Characterization of SARS-CoV-2 different variants and related morbidity and mortality: a systematic review

Introduction Coronavirus Disease-2019 (SARS-CoV-2) started its devastating trajectory into a global pandemic in Wuhan, China, in December 2019. Ever since, several variants of SARS-CoV-2 have been identified. In the present review, we aimed to characterize the different variants of SARS-CoV-2 and explore the related morbidity and mortality. Methods A systematic review including the current evidence related to different variants of SARS-CoV-2 and the related morbidity and mortality was conducted through a systematic search utilizing the keywords in the online databases including Scopus, PubMed, Web of Science, and Science Direct; we retrieved all related papers and reports published in English from December 2019 to September 2020. Results A review of identified articles has shown three main genomic variants, including type A, type B, and type C. we also identified three clades including S, V, and G. Studies have demonstrated that the C14408T and A23403G alterations in the Nsp12 and S proteins are the most prominent alterations in the world, leading to life-threatening mutations.The spike D614G amino acid change has become the most common variant since December 2019. From missense mutations found from Gujarat SARS-CoV-2 genomes, C28854T, deleterious mutation in the nucleocapsid (N) gene was significantly associated with patients' mortality. The other significant deleterious variant (G25563T) is found in patients located in Orf3a and has a potential role in viral pathogenesis. Conclusion Overall, researchers identified several SARS-CoV-2 variants changing clinical manifestations and increasing the transmissibility, morbidity, and mortality of COVID-19. This should be considered in current practice and interventions to combat the pandemic and prevent related morbidity and mortality.


Introduction
In December 2019, a novel coronavirus (CoV) emerged from Hubei province in China among people visiting the Huanan seafood market in Wuhan. The virus is transmitted through human-to-human contact and rapidly spread across the world, and soon turned into a pandemic [1,2]. Its symptoms can be divided to two main groups of majors (fever, cough, dyspnea) and minors (anosmia, The influence of these deletions on the pandemic is still unknown. But some in vitro studies have shown that Δ29 replicates less efficiently and causes milder clinical illness than the wild-type [15,16]. The relations between the magnitude of nucleotide deletion in ORF8 with its virulence and the ORF8 function are still unknown. However, a recent study suggested that ORF8 mediates immune evasion by downregulating MHC-I molecules. In vitro studies have also shown that these deletions do not affect replicative fitness, but it can affect the transcription of some essential and defensive regions such as ORF6 and N genes (known as SARS-CoV interferon antagonists); thus, it can create a more fragile variant compared to the wild-type [17][18][19].
Besides ORF8, other genome parts can be affected by the mutations, and new variants could emerge. Studies have revealed that the highly mutable spike (S) protein of the virus is associated with the elevated human-tohuman transmission rate through interaction with the host's ACE2 receptor. S protein is one of the well-characterized proteins of the Coronaviridae family; this ∼ 1255 amino acid transmembrane protein helps the virus to attach and enter the host [12,20].
There are also reports about the mutations in other parts such as nsp2 and nsp12 (RdRp). The SARS-CoV-2 nsp12 is RNA-dependent RNA polymerase (RdRp) consisting of 932 amino acids located in the polyprotein, from 4393 to 5324 aa. Structurally, the SARS-CoV-2 nsp12 protein is categorized into N-terminal (1-397aa) and a polymerase domain (398-919aa). These mutations have been observed in patients from India, Germany, and Iran [20].
In this article, we aimed to report on and compare the morbidity and mortality of the different variants of SARS-CoV-2 with that of the wild-type to realize whether they could be an immense threat to humans.

Methods
This study is a systematic review of current evidence conducted in September 2020. The authors aimed to study the effect of different variants of SARS-CoV-2 on mortality and morbidity. Our study is consistent with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist to ensure reported results' reliability and validity.

Sources of data
We retrieved all the relevant papers and reports published in English from December 2019 to September 2020 through a systematic search using keywords in the online databases of PubMed, Web of Science, Scopus,  26:51 and Science Direct. We updated our search on late February 2021. Our search strategy employed multiple combinations of keywords, as follows: • "SARS-CoV-2" OR "Coronavirus" OR "COVID-19" OR "2019-nCoV" OR "Novel Coronavirus"[Title/ abstract] • "Variants "OR "Variation" OR "Strains" OR "Types" OR "Minority Variants" OR "Genomic Variants" OR "Genetic Variation" OR "Genomic Diversity"OR "Characterizations" [Title/abstract]

Selection of the study
Three independent investigators reviewed the extracted papers' full text and selected the most pertinent papers according to the eligibility criteria. Then we pulled the relevant data and organized them in some tables. We included the original and peer-reviewed English papers fulfilling the eligibility criteria in the final report. Besides, the following exclusion criteria were applied in the present study: • Non-human studies, including animal experiments, in vitro observations, or papers with a limited report on COVID-19, and those without reference to this review's keywords. • Papers with inaccessibility to their full texts. • Any duplicated and suspicious outcomes in databases.

