[Originally published: June 28, 2021. Updated: Dec. 21, 2021.]
Note: Information in this article was accurate at the time of original publication. Because information about COVID-19 changes rapidly, we encourage you to visit the websites of the Centers for Disease Control & Prevention (CDC), World Health Organization (WHO), and your state and local government for the latest information.
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.
Then, the Delta variant surfaced in the United States. First identified in India in late 2020, Delta swept rapidly through that country and Great Britain before reaching the U.S., where it quickly surged. It was, until mid-December, the predominant SARS CoV-2 variant, accounting for more than 99% of COVID-19 cases (at the time) and leading to an overwhelming increase in hospitalizations in some states. (Omicron is now the predominant variant in the U.S.)
Delta is believed to be more than twice as contagious as previous variants, and studies have shown that it may be more likely than the original virus to put infected people in the hospital. People who are not vaccinated are most at risk, and the highest spread of cases and severe outcomes is happening in places with low vaccination rates.
Inci Yildirim, MD, PhD, a Yale Medicine pediatric infectious diseases specialist and a vaccinologist, wasn’t surprised by Delta's progression. “All viruses evolve over time and undergo changes as they spread and replicate,” she says.
From what we know at this point, people who are fully vaccinated against the coronavirus continue to have strong protection against COVID-19 compared to those who aren’t, but there have been enough questions to lead the Centers for Disease Control and Prevention (CDC) to advise additional precautions, including mask guidelines regardless of vaccination status and recommendations for booster shots.
While most Delta infections have been in people who have not had a vaccine, data also has shown the variant to have increased transmissibility even among some vaccinated people.
According to the CDC, if a vaccinated person is infected with COVID-19 (in what’s called a “breakthrough” case) and they have symptoms, they can transmit the virus to others; the agency is still assessing data on whether fully vaccinated people can spread the virus if they have a breakthrough case but have no symptoms.
Here are five things you need to know about the Delta variant.
1. Delta is more contagious than the other virus strains.
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 World Health Organization [WHO] as an alternative to numerical names.)
Delta’s quick growth rate has been especially dramatic, says F. Perry Wilson, MD, a Yale Medicine epidemiologist. 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.
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, but cases are rising in other parts of the country as well. In September, health leaders in Idaho, which has one of the lowest vaccination rates in the country, expanded health care rationing throughout the state after the Delta surge led to a scarcity of resources for all hospitalized patients.
Children, teenagers, and young adults are a concern, too. “A 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. The U.S. has allowed Pfizer-BioNTech vaccinations for adolescents and teenagers since May, and, in early November, the CDC approved FDA authorization of the Pfizer vaccine for children ages 5-17.
“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—and Omicron—could lead to 'hyperlocal outbreaks.'
If Delta—and now Omicron—continue to accelerate the pandemic, Dr. Wilson says the biggest questions will be about the heightened transmissibility. The answer 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.”
So, instead of a three- or four-year pandemic that peters out once enough people are vaccinated, an uptick in cases would be compressed into a shorter period of time. “That sounds almost like a good thing,” Dr. Wilson says. “It’s not.” If too many people are infected at once in a particular area, the local health care system will become overwhelmed, and more people will die, he says. “That’s something we have to worry about a lot.”
4. There is still more to learn about Delta.
As data about Delta accumulates, scientists are working hard to learn as much as possible as quickly as they can. One important question is whether the Delta strain will make you sicker than the original virus. Early information about the severity of Delta included studies from Scotland and Canada, both cited by the CDC, that suggested the Delta variant may be more likely to result in hospitalization in the unvaccinated. A report this summer, published in The Lancet Infectious Diseases, found that people in England with Delta had double the hospitalization risk of those with Alpha, which was previously the dominant mutation in that country.
Another question focuses on how Delta affects the body. There have been reports of symptoms that are different than those associated with the original coronavirus strain, Dr. Yildirim says. “It seems like cough and loss of smell are less common,” she says. “And headache, sore throat, runny nose, and fever are present based on surveys in the U.K.”
Meanwhile, experts continue to study Delta and breakthrough cases. It’s difficult to pin down exact numbers of breakthrough infections in the U.S., where the CDC stopped counting cases that don’t result in hospitalization or death in May. The agency notes that no vaccine is 100% effective, and any rise in cases will have an accompanying rise in breakthrough infections.
There are additional questions and concerns about Delta, including Delta Plus—a subvariant of Delta, that has been found in the U.S., the U.K., and other countries. “Delta Plus has one additional mutation to what the Delta variant has,” says Dr. Yildirim. This mutation, called K417N, affects the spike protein that the virus needs to infect cells, and that is the main target for the mRNA and other vaccines, she says.
“Delta Plus has been reported first in India, but the type of mutation was reported in variants such as Beta that emerged earlier. More data is needed to determine the actual rate of spread and impact of this new variant on disease burden and outcome,” Dr. Yildirim adds.
5. Vaccination is the best protection against Delta.
The most important thing you can do to protect yourself from Delta is to get fully vaccinated, the doctors say. Recommendations for boosters and who is eligible to get one have been changing, and updated information is available on the CDC website.
Although fully vaccinated people with a breakthrough infection can infect others, the CDC reports that the amount of viral genetic material may decrease faster in vaccinated people—so, while they have been found to carry the same amount of virus in their noses and throats as unvaccinated people, studies have also found they may spread virus for a shorter time.
There are additional CDC prevention guidelines available for both vaccinated and unvaccinated people. As efforts continue to vaccinate more people in the U.S., the CDC is recommending “layered prevention strategies” for everyone, and that includes wearing face masks in public indoor settings in areas of substantial or high transmission. The agency has also recommended universal indoor masking for all teachers, staff, students, and visitors to K-12 schools. There also may be mask mandates in place depending on your geographic location.
“Like everything in life, this is an ongoing risk assessment,” says Dr. Yildirim. “If it is sunny and you’ll be outdoors, you put on sunscreen. If you are in a crowded gathering, potentially with unvaccinated people, you put your mask on and keep social distancing. If you are unvaccinated and eligible for the vaccine, the best thing you can do is to get vaccinated.”
Of course, there are many people who have not gotten the vaccine, because personal logistics or difficulties have created roadblocks—or they may have chosen not to get it. Will the Delta variant be enough to encourage those who can get vaccinated to do so? No one knows for sure, but it’s possible, says Dr. Wilson, who encourages anyone who has questions about vaccination to talk to their family doctor.
“When there are local outbreaks, vaccine rates go up,” Dr. Wilson says. “We know that if someone you know gets really sick and goes to the hospital, it can change your risk calculus a little bit. That could start happening more. I’m hopeful we see vaccine rates go up.”
Note: Information provided in Yale Medicine articles is for general informational purposes only. No content in the articles should ever be used as a substitute for medical advice from your doctor or other qualified clinician. Always seek the individual advice of your health care provider with any questions you have regarding a medical condition.