Scientists from the Centers for Disease Control and Prevention investigated on Friday whether the delta variant of the coronavirus causes more severe illness and found no significant differences in the disease progression of hospital patients during the delta wave compared to earlier in the pandemic.
However, larger and more detailed studies from a number of other countries have found that people with Delta infections are significantly more likely to be hospitalized at all – a trend the C.D.C. Study could not be treated due to data limitations. The C.D.C. The study also said the proportion of elderly hospital patients who needed critical care or died during the delta wave had shown some signs of increasing.
Delta's higher infectivity has made it far more of a challenge than previous versions of the virus, but the question of whether it also causes more serious illness has arisen as the world moves on. The alpha variant, an earlier version first discovered in the UK, appeared to be linked to a higher risk of death, although scientists have also tried to understand whether factors other than the variant were at play.
Studies in England, Scotland, Canada and Singapore suggested that the Delta variant has been linked to more severe illness, a finding that scientists say increases the risk that outbreaks of the variant in unvaccinated areas could place greater stress on health systems. In contrast to the C.D.C. Study, these studies relied on genome sequencing, which allowed researchers to differentiate Delta variant infections and track patients before they were hospitalized.
Without access to sequencing data, the C.D.C. The researchers were unable to determine which variants the patients might have been infected with. It also looked at patients who had already been admitted to hospitals, which made it impossible to determine if they were at any higher risk of needing hospital treatment.
The study, published on Friday, looked at around 7,600 Covid hospital admissions, compared July and August – when Delta dominated – with earlier months of this year and found no significant changes in the outcomes of hospitalized patients.
The study said the proportion of hospitalized patients aged 50 and over who died or were admitted to intensive care "generally trended upward over the delta period," although the differences were not statistically significant and more work was needed. In the hospitals included in the study, around 70 percent of Covid patients were unvaccinated.
The researchers said the results were in line with those of other C.D.C. Studies using similar methods that showed no significant differences in outcomes in younger patients admitted to the hospital before and during the delta rise.
External scientists questioned the reliability of the study.
Dr. David Fisman, an epidemiologist at the University of Toronto, conducted a larger study that found that people infected with the Delta variant were about twice as likely to be hospitalized as people infected with variants that have not been classified as a matter of concern. He said such analyzes would need to control a number of factors affecting the course of Covid patients and that the availability of vaccines, tests and treatments had changed during the pandemic.
"Since this is the American C.D.C. I'm really surprised at the small sample size for people with more detailed clinical information, as well as the use of such rudimentary statistical methods to deal with that data, ”he said.
The study by Dr. Fisman, which is based on 200,000 cases and released this month, also showed a significantly increased risk of intensive care unit admission and death in those infected with the Delta variant after age, gender, vaccination status, and other factors were considered .
Approximately 70 percent of those with Delta infections in the study were unvaccinated and 28 percent were partially vaccinated. Fully vaccinated people are strongly protected from Covid.
Similarly, a June study in Scotland based on 20,000 Covid cases showed Delta infections were associated with an 85 percent higher risk of hospitalization, although it allowed a high level of uncertainty about the exact number.
And data from England, pulled from 43,000 cases and published in August, found that people infected with the Delta variant were hospitalized just over twice as often as people with the Alpha variant, despite themselves The researchers in this study also weren't sure of the exact number.
About three quarters of the patients in this study were unvaccinated; the rest were only partially vaccinated.