Variant more contagious

Why is the delta variant more contagious than original strain?

Michelle Robertson

It's been widely stated that the delta variant of the coronavirus currently raging across the U.S. and world is more contagious than the original strain and even previous variants. But why?

To find out, SFGATE spoke with two experts, Stephen Morse, a professor of epidemiology at the Columbia University Medical Center, and Fenyong Liu, a professor of infectious diseases at the University of California at Berkeley.

Both stressed that what we know about the delta variant at this time remains limited, as it only recently emerged. But they can speculate as to what makes this strain so much more contagious than previous ones.

"Current evidence suggests that in the upper respiratory tract, delta replicates (grows or reproduces) faster and to higher levels," Morse said by email. "This would plausibly explain its increased transmissibility."

Morse cited a preprint study from China and collaborators that suggested that more virus was produced in infected individuals — about 1,000-fold more than the original strain. He also noted that the average time from infection to being able to readily detect the virus was about four days, compared to about six days during the original outbreak. 

"A caveat is that data are limited, may not apply to all cases, and there are still other hypothetical reasons that a variant could be more transmissible," he said. 

He stressed that more data is needed before we can state the reasons for its increased virulence "with confidence." 

He also noted that just because the viral load is up to 1,000 times higher with the delta variant doesn't necessarily mean it's 1,000-fold more transmissible. Rather, the delta variant appears severalfold more transmissible than the alpha variant. 

Liu also noted that the delta variant is much better at binding to cell receptors than the original strain. This means it can replicate much faster and be detected earlier. 

"So when a person coughs or sneezes, it potentially spreads out much more viruses than the variants or the original strain," he said. 

He added cautiously: "I don't think we understand the reasons quite yet, we just know it happens." 

Liu also stressed the importance of getting vaccinated. He noted that while the variant can still cause symptoms in vaccinated people, its effects are typically much less severe, and likely will not result in serious illness or hospitalization. 

"We have hope because we can really increase vaccination rates," he said. "We really need to stop the replication of the virus, we need to stop new infections. Because humans are the selection force of the virus." 

Liu, like Morse, also fears future variants that will be potentially worse than the delta variant as the COVID-19 virus continues to replicate. The more people who contract the virus, the more likely new variants will occur. 

"We are observing the real-time evolution of a virus," he said. "Hundreds of millions have been infected and the virus will just keep evolving." 

Morse echoed Liu's comments. "If one [variant] can win the race against earlier variants, it could become the next delta," he said. "That's why we're encouraging everyone to get immunized as quickly as possible, and to continue exercising caution, such as wearing masks when indoors."