The Omicron XBB variant emerged as a new subvariant of the SARS-CoV-2 virus, representing a continued evolution in the global pandemic. It joined other Omicron subvariants that have spread worldwide. Its appearance underlined the ongoing adaptation of the virus. Understanding its characteristics is important for public health.
Origin and Characteristics of XBB
The XBB variant is a recombinant lineage, forming when an individual was simultaneously infected with two different SARS-CoV-2 Omicron subvariants, leading to genetic exchange. Specifically, XBB originated from a recombination of two BA.2 descendants: BJ.1 (BA.2.10.1) and BM.1.1.1 (a progeny of BA.2.75). This genetic blending distinguishes it from variants arising solely through new mutations within a single lineage.
The XBB subvariant was first identified in India in August 2022. It and its descendants, such as XBB.1.5, rapidly gained prevalence in various Asian countries, including Singapore and India, before spreading globally. This recombination gave XBB unique characteristics, setting it apart from its parent strains.
Symptoms and Disease Severity
Individuals infected with the XBB variant often experience symptoms similar to earlier COVID-19 strains. Commonly reported symptoms include fever, a persistent cough, and fatigue. Other frequently observed symptoms are muscle aches, congestion, or a runny nose.
Compared to earlier variants, like the original Omicron or Delta, XBB infections may present with subtle differences. A sore throat appears more prominent with XBB.1.5, while loss of taste or smell is now reported less frequently. Despite its high transmissibility, data generally indicates XBB does not cause more severe illness in most vaccinated or previously infected individuals compared to other Omicron strains. Studies show no significant association between XBB infection and increased rates of hospitalization or the need for supplemental oxygen.
Immune Evasion and Transmissibility
The XBB variant exhibits enhanced immune evasion, contributing to its widespread transmissibility. This capability stems from specific mutations in its spike protein, such as R346T, N460K, and F486S. These genetic changes allow XBB to bypass antibodies generated from previous infections or vaccinations.
The virus’s ability to bypass the immune system means that even individuals with prior immunity may still be susceptible to infection. For example, XBB and its sub-lineages, like XBB.1.5, have demonstrated notable resistance against antibodies from breakthrough BA.2 and BA.5 infections, and even from individuals who received four vaccine doses. This enhanced immune escape, combined with an improved ability to bind to human ACE2 receptors, contributes to XBB’s high effective reproduction number.
Effectiveness of Vaccines and Treatments
The updated 2023–2024 COVID-19 vaccines, which are monovalent and target the XBB.1.5 strain, offer protection against hospitalization from both XBB and JN.1 lineages. These vaccines may not always prevent infection, but they remain effective at reducing the risk of severe disease, hospitalization, and death. Studies indicate that XBB.1.5 vaccines’ effectiveness against SARS-CoV-2 infection was approximately 63% to 67% at four to six weeks following vaccination, though this protection can decline over time.
Antiviral treatments, such as Paxlovid, continue to be effective against the XBB variant. Paxlovid works by targeting a specific protein (protease) within the virus, different from the spike protein where most mutations occur. This mechanism of action helps maintain its efficacy even as the virus evolves. For high-risk individuals, Paxlovid has been shown to significantly reduce hospitalization and death when administered within five days of symptom onset. Beyond vaccines and treatments, general public health measures like masking in crowded indoor settings and ensuring good ventilation also serve as complementary strategies to reduce transmission.