Study suggests BA.5 evolved to induce increased inflammation compared to previous Omicron subvariants

In a recent study published on bioRxiv* preprint server, researchers assessed the comparative pathogenicity of Omicron severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) subvariants BA.1, BA.2, and BA.5, in vitro and live.

Study: Comparative pathogenicity of SARS-CoV-2 Omicron subvariants, including BA.1, BA.2 and BA.5. Image Credit: Dkoi/Shutterstock


Omicron’s BA.5 subvariant recently emerged from South Africa alongside BA.4 and was subsequently detected in several countries around the world. In August 2022, it surpassed BA.2 to become the world’s dominant Omicron subvariant. Although BA.5 and BA.4 appear to be descendants of the Omiron BA.2 subvariant in genome sequencing and evolutionary analyses, they appear to acquire pathogenicity, transmission capacity, and antibody escape potential neutralizers induced by vaccination or infection. .

Additionally, BA.5 contains unique mutations in its spike (S) protein, including L542R, which confers increased fusogenicity and resistance to immunity induced by prior infection with early variants. The emergence of BA.5 raises concerns that SARS-CoV-2 is continually evolving to acquire mutations that would increase its pathogenicity; it is therefore urgent to characterize BA.5 to intervene early and mitigate its spread.

About the study

In the present study, researchers characterized the virological characteristics of the BA subvariant of Omicron.5 in vitro and live in parallel with its predecessors BA.1 and BA.2; they used an early pandemic B.1.1 isolate containing the D614G mutation as a control. To characterize in vitro growth kinetics of Omicron subvariants, they used three cell lines, namely VeroE6/transmembrane protease, serine 2 (TMPRSS2), Calu-3, and alveolar epithelial cells derived from induced pluripotent stem (iPS) cells.

Electron microscopic analysis revealed that VeroE6/TMPRSS2 cells infected with SARS-CoV-2 exhibited multiple membranous structures or annular lamellae (AL) clustered in electron-dense areas near the nucleus. The researchers used the on-chip airway method to study the LA observed in microscopic cell sections.

In addition, they analyzed the lung function of infected hamsters for a live assessment of pathogenicity of Omicron subvariants. They assessed three surrogate markers for airway obstruction: i) subcutaneous oxygen saturation (SpO2), ii) the enhanced pause (Penh) and iii) the ratio of peak expiratory time to total expiratory time (Rpef).

Finally, to study the ability of Omicron subvariants to cause inflammation in the animals’ lungs, the researchers performed histopathological scoring. The method assessed bronchitis, hemorrhage, alveolar lesions with epithelial apoptosis, macrophage infiltration, and type II pneumocyte hyperplasia. They also examined the inflammatory response upon infection with subvariants of Omicron live.

Study results

The main finding of the study was that although BA.5 is less pathogenic than the ancestral strain of Omicron B.1.1, it had nevertheless evolved to induce a stronger inflammatory response than other Omicron subvariants, in particular BA.1 and BA.2. Despite similar in vitro Growth kinetics Like other Omicron subvariants, BA.5 was more fusogenic than BA.1 and BA.2. Within VeroE6/TMPRSS2 and Calu-3 cell lines, BA.2 and BA.5 showed comparable replication to B.1.1, but BA.1 showed lower replication rate. Conversely, in alveolar epithelial cells derived from iPS cells, B.1.1, BA.1 and BA.5 exhibited high replication efficiency compared to BA.2.

B.1.1 formed larger syncytia than Omicron subvariants and simultaneously exhibited the highest S cleavage efficiency. However, among all Omicron subvariants, BA.5 exhibited the most efficient S cleavage, indicating its evolution towards efficient fusogenicity in VeroE6/TMPRSS2 cells. The airway-on-a-chip method allowed researchers to assess the ability of SARS-CoV-2 to disrupt endothelial and epithelial airway barriers. Of all the Omicron sub-variants, BA.5 possessed the highest barrier breaking capability.

In a hamster model, the dynamics of weight changes of BA.5-infected animals were significantly different from those of BA.2-infected and uninfected hamsters. Moreover, in hamsters infected with BA.1, BA.2 and BA.5, the Penh value was significantly lower and the Rpef value was significantly higher than those of hamsters infected with B.1.1. Although inferior to B.1.1, of the Omicrn subvariants, BA.5 caused the most severe inflammation. In addition, BA.5 infection caused upregulation of four interferon-stimulated genes (ISGs), namely CXC chemokine ligand 10 (CXCL10), interleukin-6 (IL- 6), ISG15 and MX Dynamin Like GTPase 1 (MX-1).


The current study has extensively examined the in vitro and live characteristics of three clinical isolates of Omicron subvariants BA.1, BA.2 and BA.5 and highlighted the importance of continued mitigation measures for coronavirus disease 2019 (COVID-19). Kawaoka et al. showed that the clinical isolate BA.5 exhibited lower pathogenicity than the progenitor Delta in hamster models. Furthermore, they showed that the weight loss dynamics during BA.5 infection was slightly higher than during BA.2 infection.

Consistent with previous findings, the results of the current study also showed that although the virulence of Omicron subvariants is lower than that of the ancestral lineage B.1.1, BA.5 acquires increased pathogenicity by evolving to enhance the inflammatory response. Additionally, BA.5 gains higher fusogenicity and more robust barrier breaking capability than other Omicron sub-variants.

*Important Notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.

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