The highly contagious variant of COVID-19 is considered at least two times more contagious than the previously dominant alpha strain, and experts say the increased transmissibility has likely fueled the surge in COVID-19 cases, hospitalizations and deaths nationwide.
But much is still unknown about delta as scientists scramble to better understand the strain.
Here’s what we know about the delta strain and how it blunted earlier momentum in the fight against the coronavirus.
Delta is more transmissible than previous COVID-19 strains
Delta’s contagiousness is considered key to its domination, having spread to at least 117 countries after first being detected in India. Like other viruses, COVID-19 is evolving, particularly through unplanned mutations.
A study from the United Kingdom in May suggested the delta strain could be 60 percent more transmissible than the alpha variant, which was already more contagious than the original strain.
But experts are split on that figure, with some saying delta could be more transmissible and others saying it could be less.
“You don’t necessarily want to attribute that all to the virus. You know, a lot of it may reflect the people as well,” said David Dowdy, an associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.
Researchers aren’t certain about what makes the delta variant more transmissible, but there are some clues.
Michael Farzan, head of the Department of Immunology and Microbiology at Scripps Research, said one of the variant’s advantages is that it can more strongly attach to a certain receptor when spreading in the body.
“This is one of the reasons why the virus … in a person gets made at a higher level, meaning that there’s a lot more being spit out or coughed out, meaning that it’s more likely to hit the next person,” he said.
The Centers for Disease Control and Prevention (CDC) has its own figures illustrating how the strain became so prevalent this summer. The agency’s latest projection is that 97.4 percent of all coronavirus cases come from all the different lineages of the delta variant, as of the week ending last weekend.
That marks an astronomical increase from the 1.6 percent estimated at the beginning of May and the 14.1 percent from the beginning of June.
Most people infected with COVID-19 at this point won’t know for sure whether they contracted the delta strain since available testing doesn’t make the distinction between strains — it only shows whether the virus itself is present.
It has a higher magnitude of viral loads
Health experts are examining the delta variant’s viral load, the measure of how much virus a person carries and can potentially transmit, compared to previous COVID-19 strains.
A study from China suggested that the strain’s viral load could be more than 1,000 times higher than the original strain, which Fauci on Thursday said “is a mechanistic reason why you have such a tremendous increase in transmissibility.”
Basically a higher viral load can make it more likely that an infected person can “shed” the virus, allowing someone nearby to contract it.
“If a little droplet that you sent out, it has more particles and that means it’s more likely to infect the next person over and it’s more likely to infect the next person over more times,” Farzan said.
Dowdy of Johns Hopkins cautioned that other variables, including people’s behavior, may be influencing how scientists understand delta’s viral load. With more people relaxing their COVID-19 precautions and interacting with others indoors, those same people could contract more of the virus than they might otherwise.
A study of a Massachusetts outbreak indicated that delta led to fully vaccinated people having a similar viral load compared to the unvaccinated, sparking the CDC to update its mask guidance late last month.
The outbreak on Cape Cod, where nearly three-quarters of confirmed cases were among fully vaccinated people, suggested that vaccinated people could potentially transmit and spread the delta variant. But researchers said at the time that microbiological studies would be needed to confirm whether vaccinated individuals can transmit the strain.
Vaccines are still effective against delta
Studies have found that at least five vaccines, including all three used in the U.S., are effective against the delta variant in lab and real-world settings, Fauci said on Thursday.
It was previously unclear whether the Johnson & Johnson vaccine, which requires only one dose instead of two, was equally effective. But a study released last week found the immune response lasted at least eight months, resulting in the first real-world data for the vaccine, Fauci said.
Recent studies have indicated that vaccines may see a very slight dip in effectiveness against symptomatic versions of the coronavirus caused by the delta variant. The COVID-19 vaccines, like any other, are also not perfect at preventing all delta infection and illness.
But scientists agree that studies have demonstrated that the vaccinated population is less likely to get infected and much less likely to be hospitalized or die from the delta variant than the unvaccinated.
“The only reason our case numbers are lower now than they were back in December is because half of our population has been fully vaccinated,” Dowdy said.
Still more to learn
Experts acknowledge there is much more to learn about the delta variant.
“A big thing is we still don’t know how much of what we’re seeing is due to the virus versus due to behavior,” Dowdy said. “That makes a big difference because things that are due to the virus, we can’t really change as a society.”
Although there’s a growing number of studies, not all scientists are certain that the variant itself necessarily causes more serious illness among the unvaccinated, leading to more hospitalizations and deaths. It’s also unclear whether the strain is sparking more severe illness among children as pediatric hospital admissions have picked up.
Additionally, scientists have more analysis to do on under-researched mutations that may give the virus more of an advantage, Farzan said.