Fauci says delta variant is the 'greatest threat' in our fight against COVID-19. Why?

A highly transmissible SARS-CoV-2 variant called "delta" has spread to nearly 100 countries around the world, including to the U.S., where it's likely to soon become the dominant variant.

The delta variant is the "greatest threat" in efforts to eliminate COVID-19 in the U.S., Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases said at a White House briefing on Tuesday (June 22). But the good news is that vaccines, (based on data from two of them) will likely be effective against the variant, he said.

"We have the tools, so let's use them and crush the outbreak," Fauci said, urging people who haven't yet been vaccinated to do so. The variant, which is thought to be the most transmissible of all COVID-19 variants to date, currently makes up 20% of new U.S. cases, up from 10% last week, according to Quartz India.

Meanwhile, India announced on Wednesday (June 23) the emergence of a so-called "delta plus" variant, which is a sub-lineage of delta carrying an additional mutation that might make it even more transmissible. But experts are saying it's too soon to draw any conclusions about that variant.

Here's what we know about the delta and delta plus variants:

What is the Delta variant?

Officials first identified the delta variant, or B.1.617.2, in India in October 2020, and the World Health organization (WHO) designated it a "variant of concern" on May 11, 2021.

Delta is one of four "variants of concern" listed by the WHO; the others are alpha (the variant first discovered in the U.K.), beta (first discovered in South Africa) and gamma (first discovered in Brazil).

But delta is "faster, it is fitter, it will pick off the more vulnerable more efficiently than previous variants, and therefore if there are people left without vaccination, they remain even at further risk," Dr. Mike Ryan, executive director of the WHO's Health Emergencies Program, said at a news briefing on Monday (June 21).

The delta variant carries 10 mutations that cause changes to the virus' spike protein, which it uses to grab and invade human cells, according to the Centers for Disease Control and Prevention (CDC). Small changes like these in a virus's genome may impact how the virus behaves, leading to changes in its transmissibility and/or virulence.

Three of the delta mutations — E484Q, L452R and P614R — allow the spike protein to attach more easily to the ACE2 receptor on human cells, according to Quartz India.

Where is the delta variant spreading?

The variant has spread to at least 92 countries, including the U.S., Maria Van Kerkhove, WHO's COVID technical lead, said during the briefing, according to CNBC.

In the U.K., delta now accounts for more than 90% of newly diagnosed cases, according to Quartz India. The delta variant currently makes up 20% of new cases in the U.S., Fauci told NBC's "TODAY" show on Wednesday (June 23), CNBC reported.

The delta variant will likely replace the alpha variant as the dominant variant in the U.S., CDC Director Dr. Rochelle Walensky said last week in an interview with ABC News on "Good Morning America."

That's because delta is much more transmissible than the alpha variant, which became the dominant variant in the U.S. within a matter of one or two months, Walensky told ABC News.

Is the delta variant more transmissible?

The delta variant seems to be around 60% more transmissible than the alpha variant, according to Nature news. The alpha variant, which is currently the dominant variant in the U.S., is 50% more transmissible than the original virus, according to the CDC.

Delta is spreading faster in U.S. counties where less than 30% of residents have been fully vaccinated than in those where more than 30% have been fully vaccinated, according to Nature.

Is the delta variant more deadly?

Early data suggests the delta variant "is associated with an increased disease severity as reflected by hospitalization risk," Fauci said during the briefing. That data, from England and Scotland, suggest that a person is twice as likely to be hospitalized if infected with the delta variant compared with the alpha variant, according to Nature.

Still, while it's established that delta is more transmissible compared with other variants, not much is known about whether it causes more severe disease.

Do vaccines work against the delta variant?

"I will say, as worrisome as this delta strain is with regard to its hyper-transmissibility, our vaccines work," Walensky told ABC News.

Both Walensky and Fauci encouraged people in the U.S. to get vaccinated in order to be protected against the delta variant. Walensky also noted that high vaccination rates will reduce the chances that the variants have time to mutate and produce even more transmissible or severe variants.

In a recent study, conducted by the Public Health England, researchers found that Pfizer's COVID-19 vaccine was 88% effective against symptomatic disease caused by the delta variant, compared with about 93% effective against the alpha variant, according to a May 22 statement from the website of the U.K. government. Similarly, AstraZeneca's vaccine was 60% effective against the delta variant compared to 66% against the Alpha variant.

Data isn't yet available on the effectiveness of many other vaccines against delta. But Fauci told The Washington Post a few weeks ago that Moderna's vaccine — which has a similar makeup to Pfizer's — would likely have a similar efficacy against the delta variant. Still, no data has been released on Moderna's or Johnson & Johnson's response against the delta variant.

The makers of Russia's Sputnik V vaccine announced on Twitter on June 15 that their vaccine was "more efficient against the Delta variant" than any other vaccine and that they submitted their results to an international peer-reviewed journal.

While COVID-19 case counts have been declining across the U.S., there are still areas — primarily those with low vaccination rates — that are at risk of "localized" outbreaks, Fauci said. "There is a danger, a real danger, that if there is a persistence of a recalcitrance to getting vaccinated that you could see localized surges," he said. "All of that is totally and completely avoidable by getting vaccinated."

Delta poses the biggest threat to countries that have little access to vaccines, such as many countries in Africa where less than 5% of the population is vaccinated, according to Nature.

What is the 'delta plus' variant?

On Wednesday (June 23), India's health ministry announced around 40 cases of what the country calls the "delta plus" variant, a sub-lineage of the delta variant with an extra mutation seen in another variant of concern, according to Reuters. Delta plus, or AY.1, was first reported in a Public Health England bulletin on June 11.

Delta plus carries the K417N mutation, also found in the beta variant, which seems to reduce the effectiveness of a monoclonal antibody cocktail in treating the virus, according to Reuters. "The mutation K417N has been of interest as it is present in the beta variant (B.1.351 lineage), which was reported to have immune evasion property," India's health ministry said in a statement.

Where is 'delta plus' found?

Officials have identified at least 197 cases of delta plus in 11 countries as of June 16, including 83 in the U.S., according to Reuters.

Is 'delta plus' more dangerous?

India has identified delta plus as a "variant of concern," citing that it is more transmissible than any known variants; it binds more strongly to lung cell receptors compared with other variants; and it might be less susceptible to treatment with a monoclonal antibody created infected patients, according to a statement cited by the BBC.

But experts say that it's too soon to draw conclusions about this variant.