrated a 20-fold, and 5-fold increase in neutralization antibody titers, respectively (Cromer et al., 2022). The previous infection-related immune responses were found to wane after 6 months and only 41% protection was noted against the Delta variant. However, the levels of protection increased to about 95% after a single dose of mRNA vaccination (Cromer et al., 2022).
The antibody concentrations were also measured among people who received heterologous vaccinations (alternative ChAdOx1-S and BNT162b2 vaccines), and single-dose vaccination in previously infected, convalescent, and people with breakthrough infections. The serum antibodies were able to efficiently neutralize most other VOCs except the Omicron variant. Neutralization antibodies against Omicron were noted among people who received heterologous ChAdOx1-S-BNT162b2 vaccines and homologous BNT162b2 as compared to a homologous ChAdOx1-S vaccine. Neutralization antibodies against Omicron were undetectable in persons after 6 months of receiving 2 doses of the mRNA-1273 vaccine. More effective neutralization antibodies Delta variant as compared to the Omicron were discovered in persons who received vaccination after being previously infected and vice versa (Rössler et al., 2022).
The plasma antibody concentrations at 6-month period as measured in 50% neutralization titer (NT50) among people with previous COVID-19 history revealed more than 60-fold reduction in the neutralizing antibodies against Omicron variant as against original Wuhan variant (Mean ± SD of 58 ± 51 Vs 32 ± 23). Six months after complete vaccination, a booster dose with mRNA vaccineinduced Omicron neutralizing antibodies (Schmidt et al., 2021). This suggests the potential benefit of booster vaccination against infection with the Omicron variant. Neutralization antibody titers against Delta, and Omicron variants among other VOCs after complete vaccination with mRNA-1273, BNT162b, and Ad26.COV2.S vaccines were analyzed. Omicron neutralizing antibodies were undetectable in all vaccinated people who received 2 doses. On the contrary, people who received a booster dose detectable neutralizing antibodies with 4-to 6-fold lower antibody titers against Omicron variant as against other VOCs (Garcia-Beltran et al., 2022). When CoronaVac and BNT162b2 vaccines were assessed for the presence of detectable neutralizing antibody titers against Omicron variant, people who received CoronaVac had no detectable antibodies as against BNT162b2 vaccines who developed some antibodies (35.7-39.9-fold lower antibodies against Omicron) . A significant reduction in the titers of neutralizing antibodies was noted among convalescent and fully vaccinated people. Nevertheless, after the third dose of the mRNA vaccine considerably improved the protection against Omicron infection (Cameroni et al., 2021). However, it is recommended that more conserved antigens be used to prepare vaccines for improved efficacy against the Omicron variant (Sievers et al., 2022).

