SARS-CoV-2 variants occur when there are changes or “mutations” in the original virus’ genetic code. These changes occur naturally over time as a by-product of replication. The World Health Organization (WHO) tracks variants of SARS-CoV-2 around the globe and designates them as variants of concern (VOCs) when they meet specific criteria demonstrating that they spread much faster, are better adept at infecting people and/or impact the effectiveness of vaccines and therapeutics.
Currently there are four VOCs: Alpha, Beta, Gamma, and Delta. The Alpha variant, also known as the B.1.1.7 variant, was first identified in the United Kingdom where a mutation occurred in the receptor binding domain (RBD) of the spike protein. This resulted in a change of the genetic code, specifically, the amino acid asparagine (N) was replaced with tyrosine (Y) at location 501 (N501Y).
The Beta variant, also known as B.1.351 originated in South Africa, the Gamma variant, also known as P.1, originated in Brazil and Japan, and the Delta variant, also known as B.1.617.2 and by far the most menacing, originated in India. Beta and Gamma variants also have the above N501Y mutation, but they have several others, like E484K. The Delta variant has a more diverse repertoire of mutations than other variants. It lacks the N501 mutation, has a different mutation in position 484 (E484Q), and has several other unique mutations (including P681R and L452R).
The WHO also tracks variants of interest (VOIs), defined as variants with mutations that could affect transmissibility, disease severity, and the effectiveness of immune responses and therapeutics. Currently, there are five VOIs: Eta (B.1.525), Iota (B.1.526), Kappa (B.1.617.1), Lambda (C.37), and Mu (B.1.621).
When a person is infected with the virus such as SARS-CoV-2, the virus replicates many times in the host (humans). These viruses can be passed on from person to person through respiratory droplets (sneezing and coughing). With rapid virus replication and immune pressure to control the virus, this can cause the virus to mutate, generating a new variant. If the mutation found in the variant confers it some advantage, it will spread better in the population. Image credit: Mariana Bego
Individual mutations can be mapped to a common ancestor in this tree diagram. Several mutations can give rise to new viral variants and these mutations can confer advantages to the virus (for example, increased transmissibility). Image credit: Mariana Bego