XBB Variant

The XBB variant is a recombinant lineage of the SARS-CoV-2 virus, responsible for COVID-19. It emerged in 2022 as a result of recombination between two Omicron subvariants (BA.2.10.1 and BA.2.75) and rapidly gained global attention due to its high transmissibility and ability to evade immune responses. The World Health Organization (WHO) classified XBB and its sub-lineages as variants under monitoring and, later, as variants of interest.

Emergence and Nomenclature

• Origin: The XBB variant was first detected in India in mid-2022.
• Recombinant Lineage: Unlike simple mutations, XBB arose through recombination, where genetic material from two Omicron lineages combined during co-infection in a host.
• Designation: Following Pango lineage nomenclature, it was named XBB, marking it as a recombinant strain of Omicron.

Genetic Characteristics

XBB inherits mutations from both parent lineages (BA.2.10.1 and BA.2.75), particularly in the spike protein, which mediates entry into human cells. These mutations provide:

• Enhanced Immune Escape: Strong ability to evade neutralising antibodies from prior infection or vaccination.
• Increased Transmissibility: Higher binding affinity to the ACE2 receptor compared to earlier Omicron subvariants.

Sub-lineages such as XBB.1.5 and XBB.1.16 have attracted further attention due to additional spike mutations that enhance transmissibility.

Epidemiology

• Global Spread: Following its initial detection, XBB spread rapidly across Asia, later becoming dominant in several regions worldwide.
• XBB.1.5 (“Kraken”): First identified in the United States, it became one of the most transmissible sub-lineages observed during 2023.
• XBB.1.16 (“Arcturus”): Noted for a rise in India and linked to increased paediatric cases, with symptoms including conjunctivitis in some reports.

Clinical Features

The clinical presentation of XBB infections remains similar to other Omicron variants, generally causing mild to moderate symptoms, especially in vaccinated individuals.
Common symptoms include:

• Fever, sore throat, cough, and nasal congestion.
• Fatigue and muscle aches.
• Headache and loss of taste or smell (less common than in earlier variants).

Distinct observations:

• XBB.1.16 infections reported in children sometimes presented with eye irritation or conjunctivitis.
• Hospitalisation rates remained lower than during earlier waves, although risk persists for elderly populations and immunocompromised individuals.

Vaccine Effectiveness and Immunity

• Immune Evasion: XBB demonstrates one of the strongest antibody escape profiles among Omicron lineages, reducing the effectiveness of existing vaccines in preventing infection.
• Protection Against Severe Disease: Despite reduced neutralising antibody response, vaccines, especially with updated bivalent boosters, continue to provide strong protection against severe illness, hospitalisation, and death.
• Hybrid Immunity: Individuals with both vaccination and prior infection maintain broader and more durable protection.

Public Health Response

• Surveillance: Genomic sequencing remains crucial for tracking the spread and evolution of XBB and its sub-lineages.
• Vaccination Campaigns: Emphasis on booster doses, particularly bivalent vaccines, to improve protection.
• Preventive Measures: Standard public health practices—mask use in crowded areas, ventilation, and hygiene—remain effective in reducing transmission.

Significance

The emergence of the XBB variant underscores the adaptability of SARS-CoV-2 and the importance of continuous monitoring. Its high transmissibility and immune evasion highlight challenges in long-term COVID-19 management, but widespread vaccination and hybrid immunity have reduced its global health impact compared to earlier pandemic waves.