There are multiple severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants identified by viral genomic sequencing in different parts of the world. Based on the potential threats of these viral variants in terms of transmission, disease severity, immune escape, etc., they were classified into variants of concern (VOCs) and variants of interest (VOIs) by the World Health Organization (WHO). So far, four variants have been defined as VOCs—Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), and Delta (B.1.617.2).

In late December 2020, the Alpha variant was reported in the United Kingdom (1), followed quickly by the detection of the Beta variant, which carried three mutations including K417N, E484K, and N501Y at important locations in the Spike protein receptor-binding domain (S-RBD) in South Africa (2). In early January 2021, the Gamma variant carrying three mutations consist of K417T, E484K, and N501Y in the S-RBD was reported in Brazil (3). In December 2020, the Delta variant carrying mutations called 452R and 478K was first isolated in India (4).

Three key concerns of SARS-CoV-2 VOCs are viral transmissibility, disease severity, and the impacts on vaccine efficacy. For viral transmissibility, the reported studies have yielded good evidence that all VOCs are more transmissible than the wild-type virus (5–13). Risk of transmission, reported in 15 studies, was 45–71% higher for Alpha variant than the wild-type virus, while the basic reproduction number R_0t was 75–78% higher than the wild-type virus, and the reported effective reproduction number R_t ranged from 1.1 to 2.8 (8). For Beta variant, the R_t was 1.55 (95% confidence interval [CI]: 1.43–1.69) and ~50% more transmissible than the previously circulating variants (11, 12). Using dynamic modeling that integrates genomic and mortality data, Faria et al. (13) estimated that the transmissibility of the Gamma variant could be 1.4–2.2 times higher than that of the wild-type virus. A statistically significant increase in R_t relative to wild-type virus of Delta variant at 97% (95% CI: 76–117) (9). For impacts on vaccine effectiveness, the effects of the viral variants on the vaccine's protection of infection, symptomatic disease, and severe disease have been considered. The Alpha variant had less impact on the vaccine, and the vaccine was therefore still protective (14, 15). For Beta variant, the protection offered by the vaccine against symptomatic disease was reduced (16–18). The conclusion on the impact of Gamma variant on the vaccine was not yet clear. Delta variant likely reduced the protective effect of the vaccine with respect to infection and symptomatic disease (19).

Based on the newest report from WHO, the conclusions on disease severity were most uncertain among the reviews focusing on the phenotypic effects of SARS-CoV-2 VOCs. There were few reports on the disease severity of the variant viruses. Clinical outcomes were influenced by factors such as the use of health-care resource, demographic changes, and trends in social behavior (20). To date, we have found few reports of disease severity analysis based on clinical outcomes of the VOCs. By comparing four studies with datasets on the disease severity of infected persons, it was concluded that Alpha variant may not increase the risk of disease severity (21). A meta-analysis of these four studies indicated significantly increased hazard of mortality among patients with COVID-19 infected with Alpha variant relative to those infected with the wild-type virus (22). Alpha, Beta, and Gamma variants had a 1.7-, 3.6-, and 2.6-fold increased risk of hospitalization, and a 2.3-, 3.3-, and 2.2-fold increased risk of admission to the ICU, respectively (23). However, further confirmation in larger studies of Alpha variant as well as other viral variants are needed.

This study is a systematic review of current evidence conducted in June 2021 to determine the effects of SARS-CoV-2 VOCs on disease severity and clinical outcomes. The study was guided by the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P) to ensure reliability and validity of the reported results (24).

We identified a total of 1,745 (1,076, PubMed; 448, MEDLINE; 183, Embase; and 38 from other sources) relevant articles, and 1,123 studies were left after removing the duplicates. After excluding 775 articles by title and abstract screening, 348 articles met the conditions for full-text screening. Based on the above exclusion criteria, a further 322 articles were excluded. Ultimately, 26 studies that met the inclusion criteria were selected in this review for further analysis. Each study's main findings are summarized in Table 1 (25–50). Among the 26 studies, most were related to variant Alpha, followed by Beta, Gamma, and Delta.

Through meta-analysis, the data provided by the retained studies were integrated; the values of total random effects were retained; and the risk of hospitalization, ICU admission, and mortality of patients infected with VOCs compared with wild-type virus were obtained to analyze the disease severity of the VOCs. The main process of meta-analysis of variants Alpha, Beta, Gamma, and Delta are, respectively, shown in Figures 2–5, and the main results of the meta-analysis are summarized in Table 2.

In the case of Alpha variant compared with the wild-type virus, most studies concluded that the risk of hospitalization, ICU admission, and mortality were increased. Only Frampton et al. (25) and Stirrup et al. (49) reported that the risk of ICU admission were equivalent. In addition, the differences in the risk of mortality mainly came from Funk et al. (31) and Martínez-García et al. (43) wherein they concluded that the mortality rate was reduced, and Stirrup et al. (49) concluded that the mortality rate was equivalent. For the Beta variant compared to wild-type virus, Funk et al. (31), Fisman and Tuite, (35), and Veneti et al. (46) found that it increased the risk of hospitalization and ICU admission. In addition, Funk et al. (31) and Fisman and Tuite, (35) reported that Beta variant also increased the risk of mortality. It is worth mentioning that Beta variant had the highest risk of hospitalization at 2.16 (95% CI: 1.19–3.14). The only study involved in the meta-analysis regarding the Delta variant was from Fisman and Tuite, (35). The risk of ICU admission and mortality were 3.35 (95% CI: 2.5–4.2) and 2.33 (95% CI: 1.45–3.21) respectively, and it was slightly lower than that of Beta variant in the risk of hospitalization at 2.08 (95% CI: 1.77–2.39)(35).

