Cases of — and concerns about — COVID-19 are rising across the U.S. and around the world. Thousands of different variants exist worldwide, but some have become dominant variants of concern, reported the BBC. These new, more transmissible variants have begun spreading worldwide, and keeping track of them is increasingly complicated.

Here are the worst COVID-19 variants ranked.

WHO will name COVID-19 variants with the Greek alphabet to avoid stigmatization

6. The alpha variant

First detected in the U.K. in December 2020, the alpha variant is more transmissible than the original coronavirus strain detected in Wuhan, China. Alpha is anywhere from 30% to 50% more infectious than other variants currently circulating, reported The New York Times.

  • The alpha variant is likely to be deadlier than the original strain — about 55% deadlier, per The New York Times
  • Vaccines still work effectively against the alpha variant, reported Popular Science.

The alpha variant has been detected in more than 110 countries and became the dominant variant worldwide by this spring, per The New York Times.

Why the alpha variant (originally found in the U.K.) has been so strong

5. The possible ‘zeta’ variant, or B.1.621

First detected in Colombia and more recently detected in South Florida, the B.1.621 variant has not yet received a Greek-letter designation, reported The Washington Post.

  • Some doctors have taken to calling B.1.621 the “zeta” variant, per the Deseret News.
  • So far, there is no evidence that this variant causes more severe disease or evades vaccine efficacy, per The Washington Post.
  • Even in South Florida where “zeta” variant cases have surged, the variant still does not account for the majority of cases, per The Washington Post.

This variant is under further investigation with many questions unanswered. Current data on the B.1.621 or “zeta” variant remains preliminary, reported The Washington Post.

This new Florida variant may be the ‘zeta’ variant. Here’s why

4. The delta plus variant

A mutation of the delta variant, the delta plus variant, was first detected in Europe in March but is believed to have originated in India alongside the original delta variant, reported The Washington Post.

  • The delta plus variant has been labeled a “variant of concern,” per The Washington Post.
  • However, not enough cases of delta plus infections have been identified or studied to know if this variant is worse than the original delta variant, per the Deseret News.

The Centers for Disease Control and Prevention does not consider the delta plus variant to be independent of the delta variant, per The Washington Post. So fully considering the danger of the delta plus variant requires understanding the danger of the delta variant.

COVID-19: What you need to know about the delta plus variant

3. The delta variant

Currently surging around the world, the delta variant was first detected in India in October 2020. The strain has been found to be more transmissible and cause more severe disease than other variants, reported the Deseret News.

  • The delta variant is more transmissible than the common cold and about as transmissible as chickenpox, reported The New York Times.
  • Studies have found that Pfizer and Johnson & Johnson vaccines are less effective against the delta variant, compared to the alpha strain, per Popular Science.
  • However, vaccinated individuals are still more protected than unvaccinated individuals against the delta variant, per The New York Times.

The delta variant has become the fastest-growing variant of concern and the dominant variant in the U.S., per The New York Times. Infections are surging across the U.S., driven by the delta strain of coronavirus.

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2. The lambda variant

First detected in Peru in August 2020, the lambda variant is thought to be more dangerous than the original strain of coronavirus, reported Popular Science. The variant has now been detected in 29 countries, including the U.S., per Newsweek.

  • Like the delta variant, the lambda variant is highly transmissible and thought to be more resistant to vaccines, reported Newsweek.
  • Studies have found that Pfizer, Moderna and Coronavac’s Sinovac vaccines are less effective against lambda than against the alpha variant, reported Popular Science.
  • “As of now, though, there hasn’t been sufficient data regarding exactly how effective current vaccines are against preventing infection from lambda,” reported Newsweek.

Less research has been conducted on the lambda variant, leaving many questions about the strain and many unknowns, per Newsweek. Lambda has not yet been labeled a “variant of concern,” but further research is needed.

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1. A currently unknown variant

As variants such as delta and lambda have emerged, experts have begun warning that future coronavirus mutations could lead to more transmissible and more vaccine-resistant variants, reported the Deseret News.

  • A variant that evades vaccines and spreads quickly would be a “doomsday” variant, per the Deseret News.
  • While new variants are expected, it’s “impossible” to predict what form future variants will take, reported Vox.
  • The next big variant may come from one single person, reported Newsweek.

At the end of the day, “this is a novel coronavirus,” said Preeti Malani, of the Division of Infectious Diseases at the University of Michigan, per The Washington Post. “We’re still learning about it. Each of these variants bring new challenges.”

A variant worse than delta could be coming soon, Fauci says