A new, peer reviewed study estimates that the delta variant of COVID-19 doubles the risk of being hospitalized compared to the prior alpha variant among unvaccinated people.
The delta variant is the most highly transmissible strain seen yet, first emerging in India in late 2020 and quickly sweeping the globe. But scientists have debated whether this variant is also deadlier.
Preliminary studies from Scotland and Canada hinted that this version of the virus might be making people sicker, but some researchers said this could also be explained by the variant's hyper-transmissibility, which leads to massive COVID surges that overwhelm hospitals.
This new study, published in the peer-reviewed journal Lancet Infectious Diseases, adds increased evidence that the delta variant is more likely to send people to the hospital than the previously dominant alpha variant.
This doesn't apply if you're vaccinated, researchers say. Vaccines dramatically reduce the risk of hospitalization and death for both the alpha and delta variants. Most of the people in the U.K. study were unvaccinated.
"Our analysis highlights that in the absence of vaccination, any Delta outbreaks will impose a greater burden on healthcare than an Alpha epidemic," said Dr. Anne Presanis, one of the study's lead authors and senior statistician, MRC Biostatistics Unit, University of Cambridge, in prepared remarks.
In one of the largest studies yet looking at this question, U.K. researchers analyzed medical records of more than 40,000 COVID cases from March to May, roughly 20% of which were delta variant infections. By measuring what happened to people within 14 days of testing positive, researchers found that people infected with delta were more likely to seek medical care at a hospital or emergency room compared to people infected with the alpha variant.
"This is a large study that suggests a slight increase in [emergency department] visits and hospitalizations among unvaccinated persons infected with delta versus alpha," said Dr. Carlos Del Rio, executive associate dean of the Emory School of Medicine and Grady Health System in Atlanta.
"But to me, what the paper says more about delta is the fact that vaccines work," Del Rio said.
In the United States, the delta variant was first identified in March and had become the dominant variant by July. It has led to a massive surge among mostly unvaccinated people, including many young adults and children, who are less likely to be vaccinated than older adults.
"I know from anecdotal reports here in the U.S. that we are seeing more serious infections with the delta variant than the alpha variant, and these data support that," said Dr. Anna Durbin, an associate professor at the Johns Hopkins University School of Medicine.
"I think this is a trend we are seeing in the U.S. where pediatric ICUs are filling up and we are seeing young adults requiring intubation at a much higher rate than with the alpha peak in early 2021," Durbin said. "I am confident that we are seeing more severe illness in younger people with the delta variant."
In early August, National Institutes of Health Director Dr. Francis Collins said there wasn't enough data yet to be confident that the delta variant is more serious for children, but preliminary evidence so far is "tipping in that direction."
The Pfizer vaccine is currently authorized for children ages 12 and older, with authorization for children 5 to 11 expected sometime this winter. Roughly half of children ages 12 to 17 have received their first shot, according to White House Coronavirus Response Coordinator Jeff Zients, speaking at Friday press briefing.
ABC News' Cheyenne Haslett contributed to this report.