Extraction of data
The authors' names, publication date, article types (e.g., case reports), country of origin, sample size, gender, age, and clinical symptoms were recorded in an information sheet. This information was collected by two independent researchers and subsequently organized in tables. All authors cross-checked the selected articles to avoid any duplications or overlap in the content.

Assessment of quality
This study adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist to ensure the quality and accuracy of selected papers and outcomes. Two independent researchers examined the quality of the articles and the probable risk of bias. A third researcher addressed any disagreement.
The full text of selected articles was read, and the key findings are summarized in tables.

Results
We identified a total of 56 relevant articles by title and abstract. Of 56 articles, 50 were related to the genomic variations in SARS-CoV-2. The included studies were conducted in 16 countries, and one of the articles was a report on multinational scientific collaborations [1] ( Table 1 illustrates a summary of the findings). We summarized each study's main findings in the two categories: genomic variants and other results. Studies have revealed that the highly mutable spike (S) protein of the virus is associated with the elevated human-to-human transmission rate through interaction with the host's ACE2 receptor. A review of identified articles has shown three main genomic variants, including type A, type B, and type C. we also identified three clades including S, V, and G. Studies have demonstrated that the C14408T and A23403G alterations in the Nsp12 and S proteins are the most prominent alterations in the world, leading to miserable mutations.The spike D614G amino acid change has become the most common variant since December 2019. From missense mutations found from Gujarat SARS-CoV-2 genomes, C28854T, deleterious mutation in the nucleocapsid (N) gene was significantly associated with patients' mortality. The other significant deleterious variant (G25563T) is found in patients located in Orf3a and has a potential role in viral pathogenesis.

Discussion
Since the emergence of COVID-19, understanding the virus behavior has been received much attention from the scientific community. The different viral behavior has been attributed to the virus's difference in types and strains [75]. Therefore, in the present review, we characterized the different variants of SARS-CoV-2 and discussed the findings in four sections, including the different types of SARS-CoV-2 variants, their effects on viral transmission, clinical manifestations, morbidity as well as mortality, and the other relevant findings.

Different variants and strains
This review has focused on the different variations of SARS-CoV-2 and their impact on virus behavior. Alternation of the SARS-CoV-2 genome, through mutation and recombination, potentially leads to changes in the viral life cycle, including transitivity, cellular tropism, and severity of the disease. The diverse clinical outcomes in COVID-19 patients happen due to SARS-CoV-2 genome across different proteins showed co-occurrences, suggesting their structural and/or functional interaction among different SARS-COV-2 proteins and their involvement in adaptability and viral transmission. Analysis of protein structure stability of surface glycoprotein mutants indicated the viability of specific variants and are more prone to be temporally and spatially distributed across the globe 27 Latini [43] Research article Italy COVID-19 patients ACE2, TMPRSS2, PCSK3, DPP4, and BSG genes It is known that ACE2 acts as a receptor for this pathogen, but the viral entry into the target cell also depends on other proteins. In the PCSK3 gene, we observed a missense variant (c.893G > A) statistically more frequent compared to the EUR GnomAD reference population and a missense mutation (c.1906A > G) not found in the GnomAD database. In the TMPRSS2 gene, the authors observed a significant difference in c.331G > A, c.23G > T, and c.589G > A variant alleles in COVID-19 patients, compared to the corresponding allelic frequency in GnomAD. Genetic variants in these genes could influence the entry of the SARS-CoV-2. These data also support the hypothesis that host genetic variability may contribute to the variability in infection susceptibility and severity 28 Lau [44] Research article China