The results showed that in the risk of hospitalization, ICU admission, and mortality, all the SARS-CoV-2 VOCs had different degrees of increase compared with wild-type virus; Delta variant had the highest risk of ICU admission and mortality, and Beta variant had the highest risk of hospitalization.

Since the rapid spread of the SARS-COV-2 pandemic, many variant viruses including Alpha, Beta, Gamma, and Delta have emerged. However, the conclusions regarding disease severity of these variant viruses are not consistent. Accordingly, we searched for studies in the relevant field and recorded their clinical data. A meta-analysis was used to combine the information of different studies. Finally, we found that all VOCs increase the risk of hospitalization, ICU admission, and death compared with the wild-type virus, and variant Delta and Beta carried a much higher risk than other variants.

By comparing the results from different studies, we found that most of the conclusions stated that Alpha variant had a higher risk of disease severity than the wild-type virus, but Frampton et al. (25), Funk et al. (31), Martínez-García et al. (43), and Stirrup et al. (49) have expressed different opinions. However, the sample size of Frampton et al. (25) was very small, which is why their results were likely not very convincing. Although Frampton et al. (25) used whole genome sequencing to identify Alpha variant, while other reports used PCR detection of S-gene target failure (SGTF) as an alternative detection method, it did not make much of a difference to the results. Funk et al. (31) found that Alpha variant showed significantly higher risk of hospitalization rate and ICU admission, but lower risk of mortality than the wild-type. The clinical drugs for the Alpha variant in EU/EEA were more effective or some of the reported cases may have been vaccinated. The previously reported increased binding affinity between the spike receptor-binding domain and the angiotensin-converting enzyme 2 (ACE2) receptor in the Alpha SARS-CoV-2 strain may have led to further down-regulation of ACE2 if an individual got infected by this new variant compared with other variants. ACE2 was suggested to have a protective effect on lung injury in patients with COVID-19 (51, 52). Patients were aged ~70 years, and there was a large gap with the age of other studies' patients, which led to certain limitations regarding the conclusion of mortality risk (43). Stirrup et al. (49) concluded that female rather than male patients infected with the Alpha variant would have a higher risk of severe disease. In summary, it can be argued that Alpha variant was more threatening than the wild-type virus and can cause higher risk of more severe disease. The explanation for the conclusion from Funk et al. (31) that variants Beta and Gamma may also have a lower risk of mortality can be consistent with the explanation for Alpha variant. In addition, Hoang et al. (39) speculated about the risk of hospitalization and ICU admission by directly comparing variants Beta, Gamma, and Alpha. The risk of Beta variant was significantly higher than that of Alpha variant, while the risk of Alpha variant was similar to that of Gamma variant, which was consistent with our conclusion.

Although the sample size on the Delta variant was limited in conducting meta-analysis, those studies that directly compared the disease severity of variants Delta and Alpha supported our conclusions from the side. Using stratified Cox proportional hazard regression, there was a significantly increasing risk of hospitalization and emergency care attendance for Delta variant cases compared with Alpha variant cases after adjustment for confounders, which were 2.16 (95% CI: 1.56–4.36) and 1.67 (95% CI: 1.25–2.23), respectively (32). Ong et al. (42) calculated that the risk of ICU admission and mortality were 1.88 (95% CI: 0.95–3.76) and 1.88 (95% CI: 0.95–3.76), respectively. Among similar studies, the main debate involved the study from Frampton et al. (25), but as mentioned earlier, their work was limited by a much smaller sample size, which is why their conclusions were not very persuasive (21). The result of Kow et al. (22) in the risk of mortality for Alpha variant compared with the wild-type virus was 1.45 (95% CI: 1.18–1.78), which was close to our meta-analysis result at 1.37 (95% CI: 1.15–1.60). A recent review concluded that variants Alpha, Beta and Gamma all had a higher risk of hospitalization and ICU admission compared with the wild-type virus, and the risk of Beta variant was much higher (23), which supported our conclusion to some extent.

To our knowledge, this is the first study to compare the disease severity of VOCs with the wild-type virus and draw specific conclusions. We believe that our results present the threats of VOCs more clearly to the public, particularly the variants Beta and Delta. Although several different types of vaccines have been developed, further research is required regarding the protection rate of the viral variants. The fact that we did not further analyze the influence of age, sex, and geographic parameters is the limitation of the study. However, we performed meta-analysis, which is known to better reduce the impact of each study.

In this meta-analysis, we analyzed the results of studies that reported on the disease severity of SARS-COV-2 VOCs from June 1, 2020 to October 15, 2021 and processed the relevant data. By comparing with the wild-type virus, in terms of the risk of hospitalization, ICU admission, and mortality, the variants Beta and Delta have a higher risk than the variants Alpha and Gamma, and all SARS-COV-2 VOCs have a higher risk of disease severity than the wild-type virus. This is the first comprehensive study that compared the disease severity of variants Alpha, Beta, Gamma and Delta with wild-type virus and drew specific conclusions. We hope that this report can increase the awareness of the disease severity of SARS-COV-2 VOCs, particularly of variants Beta and Delta, and make the public aware of routine precautions and the importance of vaccination.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

This work described in this paper was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (HKU C7123-20G).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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