CDC: Delta Is a 'Variant of Concern'

Illustration of COVID virus particles in front of a world globe.

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Key Takeaways

  • Cases of COVID-19 linked to the Delta variant are increasing in the United States.
  • The Delta variant may spread more easily than the initial strain of the COVID virus and may also cause more severe illness (leading to hospitalization and even death).
  • Being fully vaccinated against COVID is the best protection against the Delta variant.

As many communities begin to loosen COVID-19 restrictions, the Centers for Disease Control and Prevention (CDC) has sounded the alarm of the Delta variant (B.1.617.2) of the COVID-19 virus, which originated in India and now accounts for most cases in the United Kingdom.

On Thursday, the CDC raised its formal classification of Delta from “variant of interest” to a more serious category—“variant of concern.”

Why the Change?

Stephen Kissler, PhD, a research fellow in immunology and infectious diseases at the Harvard T.H. Chan School of Public Health, tells Verywell that the Delta variant is more of a concern because it can spread more easily and cause more severe disease—particularly in people who are not fully vaccinated.

A CDC spokesperson tells Verywell that what moved the needle for the classification change in the U.S. was an increase in COVID cases linked to the Delta variant from mid-May to early June—from 2% to almost 10%.

In the U.K., the Delta variant was responsible for 90% of cases as of June 9. According to a recent report published in the British Medical Journal, the number of hospitalized COVID-19 cases topped 1,000—after having fallen to the hundreds in the middle of May. The increase might be linked to the increase in COVID cases caused by the Delta variant. 

While research is ongoing, Kissler says that one factor that may make the Delta variant more serious is that it increases the number of virus particles people are infected with, and that "higher amounts of virus can increase spread others as well as how sick people get who contract the virus."

Vaccination Is the Best Protection  

Gregory Poland, MD, an infectious disease specialist and head of the Vaccine Research Group at the Mayo Clinic, tells Verywell that the best protection against the Delta variant—or any coronavirus—is vaccination.

A recent study published in Nature found that when 20 people received two doses of the Pfizer vaccine, they had high enough levels of antibodies to protect against several variants of the COVID virus, including the Delta variant.

A person is considered fully vaccinated against COVID-19 two weeks after the second dose of a two-dose vaccine (Moderna or Pfizer), or two weeks after a single-dose vaccine (Johnson & Johnson). 

However, according to a study in The Lancet published in June, people need “full vaccination” to be protected. The study found that people were less likely to develop enough antibodies to protect against the Delta variant after just one dose of a vaccine.

Kisser says that this is “why we are urging not just vaccination, but full vaccination."

Poland adds that vaccination can also prevent variants from occurring in the first place.

“When the virus spreads it can mutate into other variants—some of which, like Delta, can be more dangerous than the original," he says. "But if you stop the spread of the virus through vaccination, you also stop mutations.” 

Precautions Even for The Vaccinated 

Poland says that healthy people who are fully vaccinated run a small risk of reinfection and likely would have only a mild case of COVID if they were to be infected by the Delta variant. Later this summer, Poland anticipates that research that is underway now will let us know what level of antibodies we need for protection against the virus and variants.

That said, Poland says that “people with weak immune systems, including people undergoing cancer treatment or organ transplant, are at greater risk of severe disease and hospitalization if infected by the Delta variants.” 

Anyone at risk should talk with their provider about any precautions that they should take as COVID restrictions lift (just as the Delta variant appears poised to increase). According to Poland, these recommendations could include continued mask-wearing, social distancing, and avoiding crowds, especially indoors.  

Winter Could Increase Variant Risk 

While we might get through the summer, there are concerns that the Delta variant could pose more of a threat in the fall and winter.

“Cases of respiratory viruses, of which the coronavirus is one, rise in fall and winter,” Kissler says. “We’re not sure why. It may be humidity, weather, or even that more people gather together indoors in colder weather.” 

According to the CDC, almost 150 million people, (about 45% of the total U.S. population) have been fully vaccinated as of June 1. That's nowhere near the 70% to 90% threshold that has been discussed as being necessary to reach herd immunity.

Amesh Adalja, MD, a senior scholar at the Johns Hopkins Center for Health Security, tells Verywell that “the Delta variant and other variants offer some level of persuasion for people not yet immunized against COVID-19 to get vaccinated, but it may not be enough for others.”

Adalja says that many people continue to worry about side effects and long-term health concerns, and may face barriers to getting the vaccine.  

If the Delta variant becomes entrenched in the U.S., Adalja says that would make vaccination even more important than it already is. To ensure that people get vaccinated, Adalja says that it's crucial that we take "a one-on-one approach to talk to people about the vaccine and their concerns."

What This Means For You

While many parts of the U.S. are lifting COVID restrictions as more people get fully vaccinated, the Delta variant of the virus still poses a threat—especially to people who are not yet vaccinated.

If you are vaccinated but are still at-risk because of factors like a compromised immune system or certain medical conditions, talk to your healthcare provider about any other precautions that they would recommend you take to stay safe.

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.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Centers for Disease Control and Prevention (CDC). COVID data tracker: variant proportions.

  3. Mahase E. Delta variant: what is happening with transmission, hospital admissions, and restrictions? BMJ. 2021;373. doi:10.1136/bmj.n1513

  4. Liu J, Liu Y, Xia H, et al. BNT162b2-elicited neutralization of B.1.617 and other SARS-CoV-2 variants. Nature. doi:10.1038/s41586-021-03693-y

  5. Wall EC, Wu M, Harvey R, et al. Neutralising antibody activity against SARS-CoV-2 VOCs B.1.617.2 and B.1.351 by BNT162b2 vaccination. Lancet. 2021 Jun 19;397(10292):2331-2333. doi:10.1016/S0140-6736(21)01290-3

  6. Centers for Disease Control and Prevention (CDC). COVID-19 vaccinations in the United States.

By Fran Kritz
Fran Kritz is a freelance healthcare reporter with a focus on consumer health and health policy. She is a former staff writer for Forbes Magazine and U.S. News and World Report.