Delta is the most recent of four coronavirus mutations added to the World Health Organization's and Centers for Disease Control and Prevention's "variants of concern" lists.
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Viruses constantly mutate, and most changes aren't concerning. Right now, health officials are most concerned with the spread of the highly contagious delta but are monitoring mutations closely.
These include lambda, listed as a "variant of interest" by WHO, and delta plus, a mutation of delta itself. Here is what you need to know:
Delta plus
The delta plus variant has an additional mutation in spike, where some people are saying is resulting in increased transmissibility, Dr. Nevan Krogan, a University of California San Francisco, told KGO-TV's Luz Pena.
It was one of 18 delta variant mutations, and one of seven mutations that has a change in the spike protein.
"[Delta is] mutating compared to other viruses incredibly quickly. I think a big part of that is because there is a significant percentage of people who are getting infected and are asymptomatic," Krogan said. "The world is really a huge petri dish."
More data needs to be collected to confirm if this delta plus is more transmissible than the delta variant.
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Dr. Anthony Fauci, the nation's top infectious disease expert, told "Good Morning America" Thursday that there is no firm evidence to determine whether the spread of the delta plus mutation should be cause for alarm.
"The delta variant that we're dealing with is so capable of pushing out other variants, that we're not expecting that to take over. Having said that, we follow [delta plus] very closely," he said.
Even with a new mutation, the vaccines continue to prove effective against these variants, said Dr. Monica Gandhi, an infectious disease specialist.
"It's true that it's mutating, but the one thing to remember about the immune system is that when we get protection from the vaccines, you form T-cells across 85 parts of the spike protein. Losing 11 to 13 places on the spike protein, you still have very strong T-cell immunity from the vaccine," she said.
CDC COVID-19 Transmission Levels by U.S. County
Lambda
WHO classified lambda as a "variant of interest" in June, meaning its genetic changes could affect transmissibility and disease severity. This means health officials are watching lambda but aren't panicking.
Less than 4,000 cases of the lambda variant were documented worldwide, with a small number detected in California, Texas and Louisiana.
The lambda variant was first detected in Peru in December 2020, according to the World Health Organization and makes up 81% of COVID-19 cases sequenced in the country since April 2021, according to a June WHO report. It has since spread to other Latin American countries along with the United States.
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Despite reports of the lambda variant, experts at Houston Methodist say delta is still the primary concern in the U.S.
"The lambda is the dominant variant in Peru and Peru has had a very difficult time with COVID-19. It shares mutations in common with the alpha variants, the beta, the gamma, which is the dominant variant in Brazil," Dr. Wesley Long, medical director of Diagnostic Microbiology at Houston Methodist, told ABC News.
"I don't think there's sufficient evidence at this point that we should be more concerned about lambda than delta, I still think delta is the primary concern for us. There's a lot more evidence that we have that delta is much more contagious, the viral loads are much higher," he added.
Since the delta variant is so hyper-contagious, it is likely to prevent lambda from taking over.
The quicker people develop immunity to the virus the less likely there are for more variants to develop or take hold.
Click here to monitor the latest information on variants from the CDC.
Reporting based on interviews, research and analysis from the ABC News' medical unit and KGO-TV