5 Things To Know About the Delta Variant

Article by: 5 Things To Know About the Delta Variant > News > Yale Medicine



Even as people began to feel some hope—or at least cautious optimism—early this summer that the pandemic could recede to the background, there was still the threat that new mutations of the COVID-19 virus could bring it back, and it might be even stronger. A major worry right now is Delta, a highly contagious SARS-CoV-2 virus strain, which was first identified in India in December. It swept rapidly through that country and Great Britain before reaching the U.S., where it is now the predominant variant. The Centers for Disease Control and Prevention (CDC) described Delta as more transmissible than the common cold and influenza, as well as the viruses that cause smallpox, MERS, SARS, and Ebola—and called it as contagious as chickenpox in an internal document, a copy of which was obtained by and reported on in The New York Times. The highest spread of cases and severe outcomes is happening in places with low vaccination rates, and virtually all hospitalizations and deaths have been among the unvaccinated, the CDC says. But the CDC released data in July that showed vaccinated people also can transmit Delta, which officials did not believe to be the case with other variants, and which led the agency to make a prompt revision to its masking guidelines. Inci Yildirim, MD, PhD, a Yale Medicine pediatric infectious diseases specialist and a vaccinologist, isn’t surprised by what’s happening. “All viruses evolve over time and undergo changes as they spread and replicate,” she says. From what we know so far, people who are fully vaccinated against the coronavirus continue to have strong protection against COVID-19 compared to those who aren’t. But anyone who is unvaccinated and not practicing preventive strategies is at high risk for infection by the new variant, doctors say. Here are five things you need to know about the Delta variant.

1. Delta is more contagious than the other virus strains.

One thing that is unique about Delta is how quickly it is spreading, says F. Perry Wilson, MD, a Yale Medicine epidemiologist. Around the world, he says, “Delta will certainly accelerate the pandemic.” The first Delta case was identified in December 2020, and the variant soon became the predominant strain of the virus in both India and then Great Britain. By the end of July, Delta was the cause of more than 80% of new U.S. COVID-19 cases, according to CDC estimates. The July CDC report on Delta's transmissibility came after an outbreak that occurred in Provincetown, Mass., after a crowded July 4 weekend, which quickly turned into a cluster of at least 470 cases. While the number of reported breakthrough cases in general has been very low in the U.S., three quarters of those infected in Provincetown were people who had been immunized. According to the CDC, even people with “breakthrough cases” carry tremendous amounts of virus in their nose and throat, and, according to preliminary reports, can spread the virus to others whether or not they have symptoms. The CDC has labeled Delta “a variant of concern,” using a designation also given to the Alpha strain that first appeared in Great Britain, the Beta strain that first surfaced in South Africa, and the Gamma strain identified in Brazil. (The new naming conventions for the variants were established by the WHO as an alternative to numerical names.) “It’s actually quite dramatic how the growth rate will change,” says Dr. Wilson, commenting on Delta's spread in the U.S. in June. Delta was spreading 50% faster than Alpha, which was 50% more contagious than the original strain of SARS-CoV-2, he says. “In a completely unmitigated environment—where no one is vaccinated or wearing masks—it’s estimated that the average person infected with the original coronavirus strain will infect 2.5 other people,” Dr. Wilson says. “In the same environment, Delta would spread from one person to maybe 3.5 or 4 other people.” “Because of the math, it grows exponentially and more quickly,” he says. “So, what seems like a fairly modest rate of infectivity can cause a virus to dominate very quickly.”

2. Unvaccinated people are at risk.

People who have not been fully vaccinated against COVID-19 are most at risk. In the U.S., there is a disproportionate number of unvaccinated people in Southern and Appalachian states including Alabama, Arkansas, Georgia, Mississippi, Missouri, and West Virginia, where vaccination rates are low. (In some of these states, the number of cases is on the rise even as some other states are lifting restrictions because their cases are going down). Kids and young people are a concern as well. “A recent study from the United Kingdom showed that children and adults under 50 were 2.5 times more likely to become infected with Delta,” says Dr. Yildirim. And so far, no vaccine has been approved for children 5 to 12 in the U.S., although the U.S. and a number of other countries have either authorized vaccines for adolescents and young children or are considering them. “As older age groups get vaccinated, those who are younger and unvaccinated will be at higher risk of getting COVID-19 with any variant,” says Dr. Yildirim. “But Delta seems to be impacting younger age groups more than previous variants.”

3. Delta could lead to 'hyperlocal outbreaks.'

If Delta continues to move fast enough to accelerate the pandemic, Dr. Wilson says the biggest questions will be about the heightened transmissibility—how many people will get the Delta variant and how fast will it spread? The answers could depend, in part, on where you live—and how many people in your location are vaccinated, he says. “I call it ‘patchwork vaccination,’ where you have these pockets that are highly vaccinated that are adjacent to places that have 20% vaccination,” Dr. Wilson says. “The problem is that this allows the virus to hop, skip, and jump from one poorly vaccinated area to another.” In some cases, a low-vaccination town that is surrounded by high vaccination areas could end up with the virus contained within its borders, and the result could be “hyperlocal outbreaks,” he says. “Then, the pandemic could look different than what we’ve seen before, where there are real hotspots around the country.” Some experts say the U.S. is in a good position because of its relatively high vaccination rates—or that conquering Delta will take a race between vaccination rates and the variant. But if Delta keeps moving fast, multiplying infections in the U.S. could steepen an upward COVID-19 curve, Dr. Wilson says. So, instead of a three- or four-year pandemic that peters out once enough people