COVID-19 patients
Vero-E6 cells The presence of a distinct motif in the S1/S2 junction region suggests the possible acquisition of the cleavage site(s) in the spike protein that promoted cross-species transmission. Through plaque purification of Vero-E6 cultured SARS-CoV-2, we found a series of variants that contain 15-30-bp deletions (Del-mut) or point mutations, respectively, at the S1/ S2 junction. The unique cleavage motif promoting SARS-CoV-2 infection in humans may be under intense selective pressure, given that replication in permissive Vero-E6 cells leads to the loss of this adaptive function 29 Lee [45] Research article Japan COIVD-19 patients ACE2, TMPRSS2, TLR7 Genome-wide association studies have identified genetic risk factors for severe COVID-19 cases in a segment of chromosome 3 that involves six genes encoding three immune-regulatory chemokine receptors and another three molecules. The risk haplotype seemed to be inherited from Neanderthals, suggesting genetic adaptation against pathogens in modern human evolution. Therefore, SARS-CoV-2 uses highly conserved molecules as its virion interaction, whereas its immune-response appears to be genetically biased in individuals to some extent 30 Liu [46] Research article USA

COVID-19 patients
Single-nucleotide variants (SNVs) Four signature groups of frequently occurred single-nucleotide variants (SNVs) were identified in over twenty-eight thousand high-quality and high-coverage SARS-CoV-2 complete genome sequences, representing different viral strains. Interestingly nucleotide substitutions among SARS-CoV-2 genomes tended to switch between bat RaTG13 coronavirus sequence and Wuhan-Hu-1 genome, indicating the higher genetic instability or tolerance of mutations on those sites or suggesting that major viral strains might exist between Wuhan-Hu-1 and RaTG13 coronavirus (L84S), N gene (D103Y, R191C, S197L, G238C, R203K, G204R) Our results portray circulating SARS-CoV-2 South American strains coming from Europe, North America, and Oceania; and mostly belongs to Clade G. Infectivity and pathogenicity of SARS-CoV-2 is related to S protein, mainly due to the h-ACE2 binding ridge structural changes of the RBD domain, on residues 482 to 485: Gly, Val, Glu, and Gly. Our report highlights two strains with novel variants on the S region, with no amino acid change in nt24022 (E1207E), whereas another non-synonymous alteration in nt25182 (E1207V), for Peru (EPI_ISL_415787) and Ecuador (EPI_ISL_417482), respectively. However, these changes seem far away from the critical region of S protein for h-ACE2 affinity. Due to its prevalence, Clade G strains could be more contagious than other subtypes due to nucleotide changes in ORF1ab (nt8750) and N (nt29063) genes that enhance viral replication. We found 8 (8/30) variations in both of nt8782 and nt28144 positions. Conversely, other regions seem to be hotspots in South American strains, with 11(36.67%) of these portraying changes at 5′UTR (nt241), nsp3 (nt3037), nsp12 (nt14408), and N/ORF9 (nt28881, nt28882, and nt28883) regions. This is paramount because changes in nsp1, nsp3, and nsp5 could be related to some functions of the viral incubation period and immune response evasion of SARS-CoV-2. Amino acid alterations in both of these regions, such as G392D (nsp1), A876T, A1043 (nsp3), and nsp5 (G3334S); and should be tested in further studies. Strikingly, we identified four changes-nt15324 in ORF1ab (RdRp), nt26144 in E gene, nt28580, and nt28657 in the nucleocapsid gene-in the suggested regions for primer annealing for SARS-CoV-2 specific fragments identification, according to real-time RT-PCR recommendations from the and SNP patterns in SARS-CoV-2 are more similar to MERS-CoV than SARS-CoV-1; except for significantly larger proportion of G > T changes in both iSNVs and SNPs. Third, a significant fraction of small indels fuel the genetic diversity of SARS-CoV-2. Fourth, the mutational spectra of the SNPs and iSNVs indicate that there is a complex interplay between endogenous SARS-CoV-2 mutational processes and host-dependent RNA editing. This observation is in line with several recent studies that propose APOBEC and ADAR deaminase activity as a likely driver of the C > T changes in the SARS-CoV-2 genomes. The study showed high sequence conservation within the NSP3 region, a region that is one of the most diverged from SARS-CoV-1 and MERS-CoV. The lower NSP3 mutations are due to its essential functional implications in viral replication, thus promising NSP3 as a good target for drug development. A number of convergent findings suggest de-mono-ADP-ribosylation of STAT1 by the SARS-CoV-2NSP3 as a putative cause of the cytokine storm observed in the most severe cases of COVID-19. Also, one deletion (at 28245 bp) was present in 10 samples (AF: 6%) in ORF8, a potentially important gene for viral adaptation to humans 44 Sarkar [60] Research article India 837 Indian SARS-CoV-2 strains 33 different mutations; 18 of which these were unique to India: S glycoprotein (L54F, K77M, R78M, D294D, E583D, Q677H), NSP3 (G716I, T749I, A994D, D1121G, S1197R), RdRP (A97V, L329I, G571S, V880I), NSP2 (S301F, G339S), and N (S194L) Non-synonymous mutations were found to be 3.07 times more prevalent than synonymous mutations. The A2a clade was found to be dominant in India (71.34%), followed by A3 (23.29%) and B (5.36%), but a heterogeneous distribution was observed among various geographical regions. The A2a clade was highly predominant in East India, Western India, and Central India, whereas the A2a and A3 clades were nearly equal in prevalence in South and North India. D614G/S, a characteristic mutation of the A2 clade that was first reported in Germany, has been found to correlate strongly with high infectivity 45 Shen [61] Research article  to the Wuhan-Hu-1 SARS-CoV-2 reference genome These genomes belong to all five major Next strain-defined clades, suggesting multiple introductions into the region and underscoring the diversity of the regional epidemic. We also found that clinically severe cases had genomes belonging to all of these clades. We found no clear correlation, but were limited by sample size. Similarly, patient phenotypes including sex, race, recent travel, symptoms, and comorbidities were represented across all five major phylogenetic clades, suggesting that susceptibility was independent of clade. The widely examined mutation in the viral spike protein (D614G) 28-30 is one of the key mutations differentiating the 19 and 20 clades. Notably, we see severe cases in both of these clades, though our dataset is underpowered to show significant correlations between viral genome mutations and disease severity. The diversity of virus genetics, clinical symptoms, and patient outcomes suggests that viral mutations are not the main driver of clinical presentation 49 Toyoshima [65] Research article Japan Covid-19 patients One thousand two hundred thirtyfour mutations by comparing with the reference SARS-CoV-2 sequence All replicating viruses, including coronavirus, continuously accumulate genomic mutations that persist due to natural selections. These mutations contribute to the enhancement of the ability of viral proliferation and infection and an escape from host immune attack 50 Ugurel [66] Research article Turkey Covid-19 patients Variations in SARS-CoV-2 