How deadly is the coronavirus delta variant?

A nurse checks on a patient in the ICU Covid-19 ward at NEA Baptist Memorial Hospital in Jonesboro, Arkansas, on Aug. 4, 2021.  (Image credit: Houston Cofield/Bloomberg via Getty Images)

The coronavirus delta variant is more infectious than previous versions of the virus, but is it deadlier?

Early data suggests the delta variant may cause more severe disease, but more studies are needed to know if this variant is indeed deadlier.

Several studies hint that, compared with the original strain of the virus, the delta variant can make people sicker if they are unvaccinated. 

A large study conducted in England from late March to late May 2021 found that, among more than 40,000 COVID-19 cases, those infected with the delta variant had about double the risk of hospitalization compared with those infected with the alpha variant. The study, published Aug. 27 in the journal The Lancet Infectious Diseases, took into account factors known to affect risk of infection, including age and vaccination status. The vast majority of COVID-19 cases in the study were unvaccinated or partially vaccinated.  

Another study from Scotland, published June 14 in the journal The Lancet, found that people infected with the coronavirus delta variant had a nearly twofold higher risk of being hospitalized between April and June 2021, compared with those infected with the coronavirus alpha variant, or the variant first detected in the U.K. But those who were vaccinated had a 60% reduced risk of being hospitalized with the delta variant than unvaccinated people who caught delta.

Related: Coronavirus variants: Here's how the SARS-CoV-2 mutants stack up

And a study from Canada, posted to the preprint website medRxiv on July 14, found that people infected with the delta variant were twice as likely to be hospitalized, and twice as likely to die, as those infected with a coronavirus strain that wasn't a "variant of concern" (i.e., not infected with alpha, beta, gamma or delta variants).

Still, these studies are preliminary, and more research is needed to show whether delta really causes more severe disease than previous variants. Delta may appear more serious than other variants simply because it caused outbreaks in populations with more risk factors for severe illness (such as older age or underlying conditions), or because outbreaks occurred in areas with more stress on their hospital systems, according to the American Society for Microbiology.

It's also unclear how the death rate from the delta variant compares with that from other variants. The overall case-fatality ratio (the number of deaths divided by the total number of cases) for COVID-19 in the U.S. since the beginning of the pandemic is 1.7%, according to data from Johns Hopkins University. However, this fatality rate includes deaths long before the availability of COVID-19 vaccines. Since the delta variant took off in the U.S. after the availability of vaccines, it's difficult to compare deaths from delta with historical deaths from earlier variants — the fatality rate for all variants is expected to drop as a result of vaccinations, according to The Poynter Institute.

What is clear is that current COVID-19 vaccines reduce the risk of infection, severe disease and death from the novel coronavirus, including the delta variant. Using data on the current rate of COVID-19 infections in the U.S., the CDC estimates that fully vaccinated people are eight times less likely to have a symptomatic infection, 25 times less likely to be hospitalized, and 24 times less likely to die from COVID-19 infection than those who are unvaccinated.

Editor's note: This article was updated on Aug. 27 at 6:40 pm ET to include information from the study published Aug. 27 in The Lancet Infectious Diseases.

Originally published on Live Science.  

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.