The delta coronavirus variant was not associated with an increased risk of severe Covid-19, according to a relatively small study published by CDC in the Morbidity and Mortality Weekly Report. But some experts are questioning the study's findings, which appear to contradict other research.

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Study details

For the study, researchers looked at 7,615 Covid-19 hospitalizations in the United States in 2021. They compared patient outcomes between hospitalizations that occurred between January and June—a period when the delta variant was relatively rare—and those between July and August, after the delta variant became dominant.

The researchers found no significant differences in the percentage of Covid-19 patients who were admitted to the ICU, received mechanical ventilation, or died from Covid-19 while in the hospital.

The researchers did find that the share of patients hospitalized with Covid-19 who were between the ages of 18 and 49 rose from 24.7% in the earlier time period to almost 36% in the later period. "Lower vaccination coverage in this age group likely contributed to the increase in hospitalized patients during the delta period," the researchers wrote.

The researchers also noted the percentage of patients ages 50 and older who were either admitted to the ICU with Covid-19 or died of the disease "generally trended upward in the delta period," although the differences were not statistically significant.

Experts question study results

Some experts questioned the findings of the CDC's study, noting that other studies have found the delta variant is associated with an increased risk of severe Covid-19.

For example, one study from Scotland of 20,000 Covid-19 cases found the delta variant was associated with an 85% increased risk of hospitalization. And a study from England of 43,000 Covid-19 cases found those infected with the delta variant were more than twice as likely to be hospitalized with Covid-19 as those infected with the alpha variant.

According to the New York Times, those and other studies examining the delta variant have relied on genomic sequencing, which allowed them to determine which infections were caused by the delta variant and which weren't, a method CDC's study did not use.

David Fisman, an epidemiologist at the University of Toronto, conducted a study published this month that analyzed 200,000 Covid-19 cases and found the delta variant was associated with a roughly twofold increased risk of hospitalization.

Fisman also found the delta variant was associated with significantly increased risks of ICU admission and death after accounting for patients' age, sex, vaccination status, and other factors.

According to Fisman, studies on the delta variant need to control for a number of factors that can affect a patient's illness.

"As this is the U.S. CDC, I'm really surprised at the small sample sizes for individuals with more detailed clinical information, as well as the use of such rudimentary statistical methods to deal with [this] data," Fisman said. (Mueller, New York Times, 10/22; Carbajal, Becker's Hospital Review, 10/22)