genome Despite some variations being in low-frequency rate in some continents, C14408T and A23403G variations on Nsp12 and S protein, respectively, were observed to be the most prominent variations all over the world, in general, and both cause missense mutations. It is also notable that most isolates carry C14408T and A23403 variations simultaneously, and also nearly all isolates carrying the G25563T variation on ORF3a, also carry C14408T and A23403 variations, although their location distributions are not similar Research article UK 7710 SARS-CoV-2 assemblies flagged as "complete (> 29,000 bp)", "high coverage only", "low coverage excl" were downloaded from the GISAID Initiative EpiCoV platform as of April 19 2020 Identified 198 homoplasy positions in the SARS-CoV-2 genome alignment (0.67% of all sites) which was associated with 290 amino acid changes across all genomes; 232 non-synonymous and 58 synonymous mutations. Two non-synonymous mutations involved the introduction or removal of stop codons were found (*13402Y, *26152G) One of the strongest homoplasies lies at site 11,083 in the SARS-CoV-2 genomes in a region of Orf1a encoding Nsp6. This site passed our stringent filtering criteria and was also present in our analysis of the SRA dataset. Interestingly, this region overlaps a putative immunogenic peptide predicted to result in both CD4+ and CD8+ T cell reactivity. More minor homoplasies among our top candidates, identified within Orf3a, also map to a predicted CD4 T cell epitope of note, we also identify a strong recurrent mutation in nucleotide position 21,575, corresponding to the SARS-CoV-2 spike protein (codon 5). While the spike protein is the known mediator of host cell entry, our detected homoplasy falls outside of the N-terminal and receptor-binding domains 52 Wang [68] Research article was predominantly identified in East Asia. Still, this variant outside of Asia was observed with striking, long mutational branch lengths. The G variant was rarely sampled in Asia but corresponded to the most frequent variant in Europe. In this study, G variants were identified in strains predominantly from Thailand, followed by Singapore and Vietnam. Cluster III belonged to clades outside the reported S, G, and V shared mutations distinguished clades, and at 6312C > A, 11083G > T, 13730C > T, and 19524C > T in ORF1ab, 23929C > T in spike, and 28311C > T in the N protein (n  is used as the reference The 9 newly evolved SARS-CoV-2 single-nucleotide polymorphism (SNP) alleles reported, underwent a rapid increase(7 cases) 0 or decrease (2 cases) in their frequency for 30-80% in the initial four months, which are further confirmed by intra-host single-nucleotide variation (iSNV) analysis using raw sequence data including 8217 samples. The 9 SNPs are mostly (8/9) located in the coding region and are mainly (6/9) non-synonymous substitutions. mutations. The mutation of single-stranded RNA viruses is much faster than the human genome's mutation rate, about 10 -6 -10 -4 and 10 -8 , respectively [76,77]. This leads to numerous quasi-species in each infected one, which may justify the observed difference in symptoms and disease severity [78]. Altered ACE2 binding interactions or shifted tissue tropism may happen due to a mutation among viral progeny that causes aggressive and immense infections [20]. Evolutionary benefits such as changing a primary epitope to escape from the host immune system or changing virulence factors to enhance transmission of the virus can occur due to gene mutations. Natural selection or vaccine selective pressure can cause these mutations and subsequently lead to new viral strains [79]. Preliminary studies at the beginning of the outbreak identified two major genotypes of SARS-CoV-2 among a Chinese population, type Ι, and type ΙΙ [18]. The prevalence of the aggressive form had decreased in the early months due to the start of treatment, and its mild form became the common variant [24].
Further studies reported the identification of three major variant types (A, B, C) of SARS-CoV-2, based on amino acid changes [22]. Forster et al. confirmed those three major variant types by phylogenetic analysis of 160 viral genomes [32]. Interestingly, variant A is the conventional type; type B viruses prevailed in East Asia, while both type A and C viruses have been dominant in America and Europe. After two mutations, including the synonymous mutation T8782C and the non-synonymous mutation C28144T, by replacing serine with leucine in type A, type B is formed. Type C is also derived from type B by the non-synonymous mutation G26144T, in which valine replaces glycine [32,80,81]. In other words, the S variant (Type A) with two mutations at 8782C>T and 28144 T>C was mainly identified in East Asia. Still, outside Asia, significant and long mutations were observed with the length of the branches. The G variant was dominant in Europe and was rare in Asia [70]. Bhowmik et al. reported two D and E subgrouping of the influential group A. Moreover, they stated that the SARS-CoV-2 genome is around 29,903 nucleotides. The highly mutable spike (S) protein of the virus is probably related to the increased human-to-human transmission rate through interaction with the host's ACE2 receptor [20]. Ugurel et al. reported C14408T variant on Nsp12 and A23403G variation on S protein, and both cause significant mutations and changes in virus variants worldwide [66].
Recent studies around the world have identified eight strains of SARS-CoV-2. However, they have a significant sequence similarity [50]. Also, Liu et al. have been recognized four distinct groups of common mononucleotide types (SNVs) in more than 28,000 high-quality, high-coverage SARS-CoV-2 complete genome sequences, demonstrating different viral strains [46]. These reports are consistent with the findings of two studies in Italy and the United States, where about 4-10 non-synonymous stable mutations were reported in the SARS genome [11,50]. Eke, one of the mutations in S protein (D614G), has been seen repeatedly in Europe and the United States since the onset of the infection, apparently because it has dramatically increased the transmission ability of SARS-CoV-2. Thus, it became the most common variant [41,56].
Although the mutation of the SARS-CoV-2 appears to be stable, consecutive consideration of virus mutations remains essential. A large study by Poterico and colleagues characterized two novel mutations in the S region across 691 complete viral genomes of SARS-CoV-2 from around the world. They also highlighted that the virus had acquired about 27 mutations, and most of South American countries' trains are nearly related to European viral isolates [57]. Meanwhile, a unique mutation 24351C (A930V (T)) in the spike surface glycoprotein was reported in one of the Wuhan strains in India [24]. In a study conducted in Singapore, the cause of SARS attenuation was attributed to the 382-nucleotide deletion in ORF8 of the viral genome [23]. In a survey conducted in mid-March in Mexico, evidence of local translocation of strains with an H49Y mutation in Spike protein strains was reported [63]. According to the findings of Castello et al., the first three cases of ORF amino acid are classified as S type in position 28 144; nevertheless, the fourth case is a G type in position 23 403 [29].

