Content is updated as more information becomes available.  Last updated: December 9, 2021.
It is normal for viruses to change and evolve as they spread between people over time. When these changes become significantly different from the original virus, they are known as “variants.” To identify variants, scientists map the genetic material of viruses (known as sequencing) and then look for differences between them to see if they have changed. 
Since the SARS-CoV-2 virus, the virus that causes COVID-19, has been spreading globally, variants have emerged and been identified in many countries around the world.
The current global epidemiology of SARS-CoV-2 is characterized by the predominance of the Delta variant, a declining trend in the proportion of Alpha, Beta and Gamma variants, and the emergence of Omicron, which was designated as a variant of concern (VOC) by WHO on November 26, 2021. 
A variant is considered a variant of interest if there is initial scientific evidence of mutations that are suspected  to cause significant changes, and is circulating widely (e.g., known to cause many clusters of infected people, or found in many countries).
There are 2 variants of interest (Mu and Lambda) that PAHO/WHO is continuing to monitor in case they become variants of concern. 
A variant of interest becomes a variant of concern if it is known to spread more easily, cause more severe disease, escape the body’s immune response, change virus symptoms, or decrease effectiveness of known tools – such as public health measures, diagnostics, treatments and vaccines. There are 5 variants of concern (VOC) that PAHO/WHO is monitoring to date.
For the purpose of discussing variants in the media and the public, WHO began using the Greek alphabet in May of 2021 to make it easier for people to keep track of variants without linking their names to the places where they were first identified, as variants can emerge anywhere, at any time.
In accordance with WHO best practices for naming new diseases, some letters may not be used if they cause confusion in major languages or stigmatize certain groups. These WHO labels do not replace the existing scientific names of the variants, which convey important scientific information to researchers and scientists.
The Omicron variant, variant B.1.1.529, was first reported to WHO on 24 November 2021 and was classified as a variant of concern by WHO on 26 November 2021. The classification was made on the advice of the Technical Advisory Group on Virus Evolution, based primarily on information from South Africa that showed that the variant has a large number of mutations and has caused a change in COVID-19 epidemiological trends in the country. 
To date, the Omicron variant has already been detected in 56 countries globally and in 6 countries in the Americas (USA, CAN, MEX, CHL, ARG, BRA) and in and 1 territory (Bermuda).
All variants are different. The Omicron variant has a large number of mutations which may mean the virus acts differently from other variants that are circulating.
At the moment, there is limited information about Omicron. Studies are ongoing to determine if there is a change in how easily the virus spreads or the severity of disease it causes, and if there are any impacts on protective measures, diagnosis or treatment.
It will likely take time before there will be clear evidence to determine if there is any change in the transmission of Omicron compared to other variants, how the variant responds to existing treatments, or whether infection or re-infection with Omicron causes more or less severe disease.
WHO is working with technical partners to understand the potential impact of Omicron on vaccine effectiveness. Currently, the Delta variant is dominant worldwide and COVID-19 vaccines approved by WHO are highly effective at protecting you from serious illness and death, including from infection with Delta.
Researchers will assess the performance of current vaccines against Omicron and will communicate these findings as soon as they become available
More questions and answers on COVID-19 vaccines
PAHO/WHO continues to coordinate with a large number of researchers around the world to understand more about all variants of SARS-CoV-2, the virus that causes COVID-19, including Omicron. Many studies are needed, including assessments of:
WHO’s Technical Advisory Group on Virus Evolution will continue to monitor and evaluate the data as it becomes available and assess if mutations in the Omicron variant alter the behaviour of the virus.
In the Americas, the Regional COVID-19 Genomic Surveillance Network, coordinated by PAHO, regularly reports new sequences and scientific findings on SARS-CoV-2 genetic changes, which is critical to improve the development of diagnostic protocols, generate information for vaccine development, and to better understand the patterns of evolution and molecular epidemiology of SARS-CoV-2.
More information on the COVID-19 Genomic Surveillance Regional Network 
The Delta variant is a variant of concern classified by WHO on May 11, 2021 and it is currently the dominant variant circulating globally.  Delta spreads more easily than earlier strains of the virus and is responsible for more cases and deaths worldwide. All WHO-COVID-19 approved vaccines currently in use are safe and effective in preventing severe disease and death against the Delta variant.
To protect yourself and others from COVID-19 variants:
As with all viruses, SARS-COV-2, the virus that causes COVID-19, will continue to evolve as long as it continues to spread. The more that the virus spreads, the more pressure there is for the virus to change. So, the best way to prevent more variants from emerging is to stop the spread of the virus.
So, the best way to prevent more variants from emerging is to stop the spread of the virus and to contribute together to reduce the number of COVID-19 infections.
To protect yourself and others from COVID-19:
Viruses are constantly evolving and changing. Every time a virus replicates (makes copies of itself), there is the potential for there to be changes in its genetic material. Each of these changes is a “mutation.” A virus with one or more mutations is called a “variant” of the original virus. 
Some mutations can lead to changes in important characteristics of the virus, including characteristics that affect its ability to spread and/or its ability to cause more severe illness and death.
We are still learning about the ways that variants impact vaccination. 
The data we currently have available show us that COVID-19 vaccines are still very effective at preventing serious illness and death against all of the current variants of concern.
Currently, the delta variant is dominant in the Americas and the rest of the world, and the WHO-approved COVID-19 vaccines are highly effective in protecting against severe disease and death.
Regarding the new omicron variant of concern, WHO is working with technical partners to understand the potential impact of this variant on vaccine efficacy.  
No vaccine is 100% effective. Although COVID-19 vaccines are very effective in protecting you against severe disease and death, some people will still get sick with COVID-19 after vaccination. They could also spread the virus to others who are not vaccinated. So, it is very important to continue to take protective measures, even after you have been fully vaccinated. 
More questions and answers about COVID-19 vaccines 
So far, the available evidence shows that WHO-approved vaccines are effective in reducing the likelihood of infection and preventing hospitalization and death from COVID-19. 
WHO is working with technical partners to understand the potential impact of omicron on vaccine efficacy. Currently, the Delta variant is dominant in the Americas and the rest of the world and WHO-approved COVID-19 vaccines are highly effective in protecting against severe disease and death, including delta infection. Researchers will evaluate the performance of current vaccines against omicron and will report these results as soon as they become available. 
Since no vaccine is 100% effective, it is necessary to continue with all preventive measures, even after vaccination: constant hand hygiene, use of masks, sneezing or coughing into the elbow, keeping a distance of at least 1 meter from other people, avoiding crowds and keeping spaces well ventilated, are measures that should continue to be practiced to prevent COVID-19 infection. 
More questions and answers about COVID-19 vaccines
Regional Office for the Americas of the World Health Organization
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