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Could the Next Global COVID-19 Pandemic Be Spreading Unnoticed, Fueled by Silent Variants?
A newly identified COVID-19 variant, XFG, also known by the name “stratus”, is spreading globally and has been detected in 38 countries, including the United States. The World Health Organization (WHO) has classified XFG as a “variant under monitoring” (VUM). Several Southeast Asian nations have reported a rise in cases linked to this variant. In the United Kingdom, the Health Security Agency reported that XFG accounted for approximately 30% of COVID-19 cases in England by late June. However, the full scope of its spread remains unclear due to the significant decline in COVID-19 testing compared to levels seen during the height of the pandemic.
Tracking the Rise of NB.1.8.1 and XFG as Emerging COVID-19 Variants Spread Amid Gaps in Global Surveillance
The WHO added the COVID-19 variant NB.1.8.1 to its VUM list in May after cases surged rapidly, especially in China. Since then, NB.1.8.1 has become the dominant strain in several countries, including the United States. According to data from the U.S. Centers for Disease Control and Prevention (CDC), the variant accounted for 42% of genetically tested COVID-19 samples from June 8 to June 21. In comparison, the XFG variant, another VUM, made up 14% of tested samples, ranking as the third most prevalent strain during the same period.
However, experts caution that current data may not fully reflect the actual situation. The United States still lacks a comprehensive COVID-19 surveillance system, despite the virus circulating for over five years. The CDC’s estimates rely on formal testing at healthcare facilities, a process that has significantly declined. With many individuals opting for at-home testing or skipping testing altogether, tracking infections has become increasingly difficult. Additionally, recent cuts in public health funding have further hindered data collection and monitoring efforts.
NB.1.8.1 has demonstrated the fastest growth among circulating variants over the past month. Several Southeast Asian countries have reported rising infections and hospitalizations associated with this variant. Despite these trends, the WHO currently assesses the global risk posed by NB.1.8.1 as “low.” Available evidence does not suggest that the variant causes more severe disease or increased mortality compared to other circulating strains.
The WHO has also classified the XFG variant as a VUM due to its increasing prevalence and potential public health impact. XFG has shown a moderate growth advantage over other variants across multiple regions. From early to late May, the variant’s share rose from 10.6% to 16.7% in Europe and from 7.8% to 26.5% in the Americas. In Southeast Asia, its prevalence surged dramatically from 17.3% to 68.7% in the same period.
WHO Classifies XFG as Variant Under Monitoring as Recombinant Lineage Shows Rising Prevalence
Public health authorities closely observe a VUM, a SARS-CoV-2 strain, because it may pose new risks. According to the WHO, the purpose of this classification is to determine whether a particular variant, or related ones, may present additional threats to global public health compared to those already in circulation. A listing as a VUM does not necessarily signal higher danger but alerts authorities to collect data and conduct further analysis promptly.
A VUM can progress to become a Variant of Interest (VOI) if evidence shows that it carries genetic changes affecting its behavior or potential impact on human health. These changes may influence transmission, disease severity, detectability, or resistance to treatments and vaccines. If a VOI causes more serious public health consequences, health authorities designate it as a Variant of Concern (VOC). A VOC shows increased disease severity, reduced vaccine effectiveness, or significant strain on healthcare systems.
The XFG variant, recently classified as a VUM, is part of the Omicron lineage and represents a combination of two other variants: LF.7 and LP.8.1.2. These “recombinant” variants are the result of previous variants merging, which is common due to the frequent mutations observed in SARS-CoV-2. While many new variants fade out due to lack of competitive advantage, XFG has demonstrated potential to outpace others in transmission, earning it a place on the WHO’s watch list.
XFG contains nine additional mutations in its spike protein compared to the NB.1.8.1 variant. The spike protein plays a critical role in the virus’s ability to bind to and enter human cells, meaning changes in this region could affect infectivity or immune evasion. Global health agencies are actively monitoring the variant’s rapid spread and genetic characteristics, even though the exact implications of these mutations remain unclear.
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