The effect of the variants on the viral transmission
Several studies have reported the association between the transmissibility and different variants and mutations [18,20,25,27,44,51,56,57,63,75]. Some mutations facilitate the transmission of SARS-CoV-2 between animal species and humans. The G-U transversion excess might play a role in the bat to human transmission [51]. Besides, the S1/S2 junction region's specific motif may have caused the viral exchange between species [44].
Regarding the viral transmission between humans, some fundamental facts are noteworthy. The rapid viral replication might cause rapid morbidity and mortality and hinder the viral passage to healthy individuals. Viruses causing slower replications and asymptomatic or mild disease can allow the transmission for a more extended period [27]. Furthermore, mutations altering viral structure might increase virulence or helping the pathogen escape the immune system, resulting in higher transmission rates [75]. Type II SARS-CoV-2 strains possibly spreading through the Huanan seafood market were also considered more prevalent than type I viruses, probably due to being more contagious [18]. Several mutations are proposed to increase transmissibility [20,57,63,75]. Variants possessing immensely mutable S proteins might be more contagious due to their interaction with the host ACE2 [20]. Concurrently, a specific mutation (D614G) in the S protein might speed up the viral transmission [75,82]. The role of QHD43416 p.Asp614Gly variant in many strains is controversial and not fully understood [56,82]. H49Y mutation in the S protein may also be responsible for local transmissions in earlier stages. It was proposed as a potential marker to trace the viral spreading between the countries and regions [63]. ORF1ab (nt8750) and N (nt29063) are also the identified responsible genes for the higher transmissibility of clade G strain [57].

