Here's What You Need to Know About the Delta Plus Variant

The original Delta variant is still the dominant COVID-19 strain in the United Kingdom, making up about 99.8% of the cases. However, a Delta sublineage, which some experts are dubbing "Delta plus," appears to be causing a growing number of infections in the country.

Early evidence shows that this variant—known as Delta AY.4.2— is gaining ground in the U.K. And it may be more transmissible than Delta itself.

Francois Balloux, PhD, director of the University College London Genetics Institute, suggested on social media that it may even be 10% more transmissible.

Health authorities are now monitoring the Delta plus variant, which accounted for about 6% of all sequenced cases in the week of September 27. It was elevated to the status of variant under investigation (VUI) by the UK Health Security Agency last week.

Experts say they currently have no reason to think the variant causes more severe disease or reduces vaccine effectiveness. But it does have the potential to become the dominant variant in the United States if it continues on its upward trajectory.

Are New Variants Always Concerning?

SARS-CoV-2, the virus that causes COVID-19, continuously evolves and develops small differences in its genetic code. Similar to other viruses, new variants are expected to develop as more infections occur.

“The SARS-CoV-2 virus is constantly mutating,” Scott Roberts, MD, infectious diseases specialist, assistant professor and associate medical director of infection prevention at the Yale School of Medicine, tells Verywell. “We will keep getting new variants and sublineages for as long as the virus is replicating in humans.” 

The changes in the virus are inconsequential sometimes, but they can be harmful if the variation causes the virus to spread more easily or become more difficult to treat.

“Most variants are not concerning, however some variants are more concerning than others,” Roberts says. “An example of this is the Delta variant. This variant, in comparison to the original SARS-CoV-2 strain, is able to transmit more easily and can cause more severe illness.”

In the U.S., the variants are classified based on their proportions at the national and regional levels, the severity of symptoms, impact on virus countermeasures, and ability to spread.

They're classified in the following order:

• Variants being monitored (VBM)
• Variants of interest (VOI)
• Variants of concern (VOC)
• Variants of high consequence (VOHC)

“Similar to what has been seen for other pandemic viruses, SARS-CoV-2 underwent a period of rapid emergence of multiple mutated viruses that eventually gave rise to the worldwide dissemination of the successive waves of Alpha, Beta, Gamma and Delta viruses,” Dan Jones, MD, PhD, professor of pathology and vice chair in the division of molecular pathology at The Ohio State University Wexner Medical Center, tells Verywell.

Currently, the Delta variant and its sublineage are considered VOC, while the Alpha, Beta, and Gamma variants are VBM.

“While predictions on the behavior of SARS-CoV-2 in the coming months cannot be made with certainty, other pandemics have historically shown lower rates of virus evolution and less dissemination of new variants as the infection rates have waned,” Jones says.

Currently, cases are dropping in the U.S.

Can Delta Plus Become the Dominant Variant?

Back in July, the Delta variant rapidly replaced the Alpha variant as the dominant virus strain in the U.S. While the Delta plus variant is currently rare outside the U.K., there’s a possibility that the same trend might occur.

“The rapid emergence of AY.4.2 in the U.K.—closely resembling the shift from multiple strains to Alpha late last year in the U.K.—would suggest it will become a dominant virus in the United States in the coming months,” Jones says.

Vaccines are proven to help curb the transmission of COVID-19. Because of gradually increasing vaccine availability and vaccination rates around the world, the U.S. government has decided to lift its travel ban for noncitizen travelers who are fully vaccinated starting on November 8.

Increased travel between countries could potentially circulate the variant. But even with restrictions in place variants have made their way around the globe.

“To date, travel restrictions have only had a modest effect on slowing the spread of the more transmissible variants of concern during the pandemic,” Jones says.

Delta plus has been identified in the U.S. occasionally, but the CDC hasn’t observed any increased frequency so far, Rochelle P. Walensky, MD, MPH, director of the CDC, said at a White House press briefing last week. They will continue to monitor all variants closely and observe if they affect the effectiveness of COVID-19 vaccines or available treatments.

“It is particularly concerning that the AY.4.2 lineage is able to grow in the setting of the Delta variant,” Roberts says. “This suggests that AY.4.2 is more transmissible than the Delta variant and may displace the Delta variant over time. We don’t know if the AY.4.2. sublineage causes more severe disease than the original Delta variant.”

There is still much to know about this new variant, but experts say that the best control methods are still standard COVID-19 public health measures.

“We do not have any evidence to suggest that our current tools to prevent the spread of the virus, such as masking and vaccination, won’t also work for AY.4.2,” Roberts says. “These preventative measures should be continued.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.