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What we know about how the delta variant of covid is spreading

It looks like vaccinated people can spread infections. We answer five questions about delta, what makes it more contagious, and why it’s complicated.

medical assistant administers a COVID-19 test in Los Angelesmedical assistant administers a COVID-19 test in Los Angeles
Mario Tama/Getty Images

Covid cases in the US have doubled over the past two weeks, and scientists are now racing to understand the delta variant, which appears to account for the vast majority of new infections. Disturbingly, delta is more contagious than other variants and has also been causing some symptomatic “breakthrough” cases in vaccinated people. 

While vaccines still overwhelmingly prevent severe disease and death, the delta variant has been changing how we think about coronavirus spread. Here are some answers to a few important questions about what it all means. 

1. What makes the delta variant more contagious?

According to estimates from the CDC, the delta variant is nearly twice as contagious as previous versions of the virus. Researchers are still trying to understand the mutations that account for this, but preliminary studies suggest that changes in its spike protein make it more efficient at both grabbing onto receptors and getting into your cells. 

The delta variant also seems to lead to higher viral loads than other variants. Viral load is a measure of how much virus is in your nose and throat. One study found that at the beginning of their infection, people with the delta variant had viral loads 1,000 times greater than those of people infected with the original version of the virus. People with the delta variant also reached their peak viral load faster, according to this study, which has not yet gone through peer review. 

2. How do scientists actually measure how contagious the delta variant is?

Viral load helps us understand how contagious a virus is. Coronavirus infections spread through aerosols and droplets when an infected person coughs, sneezes, or just breathes—so the more virus particles are in someone’s airways, the more likely that person is to infect someone else.

To measure viral load, researchers use a laboratory method called polymerase chain reaction, or PCR. They swab the nose of an infected person and extract any viral RNA that’s on the swab. Next, they run the reaction, which looks for genetic material from viruses and copies it over and over again, until there are enough copies for the lab equipment to detect.

We usually focus on the endgame of PCR—whether a test finds material from a virus, which produces a positive result. But researchers can also look at how long the machine took to return that positive result—how many copies were needed to get the viral material to a detectable level. The fewer copies, or cycles, needed to detect a virus, the more viral material there was to begin with.

This number of cycles—called the cycle threshold, or Ct—is the number that raised eyebrows at the Centers for Disease Control and Prevention. In a cluster of cases in Provincetown, Massachusetts, about 74% of the state residents involved were vaccinated. People who got infections tended to have similar Ct values, whether they were vaccinated or not. The CDC thought this might be an indicator that vaccinated people could transmit the virus, perhaps just as readily as unvaccinated people.

3. Can I still get sick with covid, even if I’m vaccinated? 

Yes, it’s possible, although your infection will likely be much less severe than an unvaccinated person’s. 

The vast majority of infections are still in unvaccinated people, says Liz Rogawski McQuade, an infectious-disease researcher at the University of Virginia. According to reporting from the Kaiser Family Foundation, US states that have tracked vaccination status of cases for different periods over the last year have found that between 94% and 99.9% of cases have been in people who are not fully vaccinated. And out of everyone who has been vaccinated, between 0.01% and 0.54% have experienced a breakthrough case.

Some studies have found that the efficacy of vaccines is a bit lower against the delta variant, especially if you’ve only gotten one dose of an mRNA vaccine. But so far, it looks as if vaccines still largely work, especially in preventing many cases of severe illness, says Rogawski McQuade. 

Vaccines might eventually need a little extra help against the delta variant—some companies are pushing for booster shots. But experts say there’s not yet evidence that boosters are necessary, and the WHO maintains that initial vaccines for the rest of the world should take precedence over booster shots for people in rich countries. 

4. What about transmission? Can vaccinated people spread the delta variant? 

It appears so, but research is still in early stages. 

While Ct values can be used as a proxy for viral load, there are a few problems with trying to assume too much on the basis of that number, especially when it comes to vaccinated people, according to Monica Gandhi, an infectious-disease researcher at University of California, San Francisco.

First of all, PCR picks up all sorts of genetic material, even from dead viruses. If your vaccinated immune system has started fighting off the infection, “you may have a lot of viral particles in your nose, but they may not work,” Gandhi says. In order to really know how infectious someone is, you need to take those viruses and see if they’re alive and able to infect people. The CDC has noted that this data is still outstanding, Gandhi says.

Also, researchers expect that vaccinated people, if they’re infected, might experience high viral loads for shorter periods of time. In one study from Singapore, researchers found that viral loads reached high levels in vaccinated people but dropped much faster than in those who were unvaccinated, although that research has not yet gone through peer review.

Other research, like one study in the UK, found that vaccinated people actually have lower viral loads than unvaccinated people, suggesting that the vaccine helps reduce transmission.

Gandhi says more studies are needed to help us better understand the role vaccinated people play in transmission. 

5. Should I be worried about these other variants I’m hearing about, too? 

Scientists keep track of where the virus is traveling and how it’s changing by grouping different strains into families, called lineages. When one of those lineages starts to act in a way that appears more dangerous, the WHO designates it a “variant of concern” or a “variant of interest” and nicknames it with a letter of the Greek alphabet, like delta. 

Delta is actually a family of sequences that are all related to each other.

Scientists often group such families into smaller “sublineages” for research purposes. You may have seen news articles about one of those, which journalists have been calling “delta plus.” But because there’s no evidence that this smaller group of viruses is behaving in a new way, the WHO still considers it part of the delta family.

Will new variants keep coming at us? What about lambda and gamma, for example? Viruses mutate all the time, so as long as there are places in the world where there’s unchecked spread, we’ll likely continue to see more variants that will behave differently. “But it doesn’t have to be that way,” says Rogawski McQuade. Slowing down the spread leaves the virus with less opportunity to mutate, which means fewer variants will emerge. 

That means the solution is to increase vaccination rates, and fast. But it will probably take more than just vaccines to keep the current wave under control, experts say. For example, at the end of last month, the CDC began recommending that vaccinated people wear masks indoors in most US communities

“Vaccines are by far our most effective tools,” Rogawski McQuade says, “but they clearly are not perfect, and they’re not our only tools. We really have to have the combination of interventions to make it through this.”

This story was updated on August 10 to clarify that the proportion of cases in vaccinated people has been reported by states over the period of several months.