By Dr Than Lwin Tun
THE omicron is a variant of SARS CoV-2 virus which is the virus that causes COVID-19. The omicron variant has a large number of mutations in the spike protein of the virus, which is part of the virus that allows it to enter human cells.
The omicron subvariant XBB.1.16, known as “Arcturus”, has been listed by the WHO as a variant under monitoring since 22 March 2023. Experts say this variant has a higher transmissibility rate than previous strains but doesn’t appear to be more dangerous.
All viruses, including SARSCoV-2, the virus that causes COVID-19, change over time. Most changes have little to no impact on the virus’s properties. However, some changes may affect the virus’s properties, such as how easily it spreads, the associated disease severity, or the performance of vaccines, therapeutic medicines, diagnostic tools, or other public health and social measures.
In June 2020, the WHO Virus Evolution Working Group was established with a specific focus on SARS-CoV-2 variants, their phenotype and their impact on countermeasures. This later became the Technical Advisory Group on SARS-CoV-2 Virus Evolution. In late 2020, the emergence of variants that posed an increased risk to global public health prompted WHO to characterize some as variants of interest (VOIs) and variants of concern (VOCs) in order to prioritize global monitoring and research and to inform and adjust the COVID-19 response. From May 2021 onwards, WHO began assigning simple, easy-tosay labels for key variants.
Considerable progress has been made in establishing and strengthening a global system to detect signals of potential VOIs or VOCs and rapidly assess the risk posed by SARS-CoV-2 variants to public health. It remains critical that these systems are maintained, and data are shared, according to good principles and in a timely fashion, as SARS-CoV-2 continues to circulate at high levels around the world. While monitoring the circulation of SARS-CoV-2 globally, it also remains essential to monitor their spread in animal populations and chronically infected individuals, which are crucial aspects of the global strategy to reduce the occurrence of mutations that have negative public health implications. In March 2023, WHO updated its tracking system and working definitions for variants of concern, variants of interest and variants under monitoring.
The World Health Organization (WHO) has upgraded XBB.1.16 to a “variant of interest”, due to its estimated growth advantage over other circulating COVID-19 subvariants, and its ability to evade the immune system. Because of these characteristics, “XBB.1.16 may spread globally and contribute to an increase in case incidence,” the WHO warned in its latest weekly epidemiological update. XBB.1.16 was first detected in India on 23 January 2023, where it is believed to have fuelled a recent surge in cases. The variant is also gaining traction in the United States, where it currently accounts for 7.2 per cent of COVID-19 cases.
Anecdotal reports from paediatricians in India suggest that Arcturus may be associated with “itchy” or “sticky” eyes, as well as a high fever and cough – particularly in children who are infected. Other viruses can also trigger conjunctivitis or pink eye, but until now, this symptom had been relatively rare among COVID-19 patients. In Northern Hemisphere countries, which are currently entering spring, it could also potentially be confused with hay fever.
According to Matthew Binnicker, director of clinical virology at the Mayo Clinic in Rochester, US, XBB.1.16 may also be associated with a higher-grade fever than other variants.
Variants of interest
Besides XBB.1.16, WHO is currently tracking one other COVID-19 variant, XBB.1.5, as a “variant of interest” (VOI) – one containing genetic changes predicted or known to affect characteristics such as infectiousness, immune system evasion, or disease severity. VOIs must also be demonstrated to have a growth advantage over other circulating variants in more than one WHO region, or some other evidence to suggest they pose an emerging risk to global public health.
So far, XBB.1.16 has been reported in 31 countries, and during the week ending 2 April 2023, it accounted for 4.2 per cent of SARSCoV-2 sequences uploaded to the international GISAID database – up from 0.5 per cent during the week ending 5 March 2023.
XBB.1.5 has been reported in 96 countries and currently accounts for 50.8 per cent of sequences, which is an increase from 46.2 per cent during the week ending 5 March.
Six other variants and their descendants are being monitored as “variants under monitoring” (VUM), meaning they contain genetic changes suspected to affect viral behaviour and may be showing early signals of faster spread, but the impact of these changes remains uncertain.
Speaking at a press conference on 29 March 2023, Dr Maria Van Kerkhove, WHO’s COVID-19 Technical Lead, reminded journalists that the world remains in a pandemic and a public health emergency of international concern – even though we are in a much better situation than we were three years ago.
Viral evolution
“One of the big uncertainties we face going forward is the virus itself,” Van Kerkhove said. “It hasn’t settled into a predictable pattern. It continues to evolve.
“One of the variants that we are looking at... is XBB.1.16. It’s actually very similar in profile to XBB.1.5. It has one additional mutational mutation in the spike protein, which in lab studies shows increased infectivity, as well as potential increased pathogenicity. So, it’s one that we are monitoring because it has potential changes that we need to keep a good eye out on.”
At that time, only about 800 sequences of XBB.1.16 had been uploaded to the international GISAID database from 22 countries – mostly India.
Enhanced transmissibility
Currently, there are no reported laboratory or country reports associating the VOI and VUMs with increased disease severity, although a recent laboratory study, which has not been peer-reviewed, suggested that XBB.1.16 may have an effective reproduction number (the expected number of new infections caused by an infectious individual) that is 1.27-fold higher than the original XBB variant, and 1.17-fold higher than XBB.1.5, “suggesting that XBB.1.16 will spread worldwide in the near future”, the authors said. However, the study found that the ability of these variants to evade the immune system was comparable.
While the ability of SARSCoV-2 to evolve, become more transmissible, and trigger new symptoms is alarming, at the current time there is nothing to suggest that XBB.1.16 is more dangerous than the many other Omicron variants currently in circulation. Even so, it remains important to stay vigilant, to try to avoid getting infected and to stay updated with the recommended schedule of COVID-19 vaccines.
Overall risk assessment
Based on its genetic features, immune escape characteristics and growth rate estimates, XBB.1.16 may spread globally and drive an increase in case incidence. From reports from India and other countries, no early signals of increases in severity have been observed. As XBB.1.16 has spread to 33 countries, disease severity is being monitored carefully. Taken together, available evidence does not suggest that XBB.1.16 has additional public health risks relative to the other currently circulating Omicron descendent lineages.
The severity and clinical considerations
An analysis of infections from India did not report any differences in hospitalization and oxygen requirement for XBB.1.16 as compared to other circulating lineages (Dr Rajesh Karyakarte’s, BJ Government Medical College, Pune). In terms of clinical considerations, there has been a slight rise in bed occupancy in some states in India (2-4 per cent). However, these levels are much lower compared to the level recorded during the Delta wave or Omicron BA.1/BA.2 wave (6). Disease severity is not higher compared to previously circulating variants. The antiviral Sotrovimab exhibits antiviral activity against XBB.1.16, similar to other XBB subvariants.
References
Tracking SARS-CoV-2 variants (who.int)
Arcturus: WHO upgrades XBB.1.16 to a COVID-19 “variant of interest” | Gavi, the Vaccine Alliance
XBB.1.16 Initial Risk Assessment, 17 April 2023 (WHO)
