The COVID-19 Delta Variant: Everything We Know About It

Cases of the virus have continued to fall as more people in the US are vaccinated. But in the spring of 2021, there was a lot of buzz about a COVID-19 variant called Delta that had plenty of people—including infectious disease experts—talking.

As of April 26, 2022, the US Centers for Disease Control and Prevention (CDC) considers Delta a variant being monitored," meaning it no longer poses a significant risk to public health in the US. According to CDC data, the Delta variant accounted for more than 20% of COVID-19 cases in the US as of June 19, 2021. But this number quickly rose as, over the summer of 2021, Delta became the primary variant, accounting for 99% of COVID-19 cases.

During a June 2021 briefing of the White House COVID-19 response team, Anthony Fauci, MD, director of the US National Institute of Allergy and Infectious Diseases, said that the variant had the potential to spread more rapidly. "We cannot let that happen in the United States," said Dr. Fauci, pointing to the UK, where the variant had caused a resurgence in cases, as a "powerful argument" for more vaccination.

In the UK, Dr. Fauci said, the Delta variant had become the dominant strain, replacing B.1.1.7 (now called the Alpha strain). It was also driving new cases in people ages 12 to 20.

Dr. Fauci added that the Delta variant may be "associated with increased disease severity" compared to the original SARS-CoV-2, the virus that causes COVID-19.

Eric Topol, MD, founder of the Scripps Research Translational Institute, said in a June 6, 2021, Twitter post that the Delta variant was "the worst we've seen so far."

British Health Secretary Matt Hancock also said in a press conference that Delta was around 40% more transmissible than SARS-CoV-2, meaning it could spread faster and easier than the original strain of COVID-19.

As if that wasn't enough, reports of a new version of the Delta variant, dubbed Delta Plus, started circulating in several countries, including the UK, India, and the US, in June 2021.

It was a lot to take in, especially as people were starting to get used to some sense of normalcy again.

Where Did the Delta Variant Originate?

The Delta variant, aka B.1.617.2, was originally detected in India in December 2020, according to the CDC. It's actually a mutation or sub-variant of B.1.617, the so-called "double mutant" strain that got plenty of attention in April 2020.

"As these viruses mutate and develop, they look like your family tree," John Sellick, DO, an infectious disease expert and professor of medicine at the University at Buffalo/SUNY, told Health. "This particular sub-variant has wreaked havoc in India."

What Was the Impact of the Delta Variant in the US?

The Delta variant caused a lot of concern in 2021 in the US. According to the CDC, the Delta variant was more easily transmissible than its predecessors. In a Yale Medicine report, F. Perry Wilson, MD, a Yale Medicine epidemiologist, said that Delta spread 50% faster than Alpha, which was 50% more contagious than the original strain of COVID-19.

And while being vaccinated helped to lessen the degree to which you could become sick with Delta, you could still pass it on to others.

Why Was the Delta Variant so Concerning?

The Delta variant has several mutations on the spike protein of SARS-CoV-2, which allowed it to spread more easily than previous forms of the virus. The CDC specifically said that this variant was more transmissible than previous variants, and that breakthrough infections were expected in vaccinated people. The CDC also said that vaccinated people who become infected with the Delta variant could pass it on to others, but that being vaccinated should help protect against serious illness, hospitalization, and death.

The CDC previously regarded the Delta variant as a "variant of concern" but changed that status to a "variant of being monitored" in the US on April 14, 2022, based on evidence of its significant reduction.

"A variant like this can rapidly take over and become the primary strain circulating in that region," Stanley Weiss, MD, professor of medicine at the Rutgers New Jersey Medical School and professor of biostatistics and epidemiology at the Rutgers School of Public Health, told Health. "That indicates that a variant has survival advantages. In places such as India where we've seen so many people infected and so much disease, there's evidence that this can spread quickly."

And take over it did. During the summer of 2021, Delta was showing up in 99% of COVID-19 cases in the US.

Experts were also concerned because "the variant has the potential to at least partially evade the protection of the vaccine," William Schaffner, MD, an infectious disease specialist and professor at the Vanderbilt University School of Medicine, told Health. He compared Delta to B.1.351 (now called Beta), which was originally detected in South Africa. "There is a chance that this can make you sick, even if you've been vaccinated," said Dr. Schaffner.

What Are the Symptoms of the Delta Variant?

Symptoms of the Delta variant are slightly different than those of the first variant that spread through the US. "COVID is acting differently now. It's more like a bad cold; people might think they've just got some sort of seasonal cold," Tim Spector, professor of epidemiology at King's College London, explained in a video released on YouTube by the COVID Symptom Study.

Spector explained that the number one top symptom for those with the Delta variant is headache, and this is followed by sore throat, runny nose, and fever. "Those are not the old, classic symptoms," said Spector. "Number five is cough, [though] it's rarer." Spector added that loss of smell isn't among the top 10 reported symptoms for those with the Delta variant.

The CDC doesn't list separate symptoms for each variant, and instead, refers people to the typical symptoms of COVID-19. This may be because everyone's body reacts a little differently to COVID-19 infection. If you're experiencing COVID-19 symptoms and are unsure whether to get tested or seek medical care (like, is this just a cold or COVID-19), the CDC has an interactive Coronavirus Self-Checker that can help you make these decisions.

Are Vaccines Effective Against the Delta Variant?

Yes-ish. The CDC cited data from an unpublished study of COVID-19 cases in 44 counties in California, which found that COVID-19 variants with an L452R mutation, which Delta has, can cause a two-fold reduction in neutralizing titers in people who have been fully vaccinated against COVID-19. (Neutralizing titers help target and take out a virus, and they tend to correlate with the level of protection offered by a vaccine.)

The CDC said, "COVID-19 vaccines are effective against the Delta variant and other variants with widespread circulation in the United States." But just how protective is left up to an individual's interpretation, since the CDC also stated that you could still get Delta and pass it on despite vaccination.

"Even if this variant can partially evade protection of the vaccine, the more people who are vaccinated, the less apt this is to spread," said Dr. Schaffner.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.