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Information and resources on SARS-CoV-2 (the virus that causes COVID-19)

WHO Greek Letter Designated Variants

  • First Identified: 9/1/2020 - United Kingdom
  • Other Names: VOC 202012/01; 20I/501Y.V1
  • Pango: B.1.1.7
  • VOC: 12/8/2020
  • Previous VOC: 3/9/2022

  • First Identified: 12/18/2020 - South Africa
  • Other Names: 20H/501Y.V2;  501.V2; 20C/501Y.V2; VOC-202012/02
  • Pango: B.1.351
  • VOC: 12/18/2020
  • Previous VOC: 3/9/2022

  • First Identified: 1/21/2021 - Brazil
  • Other Names: 20J/501Y.V3;  VOC-202101/02
  • Sublineage:  B.1.1.28
  • Pango: P.1
  • VOC: 1/11/2021
  • Previous VOC: 3/9/2022

  • First Identified: 3/1/2021 - India
  • Other Names: G/452R.V3; 21A/S:478K
  • Pango: B.1.617.2
  • VOI: 4/4/2021
  • VOC: 5/11/2021
  • Previous VOC: 6/7/2022

  • First Identified: California, USA
  • Other Names: CAL.20C; CA VUI1; 20C/S:452R
  • Pango: B.1.427/B.1.429
  • VOI: 3/5/2021
  • Previous VOI: 7/6/2021

  • First Identified: 4/1/2020 - Rio de Janerio, Brazil
  • Other Names:
  • Sublineage:  B.1.1.28
  • Pango: P.2
  • VOI: 3/17/2021
  • Previous VOI: 7/6/2021

  • First Identified: 12/1/2020 - Nigeria
  • Other Names: VUI-202102/03; 20A/S:484K
  • Pango: B.1.525
  • VOI: 3/17/2021
  • Previous VOI: 9/20/2021

  • First Identified: 3/13/2021 - Philippines
  • Other Names: VUI-21MAR-02
  • Pango: P.3
  • VOI: 3/24/2021
  • Previous VOI: 7/6/2021

  • First Identified: 11/1/2020 - NYC, USA
  • Pango: B.1.526
  • VOI: 3/24/2021
  • Previous VOI: 9/20/2021

  • First Identified: 10/1/2020 - India
  • Other Names: 21A/S:154K
  • Pango: B.1.617.1
  • VOI: 4/4/2021
  • Previous VOI: 9/20/2021

  • First Identified: 8/1/2020 - Peru
  • Pango: C.37
  • VOI: 6/14/2021
  • Previous VOI: 3/9/2022

  • First Identified: 1/1/2021 - Colombia
  • Pango: B.1.621
  • VOI: 8/30/2021
  • Previous VOI: 3/9/2022

  • First Identified: 11/26/2021 - South Africa
  • Other Names: BA.1
  • Pango: B.1.1.529
  • VUM: 11/24/2021
  • VOC: 11/26/2021
  • Previous VOC: 3/15/2023

WHO Variant Monitoring

WHO SARS-CoV-2 Variant Tracking
Updated working definitions and primary actions for SARS-CoV-2 variants
15 March 2023 | Technical Report

On 15 March 2023, WHO announced that, going forward, the Greek letters will only be assigned to VOCs, while VOIs will be referred to using established scientific nomenclature systems such as those used by Nextstrain and Pango (e.g. XBB.1.5/23A for the latest VOI). WHO and TAGVE will undertake regular risk assessments for both VOIs and VOCs.

Variant Under Monitoring (VUM)
  • A SARS-CoV-2 variant with genetic changes that are suspected to affect virus characteristics and early signals of growth advantage relative to other circulating variants (e.g. growth advantage which can occur globally or in only one WHO region), but for which evidence of phenotypic or epidemiological impact remains unclear, requiring enhanced monitoring and reassessment pending new evidence
Variant of Interest (VOI)
  • A SARS-CoV-2 variant with genetic changes that are predicted or known to affect virus characteristics such as transmissibility, virulence, antibody evasion, susceptibility to therapeutics and detectability; AND
  • Identified to have a growth advantage over other circulating variants in more than one WHO region with increasing relative prevalence alongside increasing number of casesover time, or other apparent epidemiological impacts to suggest an emerging risk to global public health.
Variant of Concern (VOC)

A SARS-CoV-2 variant that meets the definition of a VOI (see above) and, through a risk assessment, conducted by WHO TAG-VE, and determined to be associated with a moderate or high level of confidence, meets at least one of the following criteria when compared with other variants:

  • Detrimental change in clinical disease severity; OR
  • Change in COVID-19 epidemiology causing substantial impact on the ability of health systems to provide care to patients with COVID-19 or other illnesses and therefore requiring major public health interventions; OR
  • Significant decrease in the effectiveness of available vaccines in protecting against severe disease.

