In summer 2021, people were feeling some hope—or at least cautious optimism—that the pandemic could recede to the background, although there was still the threat that new mutations of the COVID-19 virus could bring it back, and it might be even stronger.
That’s when 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 and became 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 replaced Delta as the predominant variant, but in March 2022, the CDC still included Delta on its list of variants of concern in the U.S., even though it accounted for 0% of cases in the U.S. at that time.)
Delta was believed to be more than twice as contagious as previous variants, and studies have shown it to be more likely than the original virus to put infected people in the hospital. People who were not vaccinated were most at risk, and the highest spread of cases and severe outcomes happened 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 said.
People who are fully vaccinated against the coronavirus continued to have strong protection against COVID-19 compared to those who weren’t, but there were enough questions about Delta 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 showed the variant to have increased transmissibility even among some vaccinated people.
Here are five things you need to know about the Delta variant.
1. Delta is more contagious than the other virus strains.
The CDC 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 naming conventions for the variants were established by the World Health Organization [WHO] as an alternative to numerical names.)
Delta’s quick growth rate was especially dramatic, said 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 said. “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 said. “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 said. “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 was a disproportionate number of unvaccinated people in Southern and Appalachian states, where vaccination rates were low at the time of the Delta surge, but cases rose in other parts of the country as well. In September, health leaders in Idaho, which had 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 were 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,” said Dr. Yildirim. The U.S. had allowed Pfizer-BioNTech vaccinations for adolescents and teenagers since May 2021, and, in early November, the CDC approved FDA authorization of the Pfizer vaccine for children ages 5-17.
3. Delta—and Omicron—and the risk of 'hyperlocal outbreaks.'
If Delta—and later the Omicron variant—continued to accelerate the pandemic, Dr. Wilson said the biggest questions would 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 said. “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 said. “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 was surrounded by high vaccination areas could end up with the virus contained within its borders, and the result could be “hyperlocal outbreaks,” he said. “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 said. “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 said. “That’s something we have to worry about a lot.”
4. There is still more to learn about Delta.
As data about Delta accumulated, scientists worked hard to learn as much as possible as quickly as possible. One important question was whether the Delta strain could make a person 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 in 2021, 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 focused 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 said. “It seems like cough and loss of smell are less common,” she said. “And headache, sore throat, runny nose, and fever are present based on surveys in the U.K.”
Meanwhile, experts have continued to study Delta and breakthrough cases. The CDC has noted that no vaccine is 100% effective, and any rise in cases will have an accompanying rise in breakthrough infections.
There have been 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,” said 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 said.
“Delta Plus was reported first in India, but the type of mutation was reported in variants such as Beta that emerged earlier,” Dr. Yildirim added.
5. Vaccination is the best protection against Delta.
The most important thing you can do to protect yourself from Delta, Omicron, or any COVID-19 variant 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.
Meanwhile, experts continue to recommend infection prevention strategies, including following its latest guidelines on wearing masks, washing hands, being careful around crowds, social distancing, and testing.