| Animalmodels
The current COVID-19 pandemic has put enormous strain on health care system. Therefore, the development of animal models to understand the immune responses observed in patients infected with SARS-CoV-2 or its variants is urgently needed. Moreover, suitable and appropriate models are crucial to accelerate the testing of therapeutic drugs and vaccines. In this context, Bi et al. (2021) have summarized the details of transmission, pathology, and immunology induced by SARS-CoV-2 in various reported animal models. Mouse model is commonly used to understand the viral pathogenesis for different diseases due to its small size and low cost (Mohapatra et al., 2021f). Mouse (Mus musculus) model was widely used for other CoVs such as MERS-CoV (Cockrell et al., 2016;Li et al., 2017) and SARS-CoV (Channappanavar et al., 2016;Day et al., 2009).
As ACE2 is the principle human cellular receptor for the viral S-protein, some studies suggested that mink, ferrets, macaques, common marmosets, felines, rabbits, and hamsters are naturally susceptible to the infection (Brooke & Prischi, 2020;Chan et al., 2020;Younes et al., 2020;Zeiss et al., 2021). Due to lack of proper receptors, mice may not be naturally susceptible to SARS-CoV-2 infection. It was also confirmed by some experimental studies. SARS-CoV-2 requires human ACE2 receptor not mouse ACE2 . Several strategies have been employed to resolve this issue, such as development of genetic modification and mouse adapted virus. Moreover, the Syrian golden hamster (Mesocricetus auratus) is small mammal and was used as a well-characterized model to study the infections of various human respiratory viruses (SARS-CoV and influenza virus) (Iwatsuki-Horimoto et al., 2018;Miao et al., 2019;Munoz-Fontela et al., 2020;Roberts et al., 2005Roberts et al., , 2008Rosa et al., 2021;Rosenke et al., 2020). As ACE2 domain of human closely resembles that of hamsters, hamsters may be suitable as natural model for studying the SARS-CoV-2 infection (Chan et al., 2020).
The small animals (Syrian hamsters, human ACE2 transgenic mice, wild-type mice), and large animals (ferrets, F I G U R E 1 Animal models used to understand SARS-CoV-2 transmission [Colour figure can be viewed at wileyonlinelibrary.com] Cynomolgus macaques, Rhesus macaques) may contribute significantly as suitable animal models (Figure 1) to evaluate the vaccine efficacy and testing of therapies against SARS-CoV-2 (Takayama, 2020;Tiwari et al., 2020). The Syrian hamster (M. auratus) rapidly developed into a popular model. However, among other hamsters, Roborovski dwarf hamster (P. roborovskii) more closely mimics the disease with frequent lethal outcomes (Gruber et al., 2021). So, different hamster species should be used to study different courses of COVID-19 manifestations. Age and sex are two risk factors for COVID-19 patients. Keeping in mind, some studies confirmed age-dependent differences in SARS-CoV-2 infected Syrian hamsters. 40-80 week-old Syrian hamsters showed severe clinical signs as compared to 7to 9-week-old hamsters . In another study, middle-aged Syrian hamsters (32-34 weeks) showed more pronounced weight loss, delayed spread of virus in the lungs, and delayed virus clearance as compared with 6-week-old hamsters (Osterrieder et al., 2020).
A number of waves of this ongoing novel COVID-19 pandemic hit several counties since last two years and associated with high morbidity and mortality. There is no particular drug to treat this novel disease. So, to counter it vaccines were developed at an unprecedented speed and their efficacy is still debatable. Also, large number of SARS-CoV-2 variants continues to emerge and some appear more transmissible and less sensitive to virus-specific immune responses. Keeping in mind, de Vries et al. (2021) have reviewed the role of animal models in assessing therapeutic and prophylactic options to interrupt SARS-CoV-2 transmission. Jia et al. (2021) have discussed the mechanisms of SARS-CoV-2 infection, role of ACE2 in cell entry, along with immunopathology such as lymphocyte dysregulation, antibody responses, and cytokine storm. They have also highlighted the research progress of animal models for better understanding of the pathogenesis of COVID-19, which will be helpful for its treatment. Macaque infection has been the gold standard to illuminate vaccine efficacy and immune response patterns after infection by VOC (Altmann, 2021;Corbett et al., 2021;McMahan et al., 2021;Yu et al., 2020). Some recent studies suggest little differences in infectivity and virulence of emerging VOCs of SARS-CoV-2 in Syrian hamsters (Abdelnabi et al., 2021;Nunez et al., 2021;Yadav et al., 2021). However, studies are limited.

| CONCLUSION
Multiple waves of this ongoing COVID-19 pandemic hit several counties (in a more general way we can say the whole globe) since last two years after its emergence and associated with high morbidity and mortality. As there is no particular drug to treat this novel disease so, vaccines were developed at an unprecedented speed and their efficacy is still debatable. Large number of SARS-CoV-2 variants continues to emerge and some appear more transmissible and less sensitive to virus-specific immune responses. Therefore, scientists, researchers, physicians, and several agencies are in tremendous pressure to understand the way to counter it. In this mean time, we may only recommend the general public to obey the COVID-19 guideline as much as possible and stay up to date with their vaccine doses. Prior immunity from natural infection, vaccination with booster doses, compulsory mask use, and implementation of adequate prevention and control measures may contribute to less severe outcomes. Moreover, vaccinating the unvaccinated and weaker individuals and providing booster doses may prevent death and hospitalizations. Some more suitable animal models may be studied to evaluate the vaccine efficacy, and testing of therapies against SARS-CoV-2. It is also highly recommended to understand the immune response patterns after infection by VOC.

ACKNOWLEDGMENTS
All authors are thankful to their respective institutes and universities.

CONFLICTOFINTEREST
There is no potential conflict of interest.

AUTHORCONTRIBUTIONS
All the authors substantially contributed to the conception, design, and interpretation of data and approving the final version of the manuscript.