Recently, a team of Japanese researchers has identified the SARS-CoV-2 Omicron XBB.1.5 variant prevalent in the Western hemisphere, which has high transmissibility and infectivity, and is a cause of concern for public health. The Omicron XBB.1.5 variant was first detected in October 2022 and has a novel mutation in the spike (S) protein, which enables the virus to anchor itself to the human angiotensin converting enzyme-2 (ACE2) receptor and invade human cells. This mutation is of concern as it can help the virus evade the immune response.

The team conducted several experiments to gain mechanistic insights into the infectivity, transmissibility, and immune response associated with XBB.1.5. Their epidemic dynamics analysis showed that the XBB.1.5 variant is 1.2-fold more transmissible than the parental XBB.1 variant. In addition, the XBB.1.5 variant is rapidly outcompeting the BQ.1.1 variant, the predominant lineage in the United States.

The researchers also conducted a yeast surface display assay to study the virological features of XBB.1.5 and its interaction with the human ACE2 receptor. The results showed that the XBB.1.5 variant binds to the human ACE2 receptor with very high affinity. Further experiments using lentivirus-based pseudoviruses showed that XBB.1.5 has approximately 3-fold higher infectivity than XBB.1. The study also found that XBB.1.5 S protein is highly resistant to neutralization antibodies elicited by breakthrough infection with the BA.2/BA.5 subvariants, which may limit the protective immune response in patients with prior infections.

The team concluded that the SARS-CoV-2 Omicron XBB.1.5 variant exhibits enhanced transmissibility and has become a looming threat. They suggest that imminent prevention measures are needed to safeguard public health. Co-first-author Jumpei Ito from the Division of Systems Virology remarks,