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Omicron, Delta, Alpha, and More: What To Know About the Coronavirus Variants

BY KATHY KATELLA August 31, 2022

A quick guide to the coronavirus variants that are top-of-mind right now.

[Originally published: Dec. 10, 2021. Updated: August 31, 2022]

Note: Information in this article was accurate at the time of original publication. Because information about COVID-19 changes rapidly, we encourage you to visit the websites of the Centers for Disease Control & Prevention (CDC), World Health Organization (WHO), and your state and local government for the latest information.

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. 

Although new variants are an expected part of the evolution of viruses, monitoring each one that surfaces is essential in ensuring we—in the U.S. and globally—are prepared. This is especially true if a new variant is more aggressive, highly transmissible, vaccine-resistant, able to cause more severe disease—or all of the above, compared with the original strain of the virus.

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.

Below is a list of—and information about—some of the variants that have been top-of-mind. 

Omicron and its subvariants

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. At the end of August 2022, one of those subvariants, BA.5, made up more than 88% of cases in the United States, making it the predominant variant in this country. A related variant, BA.4, was also on the rise, making up about 8% of cases. 

How contagious is it? Omicron’s subvariants are considered to be especially efficient spreaders of the disease, and the BA.5 and BA.4 variants appear to be better than other subvariants at evading protection provided by vaccines and previous infection (although doctors say the vaccines still provide important protection against severe disease and death).

The original strain of Omicron was more transmissible than Delta was. One explanation was that more than 30 of Omicron’s mutations are on the virus’s spike protein, the part that attaches to human cells, and several of those are believed to increase the probability of infection.

Severity: Scientists are still working to learn more about whether the BA.5 and BA.4 subvariants can cause more severe disease than their predecessors. Data has suggested that the original Omicron strain was less severe, in general, than previous variants, according to the CDC. But it has also noted that surges in cases may lead to significant increases in hospitalizations and deaths, as they did during the variant’s spread in the beginning of the year, when the estimated death rates went as high or higher than they were at the time of the Delta variant surge last autumn. 

Can vaccination prevent it? The CDC says that while breakthrough infections in vaccinated people are expected, getting vaccinated and staying up to date with your vaccine and a booster shot is the best protection against Omicron. 

In March, while data was still emerging about how long the first booster shot will last, the U.S. approved second booster shots of one of the mRNA vaccines for adults ages 50 and older—and all adults who got the Johnson and Johnson vaccine and booster—giving them the option of choosing a fourth shot. A similar authorization was made for people with certain immune deficiencies. 

In August 2022, the FDA authorized both a bivalent Pfizer-BioNTech booster shot for people 12 years of age and older, and a bivalent Moderna shot for adults 18 and older. These boosters are designed to protect against disease caused by the original strain of the SARS-CoV-2 virus, as well as the Omicron variant. Individuals can get the booster shot starting two months after completing the two-dose primary series or receiving a previous booster dose.


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. 

How contagious is it? It’s estimated that Delta caused more than twice as many infections as previous variants—in Connecticut, it was estimated to have been 80 to 90% more transmissible than the Alpha variant. In the U.S., in June 2021, after a steady decline in COVID-19 cases and hospitalizations, the arrival of Delta coincided with a rapid reversal of that trend. In the fall of 2021, there were surges even in the most vaccinated states, prompting experts to urge people to get their booster shots.

Severity: Delta caused more severe disease than other variants in people who weren’t vaccinated. Early studies from Scotland and Canada, both cited by the CDC, suggested Delta was more likely to result in hospitalization in the unvaccinated. A report in the Lancet this past summer found that people in England had double the hospitalization risk with Delta than they did with Alpha, the previously dominant variant in that country. 

Can vaccination prevent it? All three vaccines in the U.S. were considered highly effective against severe illness, hospitalizations, and death from Delta. No vaccine is 100% effective, and Delta caused breakthrough infections in some fully vaccinated people. Also, infected vaccinated people could spread the virus to others, although likely they were infectious for a shorter time. 

Delta also prompted the CDC to recommend “layered prevention strategies” for both the vaccinated and the unvaccinated. That means that, in addition to staying up-to-date with their vaccines, people were advised to practice such strategies as washing hands, wearing masks, and maintaining a physical distance from one another, especially when indoors in places where there was substantial or high transmission.

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.

How contagious is it? While the data is limited, it was thought to be 10 to 20% more transmissible than Delta.

Severity: It did not appear to pose a greater chance of hospitalization or death.

Can vaccination prevent it? There was some evidence to show vaccines were effective against AY.4.2. Experts also recommended masking, physical distancing, and other mitigation strategies.

*AY.4.2 is technically an offshoot of Delta and not itself a coronavirus variant.


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.

How contagious is it? The CDC said Beta was about 50% more contagious than the original coronavirus strain.

Severity: There was evidence to suggest that Beta may have been more likely than other variants to lead to hospitalization and death.

Can vaccination prevent it? South Africa stopped offering the AstraZeneca-Oxford vaccine (which is not available in the U.S.) early in 2021 after clinical trials showed it did not provide strong protection against mild and moderate disease from the Beta variant. Pfizer-BioNTech, Moderna, and Johnson & Johnson also reported less protection against Beta.


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.

How contagious is it? Some mutations in Alpha’s spike protein were thought to make it more infectious. The B.1.1.7 lineage was believed to be 30 to 50% more contagious than the original SARS-CoV-2 strain. In the U.S., in mid-April 2021—before Delta became predominant—Alpha comprised 66% of cases, according to a study released in June by the CDC.

Severity: Studies have suggested the B.1.1.7 lineage was more likely to land infected people in the hospital and was deadlier than the original virus.

Can vaccinations prevent it? Pfizer, Moderna, and Johnson & Johnson all said their vaccines were effective in preventing severe disease and hospitalization in Alpha cases.  

Are there other variants to be concerned about?

The CDC is currently focused on Omicron and it’s subvariants as a variants of concern in the U.S. That includes B.1.1.529, BA.1, BA.1.1., BA.2, BA.3, BA.4, and BA.5. The classification is given to variants that show increased transmissibility, could cause more severe disease, may be resistant to antibodies from previous infections or vaccination, and/or show an ability to evade diagnostic detection.

The agency is still monitoring for other variants that are either no longer detected in the U.S. or are spreading at a slow enough pace that they don’t pose a serious risk. In addition to Alpha Beta and Delta, they include the Gamma, Epsilon, Eta, Iota, Kappa, Mu, and Zeta, as well as the 1.617.3 variant. 

Experts say one of their concerns is that limited access to vaccines around the world will drive surges in COVID-19 cases, and this will increase the chances that concerning variants will continue to emerge.

This article was medically reviewed by Yale School of Public Health epidemiologist Nathan Grubaugh, PhD.

Note: Information provided in Yale Medicine articles is for general informational purposes only. No content in the articles should ever be used as a substitute for medical advice from your doctor or other qualified clinician. Always seek the individual advice of your health care provider with any questions you have regarding a medical condition.