Variant effects on symptoms, morbidity, and mortality
Pneumonia and lung involvement is often the main clinical sign of COVID-19. Recent evidence also demonstrates gastrointestinal symptoms and asymptomatic infections [83]. The percentage of people infected with the coronavirus who remain asymptomatic during infection has not yet been accurately assessed and reported. Symptomatic patients often have clinical symptoms of fatigue, cough, nasal congestion, fever, and other signs of upper respiratory tract infections that usually appear after a week. The condition can develop into severe disease with dyspnea and severe chest symptoms [84,85], and the respiratory tract infections are known as the primary clinical signs of COVID-19 [86].
In COVID-19, pneumonia usually manifests in the second or third week of symptomatic infection. Prominent signs of viral pneumonia include reduced oxygen saturation, blood gas deviations. Changes can be seen through chest X-rays and other imaging techniques, leading to the deterioration of vital signs and death. Lymphopenia (abnormally low level of lymphocytes in the blood) is common in these patients, and inflammatory markers (C-reactive protein and pro-inflammatory cytokines) could also increase [85,87]. Furthermore, specific genetic mutations in the coronavirus can even increase mortality [85,87].

Other relevant findings
COVID-19 mortality rates differ substantially depending on the country. This difference in mortality rates depends on various factors in each country, including the adequacy of health care delivery, political decisions, and epidemiological characteristics of the affected population. The frequency of diagnostic and screening measures in asymptomatic or mildly symptomatic patients may also affect morbidity and mortality [88,89]. Studies have demonstrated a steadfast and transparent pattern of age-based exponential enhancement in mortality, regardless of geographic area, in patients with COVID-19. Age-related mortality changes are relatively common for COVID-19 because other significant causes of mortality, especially chronic diseases such as cardiovascular disease, could also be increased by advanced ages [90]. Promislow et al. have shown that the mortality rate doubling time (MRDT) of all-cause fatality 9 years in the United States was close to that of COVID-19 in New York City [91]. In other words, there is no significant relationship between age and increased death in patients with COVID-19. However, many scientists and the media have paid particular attention to age as a risk factor for mortality in COVID-19. Nonetheless, the age-related pattern of death from COVID-19 is different from other respiratory viral infections. The pattern of morbidity and mortality is higher in the elderly than in young people [90].

Limitations
Although this systematic review produces valuable knowledge regarding the COVID-19 variants and related morbidity and mortality, there were some shortcomings. First, the number of published reports is still limited. The knowledge regarding the different strains and variants and their effects on the symptoms, morbidity, and mortality is not entirely described yet. Furthermore, various countries ought to report their data to identify the worldwide distribution of these variants. Researchers might also strive to discover various novel mutations resulting in different viral behaviors in the future.

Conclusion
Overall, researchers identified several SARS-CoV-2 variants changing clinical manifestations and increasing the transmissibility, morbidity, and mortality of COVID-19; however, many observations produced controversial results. Variants with asymptomatic disease or milder disease can increase their transmission by extending the duration of contact between sick and healthy people. Mutations causing increased virulence and immune escapes might also cause an elevated transmissibility level. As the vaccine inoculations are increasing worldwide, we encourage researching for potential mutations that might escape vaccine-induced immunity. The current practice and interventions should consider these findings to combat the COVID-19 pandemic and prevent related morbidity and mortality.