WHO Technical Advisory Group on SARS-CoV-2 Virus Evolution (TAG-VE)

XBB.1.16 Initial Risk Assessment
17 April 2023

XBB.1.16 is a descendent lineage of XBB, a recombinant of two BA.2 descendent lineages. XBB.1.16 was first reported on 09 January 2023, and designated a variant under monitoring (VUM) on 22 March 2023. On 17 April 2023, XBB.1.16 was designated a variant of interest (VOI). XBB.1.16 has a similar genetic profile as the VOI XBB.1.5, with the additional E180V and K478R amino acid mutations in the spike protein compared to their parent XBB.1.

As of 17 April 2023, 3648 sequences of the Omicron XBB.1.16 variant have been reported from 33 countries (GISAID, XBB.1.16 searched using variant defining nucleotide mutations T12730A, T28297C, A28447G). The majority of the XBB.1.16 sequences are from India (63.4%, 2314 sequences). The other countries with at least 50 sequences include the United States of America (10.9%, 396 sequences), Singapore (6.9%, 250 sequences), Australia (3.9%, 143 sequences), Canada (2.6%, 94 sequences), Brunei (2.4%, 89 sequences), Japan (2.0%, 73 sequences) and the United Kingdom (2.1%, 75 sequences).

Globally, there has been a weekly rise in the prevalence of XBB.1.16. During epidemiological week 13 (27 March to 2 April 2023), the global prevalence of XBB.1.16 was 4.15%, an increase from 4 weeks prior (epidemiological week 9, 27 February to 5 March 2023), when the global prevalence was 0.52%.

  Indicator Level of Risk Level of Confidence
Growth Advantage

Comparing the month of February and the month of March 2023 in India, the proportion of XBB1.16 relative to other circulating variants rose from 15.3% (137/895) to 58.6% (2,130/3,636). Similarly for countries with more than 100
sequences, the prevalence of XBB.1.16 rose from 0.04 to 1.09 for the United States of America (USA), 1.2% (12/992) to 11.6% (223/1,929) for Singapore, and 0.07% (2/3,331) to 3.7% (154/4,180) for Australia.

From WHO’s internal variant growth rate analysis, similarly used by the UKHSA, XBB.1 family of variants, which includes XBB.1.16, have the fastest growth over
other circulating variants in AMRO, EURO, SEARO and WPRO. US CDC Nowcast model-based projections predict a rise of the XBB.1.16 variant to 7.2% (95% predictive interval 4.5- 1.3%) by 15 Apr 20236.

Nextstrain has designated XBB.1.16 a new clade 23B based on a criteria of “>0.05 per day growth in frequency and >5% regional frequency”, whereby from their
estimates XBB.1.16 represented ~10% of all sequences collected mid-March in Asia, had a simple logistical growth rate advantage of ~9% in all of Asia, and relative growth against all of XBB* variants of ~5% in India.

Moderate High
Antibody Escape

Similar to XBB.1 and XBB.1.5, XBB.1.16 neutralization assays have demonstrated resistance to BA.2 and BA.5 breakthrough infection sera.

The sensitivity of XBB.1.16 to convalescent sera of XBB.1-infected hamsters was comparable to those of XBB.1 and XBB.1.5, which points at a similar ability of these variants to evade immunity.

Moderate Low
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%). However, these levels are much lower compared to the level recorded during the Delta wave or Omicron BA.1/BA.2 wave.

Disease severity is not higher compared to previously circulating variants. In India, >70% of the population have received a booster vaccine dose.

The antiviral sotrovimab exhibits antiviral activity against XBB.1.16, similar to other XBB subvariants.

Low Moderate

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.