One thing we know for sure about SARS-CoV-2, the virus that causes COVID-19, is that it is changing constantly. Since the beginning of the pandemic, we’ve seen a number of prominent variants, including Alpha, Beta, Delta, and Omicron.

The World Health Organization (WHO) names new coronavirus variants using the letters of the Greek alphabet, starting with the Alpha variant, which emerged in 2020.

Omicron

The original Omicron strain (BA.1) was first identified in Botswana and South Africa in late November 2021, and cases quickly began to surface and multiply in other countries. By December, Omicron was causing daily case numbers in the U.S. to skyrocket to over a million, and it began to spawn subvariants. One of those was BA.5, which became the predominant virus strain in the U.S., only to be replaced in November 2022 by two new subvariants known as BQ.1 and BQ.1.1.

Delta

Delta (B.1.617.2) was first identified in India in late 2020; it soon spread throughout the world, becoming what was the predominant version of the coronavirus—until Omicron took its place in mid-December.

Delta AY.4.2

Delta AY.4.2, sometimes referred to—incorrectly—as Delta Plus, was actually the most prominent of a number of Delta offshoots, some of which had mutations new to Delta but that were found in other variants. AY.4.2 had two mutations to its spike protein, AY145H and A222V, that were considered to be key, but they were not located in a place where they would inhibit vaccines or treatments. This variant was thought to be slightly more contagious than Delta itself, but while Great Britain was tracking a steady rise of AY4.2, it did not rise as quickly in the U.S.

Beta

This variant, or B.1.351, was identified in South Africa at the end of 2020 and spread to other countries. Experts had been concerned about its several mutations and its potential to evade antibodies. Beta was not common in the U.S.

Alpha

Alpha (B.1.1.7) was the first of the highly publicized variants. Alpha first appeared in Great Britain in November 2020 and infections surged in December of that year. It soon surfaced around the world and became the dominant variant in the U.S., where the CDC classified it as a variant of concern. Then, Alpha faded away with the rise of the more aggressive Delta variant.