Scientists from the Centers for Disease Control and Prevention on Friday took aim at the question of whether the Delta variant of the coronavirus causes more severe disease, finding no significant differences in the course of hospitalized patients’ illnesses during the Delta wave compared to earlier in the pandemic.
But larger and more detailed studies from a number of other countries have found that people with Delta infections were considerably more likely to be hospitalized in the first place — a trend that the C.D.C. study was unable to address because of limitations in its data. The C.D.C. study also said that the proportion of older hospitalized patients needing intensive care or dying had shown some signs of increasing during the Delta wave.
Delta’s higher level of infectiousness has made it a far greater challenge than earlier versions of the virus, but the question of whether it also causes more serious disease has loomed as it swept around the world. The Alpha variant, an earlier version first detected in Britain, appeared to be linked to a higher risk of death, though scientists have also tried to understand whether factors besides the variant were playing a role.
Studies in England, Scotland, Canada and Singapore suggested that the Delta variant was associated with more severe illness, a finding that scientists have said raises the risk that outbreaks of the variant in unvaccinated areas may put a bigger burden on health systems. Unlike the C.D.C. study, those studies drew on genomic sequencing, allowing researchers to distinguish infections with the Delta variant and to track patients from before they enter a hospital.
Without access to sequencing data, the C.D.C. researchers could not determine which variants the patients may have been infected with. It also examined patients already admitted to hospitals, making it impossible to determine whether they were at higher risk of needing hospital care in the first place.
The study, released on Friday, examined roughly 7,600 Covid hospitalizations, comparing July and August — when Delta dominated — to earlier months this year, and found no significant change in hospitalized patients’ outcomes.
The study said that the proportion of hospitalized patients aged 50 and older who died or were admitted to intensive care “generally trended upward in the Delta period,” though the differences were not statistically significant and further work was needed. At the hospitals included in the study, roughly 70 percent of Covid patients were unvaccinated.
The researchers said the findings matched those of other C.D.C. studies using similar methods that showed no significant differences in the outcomes of younger people hospitalized before and during the Delta surge.
Outside scientists questioned the reliability of the study.
Dr. David Fisman, an epidemiologist at the University of Toronto, ran a larger study that found that people infected with the Delta variant had roughly twice the risk of hospitalization as people infected with variants that had not been labeled a concern. He said that such analyses needed to control for the range of factors that affect the course of Covid patients’ illnesses, and that the availability of vaccines, testing and treatments had all been changing during the pandemic.
“As this is the U.S. C.D.C., 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 these data,” he said.
Dr. Fisman’s study, drawing on 200,000 cases and published this month, also showed significantly increased risks of intensive care admission and death among those infected with the Delta variant, after accounting for their age, sex, vaccination status and other factors.
Roughly 70 percent of people with Delta infections in the study were unvaccinated, and 28 percent were partially vaccinated. Fully vaccinated people are heavily protected from Covid.
Similarly, a study in Scotland from June based on 20,000 Covid cases showed that Delta infections were associated with an 85 percent higher risk of hospitalization, though it allowed for a wide degree of uncertainty about the precise figure.
And data from England, drawn from 43,000 cases and published in August, found that people infected with the Delta variant were just over twice as likely to be hospitalized as people with the Alpha variant, though the researchers in that study, too, were unsure of the precise figure.
Roughly three-quarters of the patients in that study were unvaccinated, and most of the rest were only partially vaccinated.